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Amedee, Former ASE Master Tech
Category: Car
Satisfied Customers: 26066
Experience:  ASE certified tech advanced level specialist. Wisconsin certified emissions state inspector
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Voyager: plymouth..voltage jumps from 12v at the alternator to 18v

Customer Question

I have a 1990 Plymouth Voyager. Recently the Alternator has been overcharging the battery. The voltage jumps from 12v at the alternator to 18v at the battery. Consequently the battery can overcharge and boil. Currently we have to drive it with all the appliances on, however this is a short lived fix. It has been in the garage for a week and they have changed the battery, changed the alternator, changed a relay (situated just above and to the right of the batter), cleaned up any corrosive relays and have checked the integrity of the wires from the alternator to the battery (which is fine). This problem is getting annoying as we have to drive it from Banff-Vancouver Island. What could it be??? Also, is there something we can plug to drain the battery whilst driving, so that we can safely get it to the island where my partners uncle (a mechanic) can spend time working on it???
Kind regards
Submitted: 6 years ago.
Category: Car
Expert:  Amedee replied 6 years ago.
Hello and Welcome to Just Answer!

My name is XXXXX XXXXX I have been working on cars for over 10 years.

If the battery and alternator have already been replaced, chances are it is going to be a bad PCM or even a wiring problem from the PCM to the alternator. You can clip the signal wire at the pcm that control the alternator and see if the problem continues or not.
Amedee and 8 other Car Specialists are ready to help you
Customer: replied 6 years ago.
How do I find the PCM and identify the right wire? If I clip with wire and it resolves the problem can I then continue to drive it, or will this indicate that the PCM needs to be replaces?
Expert:  Amedee replied 6 years ago.
You will need to get a pin out chart for your computer to see which wire it is.

If this takes care of the problem, then chances are the computer is at fault. If this is the case, the PCM will need to be replaced.
Customer: replied 6 years ago.
If I were to replace the PCM where would I find one. Also, what do I look for (year - Does it have to be specific for a 1990, or is there a time period - can it also take a dodge caravan PCM too?)
Expert:  Amedee replied 6 years ago.
I would look specifically for one for a 1990 vehicle with the exact same engine.

You can find these PCM's all over. Ebay, the dealership, or even at junk yards