Multiple sensor and injector failure codes

At this point I'm going to have to defer to some generalizations and get involved with some of the things that throw experienced mechanics. First of all, reading voltages is a lot more accurate than taking resistance readings of wires, and reading sensor signal voltages on a scanner is much better than using a voltmeter, at least at first. One of the problems with reading resistances is all it takes is one good strand of wire to read 0 ohms. Admittedly, in a sensor circuit with almost no current flow, that's good enough, but when we get to high-current circuits, you know that even though you measure 0 ohms through that one strand, you won't get enough current to flow through it to run a power window motor or heater fan motor. All those other broken strands would cause a huge voltage drop, but only when current is trying to flow. (Think of standing on a garden hose and partially blocking it. You'll still have full pressure [voltage] as long as the nozzle is turned off, but open it up and try to get some water to flow, [current], and that blockage will cause a drop in pressure at the nozzle).

Getting back to the sensor wires, reading voltages with the sensor's connector unplugged is the same as reading the water pressure at the hose nozzle with your big fat foot kinking the hose. Full pressure, or 5.0 volts for a sensor supply, will show up regardless of how much resistance is in that wire. When you plug the sensor in and take a voltage reading by back-probing the connector, you're taking the reading with current flowing. Any resistance in the wire will cause a voltage drop and you'll find less than 5.0 volts, just like with your foot on the hose, you'll find less water pressure at the nozzle only after you open it.

The only time resistance measurements have value is when you identify an open wire, with infinite resistance, (broken), or as a verification test after you've figured out the cause of the problem.

The next thing that relates to this is the value in taking voltage readings at both ends of a wire. This is especially true when a reading at the sensor doesn't agree with what's on the scanner's display. The scanner shows what the computer is seeing, and that should be the same as what YOU are seeing at the sensor. The issue here is a break in the signal wire. I'll be back shortly for part 2.

Let me stop here for a minute and share a useful tidbit. In the typical three-wire sensor circuit, like for a throttle position sensor, it is fed with 5.0 volts and ground. Ground is typically 0.2 volts but we'll call it 0.0 volts for simplicity. The mechanical movable contact inside the sensor can go from lowest to highest voltage as the throttle is opened, however, there are mechanical stops inside it that limits the travel from approximately 0.5 to 4.5 volts. (Those numbers are for discussion of theory. In actual practice you'll find perhaps 0.38 volts to 4.22 volts or something like that.) The point is, you'll never find 0.0 volts at idle or 5.0 volts at wide-open-throttle. What CAN happen though is lets say the 5.0 volt feed wire is broken, or there's a break inside the sensor. Without that 5.0 volts, you'll have 0.00 volts on the ground wire, like normal, AND on the signal wire. 0.00 volts is outside of that 0.5 to 4.5 volt range and is not an acceptable value. THAT is what sets a diagnostic fault code for, in this case, "TPS voltage too low". Similarly, if the ground wire or the connection inside the sensor were open, you'd see 5.0 volts everywhere, including the signal wire. 5.0 volts is outside the acceptable range again, so this would set the fault code, "TPS voltage too high". It's those voltages the computer uses to know when to set a fault code.

My whole reason for sharing this wondrous information is to make sense of what happens if the signal wire is open. For now, lets assume it's the wire, and not a break inside the sensor. With a good 5.0 volt feed circuit and a good ground circuit, the sensor will work properly and you'll find between 0.5 and 4.5 volts at the sensor's signal terminal, but with a broken wire, that voltage can't get back to the computer. Logically you might expect to find 0.0 volts at the computer, and in some cases you'd be right, but not for the reason you think. That signal wire is tied to all kinds of circuitry inside the computer, and when nothing is broken, all that other circuitry is irrelevant. It has absolutely no affect on the signal voltage. But, ... When the signal wire is broken, thanks to all that other circuitry, the voltage on that wire can "float" to some random value. That random value could be between 0.5 and 4.5 volts, and it could bounce around as other signal voltages change inside the computer. As long as it stays within the acceptable range, the computer will accept it and try to make fuel metering calculations based on those erroneous readings.

