Where is the voltage regulator on this truck

  • 1990 DODGE RAM
Electrical problem V8 Two Wheel Drive Automatic

We can not find the external voltage regulator on this dodge truck can you help us please
Do you
have the same problem?
Saturday, December 5th, 2009 AT 7:13 PM

1 Reply

The voltage regulator is on the fire wall near the distributor

Here is a wiring diagram so you can tell the wire color to confirm the right part. (below)

Sometimes the computer will take over the job in some models.


It's very rare for the regulators in the engine computer to fail, but if they do, you can install one of the older external regulators. It will do a dandy job of running the alternator, but on most vehicles, the "Check Engine" light will stay on and the diagnostic fault code "alternator field not switching properly" will be in memory because the computer monitors current through that field circuit. The light comes on when the fault code(s) could have an adverse effect on tail pipe emissions. Low charging system voltage will cause injectors to fire late and spark could be weak. Even though these things won't really happen once you have the external regulator installed, the computer doesn't know the system is working fine. It just knows the monitored / unused circuit is dead.

Wiring in the external regulator is very simple, but if you use one just as a test, the case must be grounded. The other two wires in the plug are for 12 volt power, (dark blue wire), and the field control wire, (dark green). The external unit has temperature compensation built in to modify charging voltage according to battery temperature. What it can not do is momentarily cut the alternator off completely during wide-open-throttle to eliminate that 5 horsepower drain on the engine while you're trying to pass the freight train or pull a trailer up a hill.

What is the symptom? You should start by measuring the voltage on the two small wires on the back of the alternator. The engine must be running because the voltage comes through the Automatic Shutdown relay which turns on when there is engine rotation, (cranking or running). 0 volts on both wires is very rare because that same circuit feeds the coil, injectors, and fuel pump or pump relay. Except for one short piece of wire, you'd be troubleshooting a no-start condition.

Normal operation is full battery voltage on one small wire, and something less on the second small wire, but rarely less than 4 volts. If that's what you find, the regulator is working. In that case, again, what is the symptom?

If you find battery voltage on both wires, you can suspect a defective, (open circuit) regulator or a break in the wire / connector pins going to the regulator. These measurements must be taken with a digital voltmeter. A test light will "complete" an open circuit and give inaccurate results.

The most common thing to find is full battery voltage on one wire and 0 volts on the other one. This is caused by worn brushes. Sometimes pushing on them will start the alternator working for a few seconds. This also usually starts out being intermittent. The brushes can be replaced in some instances without removing the alternator from the engine. I buy them from a local rebuilder for only a couple of bucks for the pair. The dealer used to sell an entire kit with two brushes, and four plastic holders. You used the two holders for your application. Don't know if the dealer still sells them.

One last test that I should have mentioned first: The large black wire that is bolted to the back of the alternator must have full battery voltage at all times. If it's missing, the alternator's output current can't get back to the battery. There should be a large "bullet" connector near the battery cables that might be corroded or unplugged.

I used a lot of Chrysler vehicles when I taught Auto Electrical because their systems are very easy to learn for young people who have a hard time visualizing what they can't see and manipulate in their hands. I also own a bunch of cars with this charging system. They're my favorite to work on even though I don't have to very often. The GM unit from the '70s to late '80s that I think you were referring to is my second favorite. They were easy to fix as you must know. They were also pretty reliable, so I guess that's why they stopped using them in favor of the piles they use now.

Was this
Monday, December 7th, 2009 AT 5:31 AM

Please login or register to post a reply.

Recommended Guides