Low beam headlights not working

If a relay is suspect, we just switch it with a different one like it. There's typically only two or sometimes three different styles of relay in any under-hood fuse box. With current labor rates, we can't justify testing parts when customers are paying us by the hour.

I'm not familiar with relay testers, so I don't know how they work to determine if one is good, but I do have one comment of value for you to add to your memory banks. All relays will have two terminals for their coil of wire inside, and on their own, they are interchangeable. On Chrysler diagrams, they designate them as terminals "85" and "86" on most models. Where the potential problem comes in is every coil of wire that is energized creates a magnetic field around it. That's how starter motors, generators, and relays work. When the coil is turned off, the magnetic field is forced to decay instantly. That "induces" a huge reverse voltage spike that could go as high as 300 volts. In some applications that is desirable, particularly with ignition coils. We use that spike to develop the voltage for the spark plugs. In relays, those voltage spikes can do damage to the circuitry controlling them. To prevent that, most relays have a diode added internally between terminals 85 and 86. A diode is a one-way valve for electrical current flow. It's placed in there backward so in effect, it's like it isn't even there when the relay is activated. It's when the relay is turned off and that spike occurs that the diode acts like a piece of wire and shorts out that spike. It's because of that diode that terminals 85 and 86 can't be interchanged. Terminal 86 gets the 12 volts, whether it's constant or switched on and off, and terminal 85 is the ground terminal, again, regardless if it's switched on and off or is there constantly. If you want to test the operation of a relay's coil and you aren't sure if it has a diode inside, or you don't know which terminal needs the positive and negative connections, use a small 12-volt light bulb in one of the test leads. If you connect the battery with the correct polarity, the relay will click and the bulb might glow just enough for you to see it. If you connect the battery backward, the relay's diode will act as a short circuit, but the light bulb will limit current to a safe level and will be normal, full brightness. That will prevent melted wires or burned fingers. This procedure is much easier to do that it is to describe, so I put together a little drawing. Any 12-volt light bulb will work. I like to use 3157 brake light bulbs because it's easy to connect clip leads to them. That bulb will allow a maximum of one amp of current flow when connected to a 12 volt battery. Even if you use a 20-volt battery from a power tool, the bulb will be brighter if the wrong wires are hooked to the relay terminals, but current will still be limited to a safe level.

Many relays commonly found in GM vehicles use a resistor instead of a diode for the same purpose. Resistors are less effective, but effective enough to dampen the voltage spikes. The advantage is since resistors don't have a polarity, the two terminals for the coil can be interchanged.

Terminal 87A is in the middle. It has no place whatsoever in this story, so don't worry about that one.

You're trying to test a relay, but there's actually two parts to it. Testing the coil is the first part of that. You were correct to use a battery to power the coil, but the confusion comes from knowing which terminals to use, hence my sad drawing. With a simple, basic relay, the coil of wire has two ends with two corresponding terminals. We can measure it with an ohm meter, or we can use the battery, then feel and listen for the click. With this basic relay coil, the positive and negative can be applied to either terminal.

I described the use of a diode across the coil to dampen the inevitable reverse voltage spike that always occurs the instant current flow through the coil is stopped. That diode is the only reason the positive and negative must be applied to the right terminals. All you have to do with your battery is connect the positive post to terminal 86 on the relay, and the negative post to terminal 85. I added those numbers to the drawing below. If the coil is good, you'll hear and feel the click inside the relay.

My only reason for adding the light bulb is for those times you don't know which terminals to use. This way, one of three things will happen:

1. No connection, no current flow, the bulb is off.
2. Proper connection, current flow limited by the bulb's resistance, bulb is dimmer than normal.
3. Solid connection, current flow is limited by the bulb's resistance, bulb is full brightness.

A solid connection is the same as if you connected the pink and green jumper wires together. If the bulb wasn't there, you'd have nothing to limit current flow. The jumper wires would smoke and melt, and you'd burn your fingers. We need something to limit current. The bulb does that while at the same time giving us a nifty visual indication of how much current is flowing.

Almost as bad as a solid connection is when the positive is applied to terminal 85, and the negative to terminal 86. Remember, that would work normally for the basic relay, but we have a diode in your relay. With this reversed connection, that diode will act like a piece of wire, or dead short. There's nothing to limit current flow, so again, the jumper wires would melt and smoke. The diode can't handle that high current so it will short. At that point the relay will never work, even when connected correctly. To prevent that, we add the light bulb. If you connect terminals 85 and 86 backward, the bulb will be full brightness and limit current flow to a safe level. More importantly, there will be no click in the relay. You can switch the jumper wires, then the relay will click since no damage occurred to it. By using the light bulb, you can place the jumper wires on any terminals until you find the two that activate the relay. You don't have to know anything about it as far as which terminal is which.

The second part of testing a relay has to do with the high-current contacts. Those are turning on, meaning connecting terminals 30 and 87 together when the click occurs. At this time, you can use an ohm meter to check if the connection occurred. The ohm meter's reading will change from "open circuit", or infinite, to close to 0 ohms. There's always some resistance in the meter's leads, so a reading of as much as 5 - 10 ohms is normal. The problem with this test method is it doesn't require the contacts to pass very much current. They may be able to pass the few milliamps from the ohm meter, but not the 10 to 30 amps needed by the horn, radiator fan, or fuel pump motor. Every time a high-current circuit is switched off, an arc occurs when the contacts separate. Over time that arcing prevents a nice solid connection from occurring. It's also not uncommon for moisture to get inside the relay and corrode the contacts.