Okay, I'm finally getting to the good stuff. To prevent those erroneous readings, they add a "pull-up" resistor between the 5.0 volt supply circuit and the signal circuit, inside the computer. That resistor is such an extremely high resistance value that it has no effect on a properly working circuit, BUT, ... When the signal wire is broken, that pull-up resistor places 5.0 volts on the signal circuit to force a bad condition that will be detected and set a fault code. This is where you would measure the proper 0.5 to 4.5 volts at the sensor's connector, but the scanner would show 5.0 volts all the time, regardless of throttle position.

Where inexperienced mechanics get derailed is they start out by checking for fault codes, they get a "voltage too high" code, and assume the break is inside the sensor, so they replace it, then have the same problem and same code. That wastes enough time if they have the sensor in stock, but if they have to order it and wait for it, or it's an expensive part, you can see where that starts to run into dollars they can't charge the customer for. Experienced mechanics know they need to verify the scanner's reading at the sensor. With one quick measurement, they will see immediately the two don't match. The only way you can have a different voltage at each end of a wire is when that wire is open, (broken).

Okay, here's part 2. I mentioned an almost broken wire with just one strand still intact. That is all it takes to get a resistance reading of 0 ohms. Sure, that would be good enough for a sensor circuit, but what about when that last strand has just cracked and there's a little carbon between the ends? Arcing or any spark leaves a carbon track behind, just like we used to see inside wet distributor caps years ago. Carbon conducts electrical current but not as well as copper wires do. The resistance of that carbon can be measured just like any other resistor, but more importantly, it can allow enough current to get through to put some voltage on the signal wire at the computer. The typical symptom is the voltage on the scanner will change as the accelerator pedal is pressed, but it will be erratic and incorrect. More noticeable is the voltage will change when you aren't doing anything with the accelerator. Flexing the wiring harness almost always causes a noticeable change in voltage on the scanner.

The next thing to consider is there are almost always a number of three-wire sensors with a 5.0 volt feed, a ground return, and a signal wire. Those will share a single 5.0 volt feed from the computer, and they'll share a common ground wire that goes to ground AFTER it goes through some circuitry inside the computer. (That's why you'll find close to 0.2 volts instead of 0.0 volts). Once the 5.0 volt wire runs from the body over to the engine, there's a splice, and individual wires run to each sensor. The same is true for the ground wires. Those splices are real good suspects for corrosion and intermittent connections. The main wire usually runs to one sensor, and those that are wrapped around it and soldered to it go to the other sensors. The clue to a corroded splice is you'll find normal 5.0 volts at one sensor's connector, and erratic or low voltage at all the others. Low voltage at the sensors' 5.0 volt feed terminal will cause a low signal voltage that will be seen on the scanner. With a throttle position sensor it will always be proportional, meaning if you have 0.5 volts at idle when you have the proper 5.0 volts on the feed wire, if you have half as much, (2.5 volts) on the feed, you'll have half as much on the signal wire, (0.25 volts). The signal voltage will always be low by the same percentage as the feed voltage is low.

That's not true with other sensors. The MAP sensor is a good example. As I recall, you said yours has four wires. I'm not sure what that fourth one is for. Typically you'll have the same ground and 5.0 volt feed wires, but the signal voltage varies depending on vacuum. The signal voltage for a mass air flow sensor varies depending on the weight of the air flowing through the sensor. With this type of sensor, it's hard to know what the signal voltage will go to when the 5.0 volt supply is just low, but if it goes to 0.0 volts, the sensor will stop working. The same thing happens when the ground wire is broken. Either condition will set multiple fault codes, like you have. That is the clue to suspect broken wires when you have multiple codes, because you know it isn't likely two or three sensors failed at exactly the same time. When there's only a fault code for one sensor, you can assume the other sensors are working since the computer didn't detect a problem with them, and that would tell you the sensor is a better suspect than the wires running to it from the splices.

