Voltmeter fluctuates from 14 to 10 volts

Jeff Barschel · Sep 24, 2018

Had not had my car running for a while and have done a lot of work around the electrical system. Cleaning connections, adding grounds, cleaned bulkhead connections, changed out cluster and added a voltmeter.

Just got it running again and noticed that the Voltmeter will run between approx 14 volts (which is where it should be, I think) and then will drop to 10. Rev it and if goes back to to 14 volts and will stay there for a while, then drop to 10 volts.

Is this an issue with the voltage regulator, alternator or other component in the charging system? I still have the ammeter connected and it appears to be working properly.

RT6PK · Sep 25, 2018

I would disconnect the ammeter and bolt the two wiring terminals together. The ammeter is the weak link in the charging system. Then I would have the alternator checked, (if any local parts stores still know how to do that.)

Glenwood · Sep 25, 2018

Sounds like there is a heavy load on the system while idling. The regulator or the alternator could be bad but the voltage should not really drop below the battery level unless a load is being applied.

65wgn440 · Sep 25, 2018

Probably should check voltage directly at battery with a good DVOM first, to make sure it reads same as in car gauge.

SDCoronet · Sep 25, 2018

I dont know much about electronics, but I think that is normal behavior in some vehicles.

My coronet, with a good working but original type low power alternator, has always done this - at idle it just doesn't have enough amps to charge the battery.

I have had two other vehicles that do this, including a 96 honda motorcycle (all carburated vehicles).

There seems to be no issue with normal use.

On the flip side, it does bug me, and I do intend to eventually get a more powerful alternator for my car.

Jeff Barschel · Sep 25, 2018

Got back at it again tonight. When I reconnected the battery and tried to start it it was dead. Charged it and the readings are the same at the battery.
One thing I did notice is that the post on the alternator was hot (would burn your finger) after I shut it off, that's likely a symptom of load or it's getting hot from the exhaust manifold (doubtful). The post connector was also very loose. The wires were tight, but you could turn the whole post assembly was loose inside the alternator.

I'm thinking that this is likely my problem, or could it be something else?

JimKueneman · Sep 25, 2018

Jeff Barschel said:
Got back at it again tonight. When I reconnected the battery and tried to start it it was dead. Charged it and the readings are the same at the battery.
One thing I did notice is that the post on the alternator was hot (would burn your finger) after I shut it off, that's likely a symptom of load or it's getting hot from the exhaust manifold (doubtful). The post connector was also very loose. The wires were tight, but you could turn the whole post assembly was loose inside the alternator.

I'm thinking that this is likely my problem, or could it be something else?

Sounds that could be it. Loose connection = high resistance which = hot with a lot of current. The connection could be coming and going causing the fluctuation. Pop it off and pull the back half of the housing and check the internal connections. Those alternators are super simple and can be repaired easily.

Jeff Barschel · Sep 25, 2018

Thanks guys... Took a closer look. When I pulled the terminals off the post and got a good look at it the plastic washer was melted onto the nut so I could not tighten the post at all it's also brown and discolored around the nut. This cant be good

Think I'll have to pull the alternator and see whats going on inside.

Jeff Barschel · Sep 27, 2018

Thanks for the input. Took the alternator in to have it looked at. Rectifier was burnt out when the output stud shorted. They replaced rectifier and tested. Alternator tested fine. Installed tonight and now have 14.5 at the Voltmeter and between 13.75 and 14.10 at the battery with no fluctuation whatsoever. Problem solved, at least this one. Added a few extra grounds to the block and firewall as well.

Voltmeter fluctuates from 14 to 10 volts

Jeff Barschel

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RT6PK

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Glenwood

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65wgn440

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SDCoronet

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Jeff Barschel

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JimKueneman

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Jeff Barschel

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Jeff Barschel

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