Your understanding of the system is on par with students who have already learned basic electrical theory, and have had this circuit explained. You can't figure it out by just looking at the wiring diagram.
You are correct about switching the two field terminals. This goes back to when Chrysler switched to an electronic voltage regulator in 1970. The two wires plugged in individually, and could be switched. Instead of the electromagnet developing a rotating north - south - north - south.... Magnetic field, you get a south - north - south - north... Magnetic field, meaning the same thing.
You are also partly correct about identifying the field terminals with the terminal block removed. One will have full battery voltage, either with the ignition switch turned on for older vehicles, or during cranking or running on newer vehicles. There will be 0 volts on the other terminal. The voltage you find on that second terminal comes through the field winding from the 12-volt terminal. The problem with this is it takes way too long. When it's our customers' time and money that's involved, and especially up here in Wisconsin where they throw a pound of salt on an ounce of snow, those tiny terminals get real rusty, and it's common for one or both of them to snap off. Now you have a lot more work to do. Even on a good day, it takes too much time to take that block off relative to the small amount of information you're going to get.
I went one step further with my students and gave them another set of clues. A digital voltmeter is powered by a battery inside it. For all practical purposes it is an open circuit, and draws no current from the circuit. It is measuring electrical pressure, just like a pressure gauge on a compressed air system. No air flows through the pressure gauge for it to do its job. For this sad story, lets say you measure 12.60 volts on one field terminal. You'll measure exactly the same 12.60 volts on the other one too. No current flows through the meter, so there's no current flowing through the field coil, therefore no voltage drop across it. You start with 12.60 volts, drop 0.00 volts, so you end up with 12.60 volts. No way to tell which is which.
As a side note that gets included a few fays later, system voltage is fluctuating thanks to the current being drawn from the battery to run the electrical system. Pulses of current for the injectors, ignition coils, solenoids turning on and off, etc, can actually show up as the feed terminal has 12.60 volts, and the second terminal has something a little higher at the time you took the reading. Wouldn't THAT add a pile of confusion to your already hurting head?
Now consider the test light. It DOES operate by according to the current flowing through it. On the feed terminal it sees 12.60 volts. When on the second terminal, a trickle of current flows through the test light AND through the field winding. That tiny current causes a tiny voltage drop, so you might see 12 50 volts on the second terminal. That.10 volts difference means the light won't be as bright, but can you see the difference.10 volts makes? You have to observe the brightness of the light at one terminal, remove the probe, move it over to the other terminal, then observe the brightness there. Your eyes are still seeing stars from the first test. The only way this might work is if you tied each terminal to a SPDT switch with the test light on the common terminal. That would let you switch back and forth instantly, then you MIGHT see the difference. The next problem is the higher-quality test lights use bulbs that draw very little current, so this method loses even more validity.
To exaggerate this for clarity, you might connect a head light bulb instead of a test light. A full-fielded field winding draws close to three amps. A low-beam 9004 bulb draws about five amps. Without even resorting to Ohm's Law, that shows the bulb has less resistance than the field winding, and it will cause enough current to flow to cause a substantial voltage drop across the field winding. You might also be sharp enough to observe that when doing this to the correct terminal, the current flowing through the field winding in enough to make the alternator charge enough that you'd see it as higher system voltage or the head lights on the truck got brighter. Can't get any better than that, ... If you have a spare head light bulb in your back pocket and the clip leads to use with it.
But there's still a faster way. You have an extremely accurate voltmeter that will show you a change of a few hundredths of a volt, but it won't cause that voltage drop that you're looking for. You have a test light that will cause a voltage drop, but our eyes are not accurate enough to see the difference between the two terminals. At this point in the class discussion, a long and thoughtful pause was in order. One by one the "ahh"s would show up as the light bulbs over their heads began to shine. Use both of them. Put the voltmeter probe on one terminal. You'll see 12.60 volts. Leave it there, then put the test light on it at the same time. If the voltmeter stays at 12.60, it's the feed terminal. If you're on the control terminal, the voltage will drop when you touch the test light to it.
See? I'm not as dumb as I look!
My technical college was one of three remote class sites Chrysler used for their training, and in gratitude, they donated us a lot of cars. Some were used to test new assembly lines and were never meant to be sold to the public. Some, like our '97 Dakota, were used by the instructors to travel to our school, and to use for training. They were poked and prodded on so much that they also were not to be sold, but they could be donated for training. We also had a number of Dodge Shadows that had the same wiring as your truck. That's why I know it as well as I do. I spent years designing and building bugs for my kids to diagnose, and they all mirrored real-life problems that I had found previously. The common one had a "customer's complaint" of "battery goes dead while driving. Problem is intermittent". Gee; does that sound familiar?
In this case the green wire going to the voltage regulator in the Engine Computer ran through a connector between the engine and the body, and that was a perfect place to find corroded terminals. For my "bug", it was a stretched female terminal that wasn't making contact. Hidden in the harness were two wires running to the switch I used to activate the defect or to bypass it. Every car had at least five charging system defects, and the kids found out that by the time they were searching for the third one, they didn't have to look at the wiring diagram any more. They had the circuit memorized, and that knowledge would transfer to dozens of other models and years.
My concern with your truck is I want to know exactly where the defect is. I'm not satisfied to simply make it work, as you have already eluded to too. To that end, what I would pursue is to connect a wire to the computer side of the connector, for that green wire, then run it to the voltmeter and place that where I could watch it while driving. I've done that with the meter clipped under a wiper arm. Depending where the intermittent break in that wire is, when the problem occurs, voltage is going to go to 0.00 volts or to 12.6 volts. That will tell you if the break is before or after that point.
As for your grade, you dun good, but it's just a start. My grading policy consisted of six pages of tiny type listing a huge pile of tasks you might do in the shop, "minor life-altering events", (quizzes), major life-altering events", (final exam), and a number of other things to factor in to your grade. Everything earned points with 1000 points earning an "A". Everyone could earn the grade they wanted if they put in the effort, and they didn't have to do a strict list of requirements, just like you never know from day to day what you'll be doing on the job. To get to your level of understanding, you would have completed enough text book reading questions and shop activities to earn perhaps 200 - 250 points at the end of week two in my eight-week-long course. While that is indeed on track to earn an "A", the final exam was worth over 200 points, and "Core Abilities", meaning things like "works cooperatively", works productively", etc, was another easy 100 points. Those didn't show up until the last day of class. Every year a few students earned in excess of 1300 points. That would probably be you.
Electronics is the downfall of most mechanics because we have to visualize it. We learn best by taking things apart, manipulating them, and seeing how the parts interact. We can't do that with electrical theory, but we have all seen it with water. I can compare anything electrical to water flowing in a hose or river. That's how I achieved success with my kids. With good electrical skills, a mechanic can go anywhere and get a job. There isn't a single system in any car that doesn't have a computer or some electronics involved.
See how far this gets you, then let me know of your progress.
Friday, July 21st, 2017 AT 12:21 AM