The scanner and / or fault codes will tell you which circuit(s) to look at, then the voltage readings at the sensor(s) will tell you whether to suspect sensors or wires, and which wires, by those voltage readings.

The next thing to consider has to do with how some computers work. We've all heard stories about how someone found out their engine runs better when they disconnect a sensor. I mean it's running poorly, but now is better once that sensor is disconnected. There will still be symptoms like a hesitation, stumble, low power, etc, but it runs better than having to walk! At issue is the computer is still using a sensor's reading to calculate fuel metering even though it's getting bad data from that sensor. Often this includes there not being a fault code set for that sensor. The most common example of this is the MAP sensor, especially those that have a vacuum hose between the sensor and the intake manifold. If there's a crack in that hose, the sensor will see lower-than-normal vacuum, and that corresponds to acceleration or higher load on the engine. The computer responds by commanding more fuel even though the engine was running okay. The result is black smoke from the tail pipe, low power, and poor fuel mileage, among other things. Sometimes the computer will see, thanks to all the other sensors and operating conditions, that the MAP sensor is sending the wrong signal voltages, so it will disregard it. It needs to factor in engine load to the fuel calculations, so when it knows it can't trust the signal voltages, it "injects" an approximate voltage and tries to run on that. It knows, for example, if you're in "park" or "neutral", you're not under load. If the throttle position sensor says you're at idle, it knows you're not accelerating. These and a lot of other factors help the computer guess at the correct MAP voltage and that's what is uses.

It guesses pretty well in most cases, and that brings us to when someone unplugs the sensor. If that sensor is sending signal voltages that are within the acceptable range, AND are somewhat close to what they should be, as in that cracked hose to the MAP sensor, the computer will believe that sensor, but since the signal voltage is actually wrong, you can expect the engine to present some symptoms. When you unplug the sensor, the computer is forced to admit it can't get a usable signal, so it reverts to that injected signal that's based on all the other sensors and operating conditions. If it sees a high throttle position sensor voltage, for example, along with a relatively low rpm, it knows you're pulling a heavy trailer up a steep hill, and manifold vacuum is going to be real low. It knows you need more fuel even though it doesn't have a signal from the MAP sensor.

This type of problem will usually set a fault code but it won't tell you why. A typical code might be "Erratic MAP voltage", "no change in MAP from start to run", or "MAP out of range". In this case, "out of range" doesn't mean out of the acceptable 0.5 to 4.5 volts. It means it knows the signal voltage should be something other than what it's seeing. If the incorrect voltage is close to correct, the computer may try to run on it but it may still set a fault code.

With the sensor unplugged, you know the computer will inject an approximate voltage to run on. That approximate voltage might be a whole lot closer to correct than what's coming from the sensor, and that's why the engine runs better. With a signal voltage that's known to be incorrect but close, the computer will set a fault code, then it MIGHT inject an approximate value to run on. Here's where the odd problem can occur. The scanner may display the injected value, not the incorrect signal coming from the sensor! That has to be a programming issue with the car's computer because the scanner only displays the information it gets from that computer. This has happened on some GM cars in the '90s but I can't be more specific than that. You could have a fault code that says a sensor's signal voltage is too high, but the voltage on the scanner appears to be perfect and changes like it is expected to. The code might have set during an intermittent problem, and you would assume it wouldn't set again once it's erased, but in fact, the code comes right back while the signal voltage looks good. I know this is an issue with some other car brands because it's common enough that we discuss it from time to time. This is where it is critical that you measure the voltage right at the sensor to verify the findings on the scanner before you buy a part.

