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Old 06-02-2023, 08:26 AM   #1
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Overcharging voltmeter

When I start a trip my voltmeter reads 14.2 volts. After I drive for a hour or two the needle will begin to creep up, first to 14.8 then 15.2 and then to 16 volts. Sometimes it will swing back and forth between 14.2 and 16. I put on my headlites to create a load but this has no effect. The chassis battery is 5 yrs old, does not use water and I keep it charged when parked. I've checked for loose connections but did not find anything so far. The alternator is about 5 years old. Not sure if the regulator is internal or external to the alternator. Not sure where to begin checking as when idling the voltage is 14.2. This is a 1994 Winnebago Vectra DP.
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Old 06-02-2023, 08:43 AM   #2
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A voltage change of that magnitude should result in a very noticeable brightening of lights. Are you seeing that?

Have you validated the voltage reading with another meter, perhaps something like this?

What engine and alternator do you have?

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Old 06-02-2023, 08:52 AM   #3
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IF (yes big IF) you have a duvac alternator, a poor connection in the SENSE WIRE/sense terminal of the alternator can falsely signal the alternator to put out high voltage. Same for a bad diode-based battery isolator.

The sense wire is small gauge and connects from the chassis battery side of the battery isolator to the sense terminal of the alternator. If for any reason it reads low, it will (and should) cause the alternator to put out excessive voltage.

As you can see, we need more details on what alternator and battery isolator you have to be of assistance.
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Old 06-02-2023, 10:45 AM   #4
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That sure seems like the voltage is high? I would expect a lead acid battery to be charged by the alternator between 13.4 to maybe as high as 14.8
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Old 06-02-2023, 11:34 AM   #5
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Good advice from Wolfe10. I can't think of any other reason the alternator voltage would go that high.

In my limited experience with this sort of problem, the sense wire came from the ON side of the ignition switch, so that it only gave feedback to the alternator if the engine was on/running. Resistance in that feedback wire causes the alternator to produce excessive voltage. Odd that the voltage grows over time, though. Some sort of heat build-up causing an increase in resistance?
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Old 06-02-2023, 03:40 PM   #6
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Replace the alternator.
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Old 06-02-2023, 04:16 PM   #7
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Check what's mentioned the sense wiring , crimp spades ect. If Leece Neville alternator ...they sell voltage regulators for them if its bad. Had car alternator go bad and over charge instead of under charging. Starting pushing 19v... boy were headlights bright ,turned on brights ....was like aircraft landing lights lol.
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Old 06-02-2023, 07:53 PM   #8
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First thing; assuming they are lead-acid type, have your batteries load-tested to insure they are in good condition. At 5 years old that is the only way to be sure of their condition.

If they pass the load-test, you know the alternator is working, have the regulator tested. If necessary it can be replaced for much less money than a new alternator will cost.
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Old 06-03-2023, 02:27 PM   #9
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A lot of good tips in this thread. In the process hook up an alternative volt meter to confirm its not the gauge. I carry a spare alternator just in case. you might do that anyway.
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Old 06-03-2023, 04:07 PM   #10
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Originally Posted by NXR View Post
A voltage change of that magnitude should result in a very noticeable brightening of lights. Are you seeing that?
And a battery that is boiled dry. You need a second meter connected directly to the battery to verify the first one.
Retired. 31 year of automotive engineering for one of the Detroit 3, specializing in Powertrain Control Systems.
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