Hey guys, have not posted in a while, but got a weird voltage fluctuation while car is driving.

With car idling, voltage is 14v, and all the usual checks with voltmeter from alternator to battery check out fine. same with car at to 2k rpm with A/C and lights on.

Drive the car for a few hundred meters and volts jump around till they settle on about 11.2, sometimes dip to 10.9, but not often. This happens whether your on it hard or just cruising, rpm's don't seem to make a difference while your driving, but knock it out of gear and coast along with revs down to idle and within 10 seconds volts pic up again.
Obviously while driving i am only going off the dash volt readout.

I have also noticed the right side of the dash goes dark when the volts drop, but then brighten up when the volts jump, whether this is a cause or symptom i don't know. Could it be a poor connection going to the right side of the dash (speedo and volt/coolant readout) and there is nothing wrong with alternator?

Anyone know what this could be?

If it was the voltage regulator, wouldn't it still fluctuate at idle?

If it was a poor connection to battery from alternator, it should still fluctuate at idle also.

It is also a brand new alternator which i put in thinking the last one was dead. (Powermaster 37803)

Any help is much appreciated.

 

Did you have this problem with the last alternator?

You might try disconnecting the alternator, take a ride and make the same checks and see if any of the same symptoms exist.

Run couple wires inside the car from the battery and measure voltage with a DVM to make sure dash board read out is the same……or not.

Depending on results, the grounding points might have need to be checked. Maybe some 89 guys can chime in and tell you where the exact problem grounds are.

 

To measure the voltage from inside the car using the DVM, it's handy
to make up a set of DVM cables with cigarette lighter plug. Radio
Shack will have all the bits, but you may need a soldering iron.

 

Thanks guys.
Pcolt94, yes this developed with the last alternator, it was about 8 years old so i figured it was about to die and replaced it.

I Will try driving with alternator disconnected at the plug terminal and see if fluctuation stops, if it does this would be an alternator problem yes?

I will also try checking voltage inside car with a dvm, this can hopefully determine if its a gauge connection fault or not, yes?

Thanks again

 

On the surface, first logical thought would be the new alternator is no good mainly the voltage regulator. Maybe it needs to be repalced again.

 

Any work done to the distributor assembly (cap, coil, rotor or wiring) before the trouble started?

 

Engle, have an aftermarket distributor in place ( DUI 50000volt street/strip coil) but was installed a few years back. Surely this would not drop volts down from 14v to 11v, especially just driving around town?

Took some readings with a DVM connected to battery while driving, and readout was basically the same as dash, so that eliminates a loose connection to the dash i think.
Disconnected alternator, and voltage sat about 11.6v on gauge and on DVM, it did not fluctuate, so it looks like the new alternator/ regulator is playing up and only wants to charge at idle for some reason. Taking it out today and getting it bench tested under load to confirm.... If it isn't this the only thing left is a loose connection between alternator output and battery.... rather hope it is the regulator.

 

No knowing anymore than I do I would suggest you either have some sort of a major current draw that is vibration/movement related or it might be the large wire on the back of the alternator might be loose as an option. If that wire is loose the battery can't charge properly and the voltage would go down over time. So when the connection is made the voltage would jump up where it should be but when vibrated loose it then drop to whatever the battery voltage is. I would bet on the latter rather the the former cause a big current draw when it isn't supposed ot generally makes smoke or pops fuses.

Let's not change parts until we look at the easy stuff. Connections on cars of our age are always suspect and generally easy to solve. Long ago I learned (and occasionally have to relearn) that if you hear hoofbeats you shouldn't think of zebras.

 

Thanks guys, got it bench tested under load and all was looking fine until the alternator warmed up, then amps and volts started jumping around like crazy. They Pulled it apart and found a fault, (can't actually remember what he said, something was marginally out of place internally) he fixed the fault and all is good. put it in the car and sure enough its charging fine.

Thanks again

 

Thanks for end of story.
Good that they found it.

 

I had this problem on mine and I went thru the usual suspects (tested alternator at 2 different stores, new battery), etc.

It ended up being a bad ground.

This was not surprising since I recently had relocated the battery and spliced in the new heavy gauge negative cable to the old cable.

I dont think it was making good connection at the splice.

Either way, I replaced all the wiring (both the old and the new) w/ heavy gauge 0/1 wire and used an engine AND frame point as a ground and it works good.