The fact that you're seeing changing voltages tells me you're in a circuit with a defect, but to have so many fault codes they would have to have something in common. Given the engine work you just did, the likely suspects, as I'm sure I must have mentioned already, are a break in the common 5.0 volt feed wire, a break in the common ground wire, or all the signal wires have breaks, meaning a harness connector is unplugged or loose.

Oh, and lest I forget, instead of a pull-up resistor, a manufacturer could elect to use a "pull-down" resistor. That works the same way, but it's connected to ground instead of the 5.0 volt supply, and will also force an unacceptable condition to be detected when the signal wire is broken. You'll find 0.2 volts for signal voltage.

Well, I have one more suggestion, and it's related to what you already did. In tv repair, we ran into integrated circuits that were similar to an automotive computer module, and part of the troubleshooting was the same. There are four things needed to make them work. You must have all the power supplies, all the grounds, all the inputs, then you'll have all the outputs as long as nothing is shorted or open in those output circuits.

For your Engine Computer, the power supplies are going to be 12 volt sources, and there are likely to be four of them. I'm going to have to defer to my Chrysler experience, but most brands are very similar. You'll need a copy of the manufacturer's service manual. Sorry to say your car isn't listed on the online service manuals I have access to.

With most cars, there is going to be a constant 12 volt supply to keep the computer's memory alive. A second 12 volt supply comes from the ignition switch and tells the computer when to turn on and start doing stuff. In the case of Chrysler products, a third 12 volt supply comes from the relay that gets turned on by the computer. That's to verify the relay turned on. A fourth 12 volt supply may come from some other source as the system voltage sensing circuit for the voltage regulator. Chrysler builds their voltage regulators into the computer. You probably won't have that since most other manufacturers built theirs right into the generator.

With the service manual, there should be a chart that lists every terminal for the computer, and at least what its function is. From that you can infer which are the 12 volt supplies and the grounds, or you can look at the diagram to see where the wires go. You can go down the list and check each terminal in order, but I prefer to check every 12 volt supply first since that's where most problems lie, then I check every ground, and then the inputs.

The grounds are a little tricky. Typically there's four of them, two "power" grounds and two "signal" grounds. Measuring the resistance in these wires is usually very inaccurate and leads to overlooked defects. The issue is if there's just a real little corrosion between a wire and the terminal that's crimped onto it, that resistance may be too small to measure accurately. That's partly due to the resistance in the meter leads. Those could have two to five ohms themselves, so half an ohm of resistance in a connection would not be noticed. What we CAN measure is the result of that resistance, and that's the voltage that is dropped across it when current tries to flow through it. We do this in high-current starter circuits all the time. There, .04 ohms of resistance is WAY too small to measure, but it's enough to cause slow cranking. We can find that by the voltage dropped across the connections.

You can do the same thing with the Engine Computer's ground wires, but it has to be done when current is trying to flow. You can start by just measuring the voltage on each ground wire, and if you're lucky enough to find one with voltage, the problem is located. However, once you found 0.02 volts or less, you have to do the tests again while cranking the engine. That's when multiple circuits get turned on and more current will be flowing. That current will result in a measurable voltage if there's excessive resistance in a wire.

I started with that description to explain why there's multiple ground wires. There's usually two "power" grounds in case one fails. By "power" they mean that is the ground circuit for things that use a lot of power. That includes ignition coils, injectors, and relays and solenoids. All of those things draw relatively high current so those will be fatter ground wires. Also, relays and solenoids create voltage spikes when they get turned off, and ignition coils and injectors do the same thing. Anything with an electromagnetic coil generates a voltage spike when it gets turned off. That's the goal in an ignition coil, but it's an undesirable byproduct of the other devices. Those items draw current that pulses on and off, and the normal resistance in the ground wires causes a voltage drop proportional to the pulsing current flow through that resistance. All of these high-power items are not affected by a little voltage drop. If an injector works fine on 14.0 volts, it will work fine with only 13.8 volts.

