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Testing voltage regulator

I recently replaced the voltage regulator on my 1983 W150 Dodge pickup. I have the old VR and am wondering if anyone knows what could be learned with a multimeter. There are two contacts and the metallic case. I can measure the resistance between any of the two contacts. Does this tell me anything useful? The reason for replacing the VR is the battery wasn’t charging and the alternator tested OK. Wiring seems OK, but it is getting old. No shorts detected, but could be intermittent. The battery seems to be charging, but I’m going to wait till it warms up a bit to continue testing. The truck is outside and it’s below zero.

Do you have gauges or idiot lights? If you have a gauge you are styling. If you have an idiot light and it is not going on most likely the alternator is fine and you are looking at a bad battery or connections, ie battery to ground would be a prime suspect. If the gauge shows 14 volts or so while running it is a battery or connection issue.

Like @Barkydog says, one fairly easy test is to measure the voltage on the battery when the car is idling. 14-14.5 volts usually means the alternator and VR are both working.

If the voltage regulator loses its ground it will fail. It is a good idea to sand the area around the mounting screw holes on the firewall to ensure that the new VR is well grounded and never start the engine without the VR being securely mounted if the harness is connected.

Those old Chrysler external regulators were almost impossible to field test…They were tested by testing the alternator by “full fielding” it. If the alternator responded normally, then the regulator was judged to be at fault. Typically, owners of these vehicles replaced both the alternator and regulator at the same time as the regulators were not very expensive. Millions of Chrysler product vehicles used them, so you could get one at a salvage yard for a couple of bucks…They were pretty reliable, failure was unusual…

I must not have made my question clear. I’m not trying to fix things, rather learn if any information can be obtained from a voltage regulator that has been removed from service. Tests on the truck show things are working, but I’m curious about what, if anything, the old VR can tell.

There probably is some kind of test you could do on the VR to bench test it, but you’d need somebody with charging system electronics expertise on 1980’s Dodges, and this person may be hard to find. Try this link, some general info on Dodge charging systems …

If I had the project to test your Voltage Regulator, I would hunt down a 0-30 volt variable power supply with at least a 200 milliampere capacity. Connect the positive lead of the power supply to the Bat contact on the regulator and the negative lead to the regulator case. Connect a 100 ohm 5 watt resistor from the power supply lead to the field contact of the regulator. As the variable power supply is brought up measure the voltage with a DVM from the field contact to the VR case. It should read ~0.7 volts. As the power supply voltage reaches ~13.5 volts, the field contact voltage should pop up to the supply voltage. That should check the operational status of the regulator.

Another test to perform is of the shunt diode that is in the regulator. If your DVM has a diode testing function, check the forward and backward resistance of the field terminal to Batt terminal. The resistance with the positive lead of the DVM on the field contact should be significantly less than with the negative lead on the contact.

That is about as much as you can do to check VR. If you decide to destructively examine the VR, remove the cover and see if there is an adjustable potentiometer on the circuit board. I have always wondered if there was one inside.

Hope this satisfies your curiosity.

Sun Equipment offered a bench tester for starters, alternators, generators and voltage regulators but it was large, heavy and required several batteries plus 220v power. I sold one to the federal government after I diagnosed the problem they were having with Dodge trucks not charging. They had a shelf full of new voltage regulators that were bad out of the box and trucks with corroded bodies and large alternator pulleys on engines that idled most of the time with all accessories running. A man named Messerschmidt was the purchasing agent for the Air Force base, believe it or not.