All of this testing is fine when we're learning how relays work, but it's not practical to diagnose a problem this way. The fastest and most effective way we do this is to first place a finger on it, then have a helper turn on the switch that activates that relay. If we feel the click, we know the coil is good, and everything that controls that relay is working. That entire half of the system is done.

For the high-current half of the system, the first step is to remove the relay, then check for 12 volts always on terminal 30. If that's there, and the relay clicks, the last step is to check for 12 volts appearing on the circuit under test. For example, for a dead head light circuit, we'd check for 12 volts at the plug for one of the head lights. If it's not there but every other test was good, we'd suspect the contacts in the relay are arced or there's a break in the wire leaving terminal 87. The next step would be to substitute a different relay as a test. There's almost always up to a half dozen identical relays used for different circuits. There's one for the AC compressor clutch, one or two for the wipers, one for the fuel pump, etc. Any of those can be used. If that gets the system working, it's pretty obvious the relay was defective and must be replaced. If that doesn't get the system working, we know the problem lies further down the circuit, so we'd follow that wire to the next test point.

Hope that clarifies the issue. Hit me with your next question.

The relay is in the lower right corner of the Junction Block, (inside fuse box).

I downloaded the front lighting diagrams. I'll have to study them and format them for uploading, if Boris K doesn't get back to you first.

You have the right idea on checking for voltage on a wire, but here's a link to an article that explains more about wire testing:

https://www.2carpros.com/articles/how-to-check-wiring

Wire testing might be appropriate if the relay was turning on, but you didn't mention if you felt the click. If that relay is not turning on, we have to shift our attention to the Body Computer. Also, did you jump the two terminals in the relay's socket? The low beams should have turned on when you did that. If they did, we know the wiring has to be okay.

No hurry. I run to town almost every day to read and post replies, so I'll be here to see what you come up with.

Hi,

I noticed you haven't been contacted in a couple of days. Have you been able to make any progress? We're interested in knowing.

Let me know and let me know if I can help.

Joe

Just got back to town. Was at the Iola Old Car show swap meet since Wed. My phone worked there for the first time, but this year I was surprised to find they discontinued their internet access. Sorry for the delay. Sorry to hear about you 'puters. I'm all too familiar with that frustration. I have four backups now on memory sticks.

I don't recognize your drawing. Is that related to your vehicle or your computer?

I'll be back tomorrow. Gotta continue to home to see if any of my cats passed out from starvation.

We need to start by checking the fuse for the low beams and making sure that is not an issue.

This is a 50-amp fuse and number 15 on the PDC in the engine bay.

https://www.2carpros.com/articles/how-to-check-a-car-fuse

If the fuse is ok, we need to go to the relay and remove it and check to find out if the BCM is grounding the relay so that it will turn the head lamps on.

You do this by putting a meter lead on battery positive and then the other on pin 86 and turn the headlights on. The meter should read 12 volts which will show the BCM is commanding the headlights on.

https://www.2carpros.com/articles/how-to-check-wiring

If it is, then jump the relay and make sure the head lamps actually come on.

Let's start with this and we can go from there.

Thanks

We need to step back and take a breath, then start anew. After rereading the replies, some things came to light that I overlooked before. First, what you're doing with a battery is a huge time waster. There are easier and faster ways of approaching this. Next, I noticed at one point you were connecting the battery to terminals 30 and 87, then to 30 and 87A. Those are the switching contacts, not the coil that makes the relay click. When the relay is not energized, terminals 30 and 87A are connected internally and will short out your battery.

In the name of speed and efficiency, if you have any reason to suspect the relay, just switch it with one of the others like it. If the malfunctioning circuit works now, the removed relay is defective. If the circuit is still dead, it's not the relay and we can move on. The formerly suspect relay will work properly in the circuit you switched it to.

Once we know the relay isn't causing the problem, there's two circuits we have to look at. Terminals 85 and 86 are for the coil that energizes the relay. Put your finger on the relay, then have a helper turn on the lights. If you feel and hear the relay, click, we're done in that circuit. When multiple relays click on and off at the same time, you may need to remove the cover on the one you're checking, then watch if the movable contact moves when your helper turns on the switch.

If the relay clicks on and off at the appropriate times, we can move on to the switched side of the relay, meaning the two contacts, terminals 30 and 87. There's two quick tests for this circuit. The first is to check for 12 volts on terminal 30 in the relay's socket. That should be there all the time. A test light is the best tool for this test. Often these circuits are fused with a fuse link wire. Those can burn open, yet a digital voltmeter can falsely detect 12 volts that isn't really there. I can explain that further later, if needed. If those 12 volts are missing, check for it on terminal 87. That is customarily not the terminal that has the 12 volts applied to it, but the circuit can work that way. Double-check rather than overlooking something weird.

The other quick check is to just jump terminals 30 and 87 together in the socket, with a piece of wire or stretched-out cotter pin. That will energize the circuit just as if the relay had switched on. The circuit, the headlight in this case, should turn on now. If it doesn't, you can do the voltage tests along the circuit until you find where the 12 volts are being lost and not make it to the light. No need to have the relay installed and no need to turn on the light switch. This same procedure works for the majority of the other relay-switched circuits on the vehicle.

Once we know which circuit isn't working, we can look at the diagrams and figure out where to go next.