The problem comes in with the "signal" ground circuit. That means engine sensors. As I mentioned earlier, it's normal to find 0.2 volts on the sensors' ground terminal, and since nothing is pulsing on and off, current flow and the voltage drop remain steady. If we were to tie all of these ground wires together into just one wire, as some people do when rewiring a car, that pulsing voltage drop from the high-power stuff would show up on the sensors' ground terminals. Now you have 5.0 volts feeding the throttle position sensor, for example, but you no longer have 0.2 volts on the ground terminal. To make the discussion easier, lets use 0.0 volts again for ground. If you put the throttle position sensor exactly in the middle of its range, there will be 2.5 volts on the signal wire. However, if the ground terminal has 0.4 volts on it, midway on the TPS would be 2.7 volts. This is just like with a tape measure. 2.5 inches is halfway between 0.0 inches and 5.0 inches, but 2.7 inches is midway between 0.4 inches and 5.0 inches.

That 0.2 volt error might seem insignificant, but that is a major deviation for a MAP sensor. Even if the computer were to learn the characteristics of that sensor, it is going to see the pulsing voltages and interpret that as a pulsing load on the engine. The bottom line is we use two different ground circuits so the sensor voltages aren't influenced by those high-current voltage drop pulses that can't be avoided.

To condense all this wondrous information about grounds, high resistance in one of those wires typically results in running problems like hesitations, or other electrical symptoms like flickering headlight brightness. They usually don't cause a no-start condition. To cause a no-start, a ground wire would be more likely to be broken or disconnected.

Most of the no-start conditions are caused by the loss of an input. Those are the crankshaft position sensor and / or camshaft position sensor. Pulses from those sensors are what tells the Engine Computer the engine is rotating and it's time to turn things on, including the fuel pump. They are also responsible for spark and injector timing.

The nice thing about inputs is a defect in just one circuit will almost always be detected by the computer, and the diagnostic fault code will tell you which circuit to look at. If a signal voltage is off by too little to set a fault code, it won't cause a no-start either.

Outputs are usually pretty evident too when there's a problem. High-current things that run through a relay, like the radiator fan motor, will blow a fuse when shorted, and they won't stop other circuits from working. Outputs that are driven directly by the computer, like ignition coils and injectors, will be shut down by the computer to protect it from over-current. This includes the 5.0 volt supply for the sensors which is another output. In most cases once a short on the 5.0 volt supply is removed, the ignition switch has to be turned off and back on to restore its operation.

Okay, now let me explain generators. Everyone will know what you mean when you call them "alternators", but they were developed by Chrysler, and they copyrighted that term. Regardless of what you call them, all generators and alternators work the same way. You need three things, a magnet, a piece of wire, and most importantly, movement between the two of them. In cars we use an electromagnet because we can vary the current flow through it and thus vary the strength of the magnetic field. We will "induce" a voltage into the wire when it moves past the magnet, but by turning the wire into loops, we induce a voltage into each loop, and those voltages add up to give us more than 12 volts coming out to run the electrical system. The movement comes when we spin the electromagnet with the belt and pulley.

There are a number of ways to vary the output. First of all, by its physical nature, the AC generator, (that's an alternator, as opposed to the earlier inefficient DC generators from the 1950s), is incapable of developing more current than it was designed for. Also, it will only develop exactly the amount of current that's needed to run the electrical system and recharge the battery. We don't have to worry about current limiting.

We do have to worry about controlling voltage. Without going into all the detail, (and please do not do this), if you disconnect the battery while the engine is running, it is possible for the output voltage to exceed 30 volts. That will destroy all the computers on the vehicle and burn out any bulbs that are turned on. The voltage regulator is responsible for holding output voltage to a safe level, but it needs the battery to help it do that.

