Voltmeter pegged

[hear]

Had a weird thing happen just now... voltmeter on the gauge cluster was pegged, reading over 19V. Drove a little bit and it went back down to the normal ~14V reading. Then went shot back up.

I THINK it was caused by my + battery terminal pushing into the rubber insulator on the upper fender. I re-aligned the terminal to not be so close to the fender, didn't have the right tools with me to tighten it down. It wasn't pushing so hard that it cut through the rubber to make good electrical contact with the fender though.

Anyway, I'm trying to figure out why this would've cause it to peg. I guess the battery was dumping charge to the body (ground), and the alternator was responding by charging more or something?

 

[Jerry Bransford]

High voltage spikes like that are commonly caused by a bad connection at the battery. The voltage regulator isn't able to sense the battery voltage to determine its state of charge so it jacks the voltage up out of the alternator.

Go buy a $4 battery brush like shown below, then remove both connectors and thoroughly clean both the battery posts and the inside of the connectors. And make sure the connectors are TIGHT when you reinstall them. If you can't get the nut on the clamp tight enough for that, let me know and I'll tell you how to get it to clamp the connector onto the battery post tightly again.

 

[hear]

High voltage spikes like that are commonly caused by a bad connection at the battery. The voltage regulator isn't able to sense the battery voltage to determine its state of charge so it jacks the voltage up out of the alternator.

That's good info, thanks.

it's a brand new battery so the brushes/cleaning aren't the problem, but the + terminal was loose enough to pivot so I'll tighten it down.

 

[hear]

Welp, I don't think it is that the terminals aren't tight. For one, they're tight. Two, they haven't moved since I tightened them per your recommendation, yet during the 2 hr trip to Crossbar this weekend it did it 3 times, each time pegging the gauge for a few minutes and then returning to normal. It didn't do it at all while wheeling, and did not do it at all on the return trip.

Can we talk about how the voltmeter even works on a 97 TJ? Is it measuring the voltage across the battery, or is it doing something clever like measuring it at the alternator? If it is not the connections at the battery, what else could be causing this intermittently?

 

[SkylinesSuck]

Not TJ specific, but I have seen bad grounds cause this. Do you notice anything else like blower motor noise changing or lights getting brighter or anything?

 

[Jerry Bransford]

Welp, I don't think it is that the terminals aren't tight. For one, they're tight. Two, they haven't moved since I tightened them per your recommendation, yet during the 2 hr trip to Crossbar this weekend it did it 3 times, each time pegging the gauge for a few minutes and then returning to normal. It didn't do it at all while wheeling, and did not do it at all on the return trip.

Can we talk about how the voltmeter even works on a 97 TJ? Is it measuring the voltage across the battery, or is it doing something clever like measuring it at the alternator? If it is not the connections at the battery, what else could be causing this intermittently?

The alternator's charge lead is connected to the battery so measuring at any point will be measuring both when the engine is running. Measuring that voltage with the engine off measures the battery voltage, measuring the voltage at any point with the engine running measures the voltage coming out of the alternator.

The cluster's volt meter being "pegged" is showing the voltage coming out of the alternator. Being pegged is normally a sign that the alternator's sense lead, which it also uses to charge the battery, is not seeing the voltage from the battery so the alternator keeps raising its voltage output trying to get the battery to charge so it can see a voltage from the battery. Any time the volt meter is showing over 12.6 volts, or 12.8 volts with an AGM battery, it's showing the alternator is trying to charge the battery.

I'd check the heavy connection between the battery's + post and the alternator which also serves to tell the alternator the battery's charge state. An exceptionally high voltage is indicating the alternator is working "overtime" trying to get a response from the battery. When the alternator doesn't sense a battery voltage, the alternator keeps raising its voltage output trying to charge the battery. At some point it can peg the volt meter if it never gets a response from the battery... often caused by nothing but a poor connection between itself and the battery's + post.

 

[CharlesHS]

Check the connections on the alternator and the grounding wires are tight to the left of the oil filter...

 

[hear]

Not TJ specific, but I have seen bad grounds cause this. Do you notice anything else like blower motor noise changing or lights getting brighter or anything?

Wasn't running any accessories at the time, so wasn't able to notice anything.

I'd check the heavy connection between the battery's + post and the alternator which also serves to tell the alternator the battery's charge state. An exceptionally high voltage is indicating the alternator is working "overtime" trying to get a response from the battery. When the alternator doesn't sense a battery voltage, the alternator keeps raising its voltage output trying to charge the battery. At some point it can peg the volt meter if it never gets a response from the battery... often caused by nothing but a poor connection between itself and the battery's + post.

I'll check the wiring there as well as the grounds and see if anything is obviously loose or corroded. Is there any short or long term damage that can be done to either the alternator or battery if this situation exists for an extended period of time?

It's still strange to me that it happens, and then goes away for no apparent reason, and it never does an on-off-on-off type of intermittent thing so it doesn't feel like it's just a bad connection that is tenuous enough to go in & out. Is it possible its a sign of an impending alternator problem?

 

[Jerry Bransford]

Wasn't running any accessories at the time, so wasn't able to notice anything.



I'll check the wiring there as well as the grounds and see if anything is obviously loose or corroded. Is there any short or long term damage that can be done to either the alternator or battery if this situation exists for an extended period of time?

It's still strange to me that it happens, and then goes away for no apparent reason, and it never does an on-off-on-off type of intermittent thing so it doesn't feel like it's just a bad connection that is tenuous enough to go in & out. Is it possible its a sign of an impending alternator problem?

It's possible that the alternator's voltage regular located inside the PCM is going bad.