89 Dodge Spirit, alternator overcharging.

Could be the regulator but Chrysler really had very little trouble with that circuit.

Measure the voltage on the two small terminals on the back of the alternator while the engine is running. One will have full battery voltage. The other one is the important one. Even if the voltage regulator circuit is shorted, it can't pull the voltage all the way down to 0 volts. If the regulator is the problem you will find around 2-4 volts on that terminal. If you find 0 volts, look for that wire grounded somewhere between the alternator and the computer.

Caradiodoc

Replacing a wire isn't the answer. If you're going to approach it that way, cut the dark green wire at pin 20 of the Engine Computer. That is in the top row of pins furthest to the rear of the car. If the system continues to overcharge, that wire is grounded somewhere. If the system remains dead, the computer is indeed shorted. If everything else works, it is possible to wire in an external regulator from the 1970s to run the alternator but the Check Engine light might be on because the computer will see the missing field current. That means you won't know when a different problem occurs because the light will already be on.

Caradiodoc

I can't find an '89 service manual. The closest ones I have use two different circuits. I think you have the newer circuit but the field wire might not be pin 20 in the computer's connector as I previously stated.

To clarify, there is no ground wire in the field circuit. If the wire going from one field terminal to the computer is grounded, the system will charge wide open as it is doing now.

Find an inexpensive digital voltmeter so we can do some tests. If you have a Harbor Freight Tools store near you, they have a perfectly fine one for less than ten bucks and it goes on sale quite often for $2.99. It is just as accurate as the $300.00 models, just be sure the leads are plugged in tightly. Walmart and Sears have them too.

I'd like to know exactly what voltages are on the two small wires on the back of the alternator when the engine is running and with the ignition switch on but the engine not running. Four readings in total.

Caradiodoc

That's a huge difference in voltage between the two terminals which means it's developing a strong magnetic field and a high output. The fact there is some voltage on the control wire proves it is not shorted to ground. The overcharge condition is being caused by the voltage regulator. I'm 99 percent sure everything is in the Engine Computer and replacing it will solve the problem, however, there are two important points to look at.

First, there is a 12 volt feed wire going to the computer that is used for the regulator to sense system voltage. I don't know which wire that is but if that voltage is missing the regulator will think system voltage is low and will keep trying to raise it. Normally that circuit also feeds other things so it can't be missing completely. If the voltage is just a little low, say due to a corroded splice, the other stuff will still work but the regulator will think system voltage is low. Based on the recent history, I'm inclined to think a used computer will solve the problem.

The second thing to keep in mind is there were some vehicles that split the Engine Computer into two parts. The Logic Module sat inside the car behind the right kick panel and received the information from the various sensors. It made the decisions, then told the Power Module under the hood what to do. It was the Power Module that ran the injectors, ignition coil, alternator field, and other relays and solenoids. In that system the voltage regulator in the Power Module can become shorted and cause the overcharge condition, or the sensing circuit in the Logic Module can fail to read system voltage properly or fail to tell the regulator when system voltage is high enough. Unless you can find a wiring problem you might have to resort to just plugging in a different computer to see if it solves the problem.

I'm pretty sure that Logic Module was used on older cars from around '85 or '86, but I don't know how long that system was around.

Caradiodoc

I only worked on one or two of these systems a long time ago. I don't know how the two modules communicate. It could be a dedicated wire between the two parts of the voltage regulator in each module, or it could be a "twisted pair" of two wires called a "data buss". A whole bunch of systems send coded computer signals over those two wires. In lieu of a valid diagnostic procedure, this one time when you might have to resort to the less desirable part substitution method.

If you are near a large city, there is likely to be a pick-your-own-parts salvage yard where you pay a buck, then grab one of their wheel barrows to haul your tool box around. I would take both modules right away because parts there are usually very cheap. I found a lift gate for my old rusty trusty '88 Grand Caravan in St. Louis, MO for $250.00. (No thanks). Found a better lift gate AND a sliding door for a total of less than $90.00 at a yard in Nashville. That was one of a chain of very clean and well-organized yards called "Pull-A-Part". Computer modules there cost $18.00.