The output voltage is determined by the number of loops of wire in the "stator" winding. That's the wire that we turned into many loops. It is not practical to run alongside the car and pull loops of wire out when the battery is fully-charged, or add loops when we want to roll the power window down. During the manufacturing process, the current output rating can be increased by adding just a few more inches of copper wire in the form of another loop or two, but remember, that only increases the capability of the generator. If the electrical system is calling for 35 amps, it doesn't matter if you have a 90 amp generator or a 120 amp unit. Either one will just develop 35 amps.

We can vary the output voltage by changing the number of windings on the spinning electromagnet, but again, you'd look funny trying to do that with the hood up and zipping down the highway. Since this one creates the magnetic field, we call it the "field" winding. There's no advantage to adding more wire to this one.

The spacing between the stationary "stator" winding and the spinning "field" winding can be changed to vary output voltage. This is how a lot of welders work when they have dials to adjust the current instead of switches. This too is not practical to adjust. GM has had some trouble with their redesigned generators on '87 and newer vehicles. They decreased the spacing so much to get the interaction stronger, but with just a little play in the bearings, the spinning field winding can catch on the stator and lock it up. I've seen some where people had to cut the belt so they could restart the stalled engine.

Another way to vary output voltage is by varying the speed at which we pass the magnetic field through the wire. Because the main ingredient to make this work is the movement, generators of every design are relatively inefficient at low engine speeds. That's why sometimes you'll see head lights dim slightly when the engine is idling. One way to increase the efficiency is to build the metal frame of the field winding with multiple fingers extending around from the north and south poles. That means there will be many north - south transitions per revolution instead of just two. Each transition makes the needed movement. By having more of those transitions, it acts like it's spinning faster.

Getting back to speed, we actually DO vary output voltage when we change engine speed, but just as with everything else up to this point, it is not practical to increase engine speed because you want to turn on the heater fan, and it's not practical to slow down to stop the battery from over-charging.

The only practical way to control output voltage is by varying the strength of the magnetic field. On the older DC generators, we passed up to about five amps through stationary field coils bolted to the housing. The output voltage and current were developed in the spinning armature, then we had to get it out through carbon brushes. If we were lucky, for that five amps in we might get 30 amps out. That was hard on the brushes too to pass that much current through them. Efficiency of these were very low, and if the engine had to idle a lot, you might have to charge the battery once every week or two.

With the AC generator, the high output current is taken off the stationary stator winding and no brushes are involved. The brushes pass the very low current to the spinning field winding. Due to the resistance of the really long wire in the field winding, the most current it can pass is close to three amps. That will give us maximum output which today can easily be over 120 amps. It is real easy to control that three amps with a wimpy, inexpensive transistor. This was another first from Chrysler. They first used the alternator on 1960 models, and the electronic voltage regulator on 1970 models. Earlier versions were electromechanical regulators.

Understand that it's the current we need to run stuff, but we vary how much we get by varying the voltage. This is equivalent to raising the level in a water tower so you get more pressure, (voltage), then that higher pressure forces more current to flow. If you want less volume to put out that house fire, you have to lower the pressure.

Once the battery becomes fully-charged, system voltage will start to creep up. Once we get to around 14.25 to 14.75 the voltage regulator adds resistance to the field circuit. That lowers current flow through it and reduces the strength of the magnetic field. A weaker magnet induces a lower voltage into the stator winding, and that lower voltage results in less current flowing out of it. If you turn on the heater fan, in the blink of an eye the system voltage will drop, the regulator will sense that, it will reduce resistance so field current increases, the magnet gets stronger, output current goes up to meet the new demand, and system voltage goes right back where the regulator is designed to want it to be.

Originally all voltage regulators were mounted on the body sheet metal and wires connected them to the generators. GM was the first manufacturer to build it into the generator starting around 1972. There were some minor disadvantages, but in general it was a real nice design and easy to repair. In my opinion it was the world's second best design, and they used it through the '86 model year. By far they have the worst design now starting with '87 models.

