The web site isn't online yet. You have it easy with the terminal block off. You can measure the 12 volt feed terminal with a voltmeter. It will have 12 volts for one second after turning on the ignition switch. The other one won't have any voltage. When your system is connected and working properly, that second terminal will have less than battery voltage. The problem comes from having a defective voltage regulator or a broken wire going to it. That will cause both terminals to have exactly the same voltage. To perform the full-field test to verify the alternator is working, you must ground that small terminal that normally has the lower voltage, but how do you tell which one that is when they both have the same voltage? You can't go by wire colors because they go through that black plastic block in your photo. I don't allow my students to poke holes in insulation to take measurements so there has to be a different way to figure out which terminal to ground. You have a 50 / 50 chance of picking the right one, but if you pick the wrong one, it means blowing a fuse if you're lucky, blowing a fuse link wire if you're unlucky, and burning out an internal connection in the Engine Computer if you're really, really unlucky. I've done all three.
Here's a copy / paste version of that web page, but you must realize it was written for automotive students who already understand basic electrical theory and they know how charging systems work. It won't make sense if electrical theory mystifies you.
Which Field Terminal Do I Ground?
So you've decided to perform the full-field test to verify a defective voltage regulator. It's an "A"-type field circuit with one field terminal already having battery voltage applied. You want to ground the other field terminal. That terminal can't be any easier to identify than on the 1970 - 1989 Chrysler rear-wheel-drive systems. The dark green wire can be grounded at the alternator or at the voltage regulator on the firewall. For the "SI" AC generators from the early '70s through 1986, GM provided the tab to ground inside the "D" hole on the back of the housing. That is real convenient, ... If you can find it and gain access to it. But as easy as the older Chrysler system is to figure out, their newer systems have a clinker thrown into the works.
Beginning with the front-wheel-drive vehicles, (and later in all Chrysler vehicles), the two field wires pass through a black plastic block that is bolted to the rear housing. Two tabs on that block bolt to the two field terminals. The two terminals are easy to tell apart when the system is working properly. One will have full battery voltage. The other one that goes to the voltage regulator will have less than battery voltage, commonly 4 - 11 volts. That's commonly called the "control" circuit and is the one to ground to do the full-field test. The problem pops up when there is a break in that circuit going to the regulator. To give the customer an accurate estimate, you want to quickly verify the alternator is ok. That means grounding one of those field terminals to perform the full-field test. But with that incomplete circuit, whether it be due to a break in the wire or an open voltage regulator inside the Engine Computer, there is no current flow through the field winding so there is no voltage drop across it. You will measure exactly the same voltage on both terminals. Hmm.
You could guess which terminal to ground, and you would have a 50 / 50 chance of being right, but you also have a 50 / 50 chance of blowing a fuse or worse yet, a fuse link wire. Fuse links were used in the '80s and early 90s. They are buried in the wiring harness and replacements must be soldered in. Repair can add an hour or more to the time you spend on this vehicle. There has to be a better way to identify the right terminal to ground. A do-it-yourselfer might pierce the insulation of the wires where they go into the terminal block but that is not the sign of a professional. Water will get in and corrode the wire. Even with this destructive approach, the same voltages will be found that could have been measured right on the field terminals. The reason do-it-yourselfers resort to this method is they can see the colors of the wires they're piercing. The problem is that most cars that use the black plastic terminal block also use two dark green wires. The feed wire has a black tracer that is almost impossible to see, so it is really not a very practical or useful way to determine which one to ground.
Rather than butchering wire insulation, a novice would be better off looking for the field wire at the Engine Computer, but for you the professional, there is an easier way.
Digital voltmeters have a very high "impedance" which simply means they have such a high resistance, it is like they aren't even there. To take their readings, they draw such an extremely tiny amount of current from the circuit that for all practical purposes, it's too small to measure. Think of a pressure gauge on a water pipe, (or compressed air line). No water flows through the gauge, yet the gauge provides the pressure reading.
Now, logic would dictate if there is no voltage drop across the field winding because there is no current flow, we have to cause current to flow to cause a voltage drop. That voltage drop will cause a lower voltage to be measured only on the regulator-side field terminal after current flows through the field winding. While the digital voltmeter is a very high impedance device, the test light has a very low resistance so it will complete the circuit and allow field current to flow through it to ground. Once again logic comes into play and you might be thinking that thanks to the voltage drop across the field winding, the test light will be full brightness on the feed terminal and dim on the control (regulator side) terminal. The problem is that while the test light does indeed complete the circuit and allow current to flow, that current will be very small and so will the voltage drop. The difference in the brightness of the test light will be almost impossible to detect. To make matters worse, you must remove the test light from one field terminal, then move over to the other one. The light goes off while you're moving the probe. Our eyes simply are not calibrated accurately enough to tell the minuscule difference in brightness between the two test points.
Now, let me stop here and point out the problem with the test light is caused by the fact it is still very high in resistance compared to the four ohms of the field winding. Closer to 40 or 50 ohms would be typical for an average test light. You would likely get usable results by using a head light bulb because it will allow more current to flow resulting in a much higher voltage drop across the field winding and a much dimmer bulb on the control terminal that our eyes can easily detect. But that kind of defeats the purpose of the story, doesn't it? Besides, you are likely to have a test light in your tool box. How often do you have a head light bulb ready to use in a hurry?
So we have two tools, neither of which is going to help us figure out which field terminal to ground. The voltmeter can read to a tenth of a volt accuracy but will read exactly the same voltage on both terminals. The test light will complete the circuit, cause current to flow and a voltage drop to occur, but we can't perceive the difference in brightness of the bulb. Do you see where I'm going with this? I'll give you a minute, ...
Time's up. You need something to cause the voltage to be lower on the control field terminal. That would be the test light. Now that the voltage is lower, you need something accurate enough to measure the slight difference in voltage. That would be the digital voltmeter. Kindly ask your instructor to smack you along side the head if you haven't figured it out yet! Use both of them at the same time! The test light will cause the voltage on the control terminal to be about 0.2 volts lower than on the feed terminal. That's not much difference but it's plenty for the voltmeter to see. Now, in less than a minute, you know for certain which terminal to ground for the full-field test without guessing or popping fuses.
Tuesday, February 1st, 2011 AT 2:54 AM