Chrysler never put the regulator inside the generator, but by the late '80s they did stick them into the Engine Computers. They had extremely little trouble with the older electronic regulators, but all they offered was temperature compensation. Charging a battery is a chemical process, and chemical processes slow down in lower temperatures. These regulators bump up charging voltage in cold weather to insure the battery gets fully charged. I don't like the idea of sticking the regulator inside the computer because if it fails, you have a much more expensive part to replace. The good news, however, is they still don't have much trouble with that circuit. The big advantage to this is now they can adjust charging voltage for all kinds of new variables. A generator can easily take over five horsepower to run. When you're passing a line of cars, ... Going up a steep hill, ... And pulling a big trailer, ... You need every ounce of power you can get. For that instant at wide-open-throttle, the computer can open the circuit to the field winding. That will remove the load and let the generator freewheel. With that extra few horsepower, you can pass a couple more cars!

If the engine is starting to run hot in hot weather, especially in stop-and-go driving, the computer may turn the generator off or reduce its output for a few minutes to lessen the load on the engine. The point is, with the regulator inside the Engine Computer, the generator's output voltage can be varied to accommodate a variety of variables beyond just air temperature.

For a long time most other manufacturers put the regulator inside or on the back of the generator. Almost all have no easy way to test their operation, and some are too complicated to replace them, so we just replace the entire generator and regulator as an assembly.

This finally brings me to your question about the wires. Every generator is going to have that really fat output wire bolted on near the back. There may be a fuse somewhere in that wire, but that wire just goes straight back to the battery's positive terminal. If you measure 12.6 volts between the battery's two posts, you had better find 12.6 volts between the engine block / generator housing / mounting bracket, and that output terminal. With the engine running it's the same thing. If you find 14.5 volts at one of those places, you had better find it at the other one too.

Only the idiots at Ford tried to use plug-in terminals for the output circuit. At least they knew to use two terminals side-by-side because one could never handle that much current, but then they warned to never unplug that connector! Doing so would weaken and degrade the connection. That would add just a tiny fuzz of resistance, and that would lead to heat buildup, and burned-up terminals. To replace the generator you were supposed to cut the wires and splice them to the plug that was already in the new generator. Needless to say, they had a pile of unhappy owners with that poor design.

Next, regardless of where the regulator lives, it has to have a wire tied directly to system voltage. Chrysler used one circuit that came from the ignition switch to provide 12 volts to everything under the hood including the power source to run the regulator. That same wire was used to sense system voltage. On the more common GM and Ford regulators that are built into the generators, there is a sensing wire that is tied straight back to the battery positive post, but with a fuse in the middle. Those will have 12 volts all the time. Yours may be like that too. If not, one wire will get full battery voltage when the ignition switch is turned on. To simplify the circuit, that sensing wire could just tap off the output terminal since that also goes right back to the battery.

There has to be a way to tell the driver when there's a failure of the charging system and the battery is going to run down. There will be a wire on the regulator for the warning light circuit. Typically the way that circuit works is 12 volts is applied to the light from the ignition switch, then the other side of the bulb is grounded by the regulator. That turns the light on for a bulb check before the engine is started. The current going into that regulator's terminal is also used to "wake it up". At that point, the wire that always has battery voltage to sense system voltage supplies the initial current for the field winding to get things started.

You'll typically find 2 volts on this "wake-up" wire. That leaves 10 volts across the dash bulb which is plenty to light it up. Once the engine is started and the generator starts producing an output, the regulator puts full system voltage back out on that wake-up wire and back to the dash bulb. With 14 volts from the regulator on one side of the bulb, and 14 volts from the battery through the ignition switch on the other side, the net difference is 0 volts, so the bulb goes off.

So finally, ... There's the three wires you must have on your regulator. The output wire with full battery voltage all the time. The sensing wire that must have full battery voltage, but either all the time or when the ignition switch is on. And the warning light wire to tell the regulator to start doing its thing.

Now to complicate the issue some more, there is a resistor across the socket for the dash light in case the bulb burns out. That resistor is much higher in resistance than the bulb, but it still passes enough current to get the regulator started. The bulb might have around 20 ohms of resistance. That extra resistor is usually in the range of over 500 ohms.

The last part to this sad story is there could be a fourth wire to the regulator. You knew there had to be a reason I shared all that history from Chrysler. By building their regulator into the computer, it can take advantage of everything the computer knows. For example, the computer turns on the radiator fan relay when it's needed, and that really high current could cause a momentary drop in voltage. Before it recovers you would see the head lights dim, then come back. The voltage regulator can anticipate this when it knows what the computer is going to turn on and off, so it can tweak the generator's output to overcome those little annoyances. There are some regulators built into the generators that have this same capability, but they need that fourth wire to get that information.

One thing I overlooked is the regulator has to have a ground. It's not sufficient to measure system voltage unless it is in relation to something. That 14 volts is between ground and the battery's positive circuit. All the older mechanical regulators had to be bolted to the body to get the ground circuit. The same was true for Chrysler's electronic regulator. It had just two wires, the power to run it and sense system voltage were done with one wire. The second one came from the field winding so that current could continue to ground through the regulator's circuitry. To get to ground the regulator had to be bolted to the body.

As a final point of interest, Chrysler's electronic regulator can be retrofitted to replace a failed regulator in the computer. That only requires snipping one wire from the computer's connector, then connecting it to the replacement regulator. The charging system will work perfectly fine, but there is one problem with doing this. The computer monitors current flow through the field winding, and with this wire cut off, it won't detect any field current, so it will set a diagnostic fault code, "Field not switching properly". That alone won't affect anything else, but when a fault code relates to something that could adversely affect emissions, it must cause the Check Engine light to turn on. The computer assumes the charging system is dead and battery voltage is going to start to drop as it runs down. Low system voltage could affect how the injectors fire, and spark voltage could decrease. The fuel pump could slow down resulting in low fuel pressure and a lean condition that the computer can't make up for. Any of these things could adversely affect emissions, so the Check Engine light will be on. We've all heard about people putting tape over that light, but either way, with the light always on, how will you know when a totally different problem is detected? A new problem could be very minor but turn into an expensive one if it's ignored. For that reason, while this modification will work, I don't recommend it.

I actually am using one of these Chrysler regulators on my New Holland skid steer. I was too cheap, (frugal), to buy a new regulator, and I had a few of the electronic ones on hand. It runs that little generator just fine.

Also, on all computers, there is a list of conditions that must be met to set a fault code, and one of those conditions is certain other codes can't already be set. That's because the computer constantly compares different things to each other to determine when one has a problem. If it already knows there's a problem with the charging system, it won't set any fault codes for things that are affected by that first problem. A second, new problem will cause symptoms, but with no fault code, where do you start looking?

The older resistors had color bands to denote the values but for today's miniature electronics, those are pretty big. Since the early '90s the industry has been using very tiny, rectangular "surface mount" resistors. They don't have lead wires. They're glued to the circuit board, then solder is draped over the ends onto the copper circuits. Most of the time they have numbers on them to indicate their values.

As long as the resistor is not physically broken, a tv repairman will be able to measure the values and find suitable replacements if they're needed. If you can't find anyone locally, as a last resort, and if the module is no good as is, you can send it to me already removed from the housing and I'll take a look at it. I've repaired a lot of broken circuit boards on radios dropped by the UPS basketball team.

A word of warning though. I used to build a lot of bugged cars for my students to diagnose, and I ran into a Chrysler Engine Computer that had four layers of copper circuits instead of two, (one on each side). I drilled through the board to run a wire to switch the alternator field on and off and didn't realize I was drilling through two hidden layers of circuits. I killed the circuit driving one of the injectors. Once I realized what I had done, I was able to find the tattered ends of the copper and reattach them, but that would hardly be a reliable repair for a daily-driven car. Any good tv repairman should be able to do the repair as well as I can.