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Battery Light at High Speed

I posted on this forum a few weeks ago about my battery light coming on when I am on the highway (1999 Chevy Cavalier). It wasn’t coming on until I hit around 70-75 mph. It comes on and goes off. Then, several seconds later, it comes back on and goes back off. I replaced the belt and the first day driving, it didn’t come back on. The next day though, it started again.

I also replaced the valve cover gasket because it was leaking oil and I thought maybe the belt was getting some oil on it. That does not appear to be the case, unless the oil is coming from somewhere else.

I have an old style voltmeter, nothing digital, and when I place the meter on the battery, it seems to have an adequate charge, but noticed something odd when I placed it on the battery while the car was idling. The thing was showing a greater voltage while running, somewhere around 14+ volts (can’t remember exact reading). But, occasionally, the needle on the voltmeter would just drop off to 0 and then suddenly come back up to around 14 volts, maybe a bit more. I don’t know if it was the voltmeter not reading it right because it does not have clamps, it is just two pin shaped ends on the red and black wires. I was holding the pins onto the battery as tight and still as possible, so if it was correct, why would the voltage drop off like that?

You might have a problem in the voltage regulator, or a diode(s) in the alternator that could cause the battery light to come on when driving.

I don’t see why the voltmeter would drop to zero if you are making solid contact. It might flucuate between 12v and 14v but should not go much below 12v if you are in contact with the battery. I suspect you have a short inside the meter or wire is broken in one of the leads causing the zero reading. Vibration and movement of the meter are likely causing the reading error.

With the engine running, put the volt meter back on the battery. At idle, 14+V is good. A healthy alternator will power the charging system between 13.5V to 14.5V. Rev the engine up to 3000 RPM a keep an eye on the meter. If it drops to 12V (battery voltage) or rises above 16V, the voltage regulator inside the alternator is bad. Replacing the alternator will fix the problem.

I also believe the reason your meter drops to zero is a bad connection with the probe. If the charging system is dropping to zero volts, the car would stall.

My plan is to get a digital voltmeter with clips so I don’t have the connection problem. Have been meaning to buy one for a while now. Don’t really have a way to tell what RPM is though so I guess just revving it up would be all I can do. What I don’t understand is that I never see the light come on until I’m going over 70 MPH.

I’ll guess you are turning something over 3,000 rpm at 70mph, perhaps close to 3,500. That is spinning the alternator at a good clip. Perhaps your battery light is coming on to indicate “overcharging” meaning too much current to the battery. Overcharging essentially “cooks” the battery and results in the a short battery lifespan.

When you test with your voltmeter you need to crank up the rpm’s to the same rate as when you are cruising along at 70mph. If your car has a tach in the dashboard, rev it up from an idle to rpm you see at 70 mph. Listen to how the motor sounds. Then do the same when you have the voltmeter attached and check the output voltage at the high rpm.

The base model 1999 Chevy Cavalier has a speedometer, odometer and temp gauge, no tach. I guess I’ll just rev it up and see what happens. Just not too fast though :slight_smile:

Agree with UncleTurbo that you may have a bad voltage regulator. It may be dropping out intermittently, which would explain the occasional warning light and also your voltage test in which the voltage sometimes dropped to zero.

I don’t know where the voltage regulator is on your car; these days they’re usually built into the alternator, as far as I know. A new alternator may solve the problem.

The first thing you want to do when there’s a battery/charging issue on a GM vehicle with the side mount battery terminals is check the terminals for corrosion.

Remove both battery cables from the battery. On the positive battery cable peel back the red rubber cover to expose the terminals. If a lot of corrosion is found under the red rubber cover, replace the positive battery cable assembly.


If you have a Sears Auto nearby, phone them up ask them if they still provide their free alternator testing service. If so, they have all the testing equipment to do it right, and it’s a quick test for them to do. They just clamp some probes on to the cables and rev the engine and read the meters. That’s the first thing I’d do for this problem.

Thanks everyone for the comments. I took the car to the alternator shop where I had the thing rebuilt in Feb 2011. He put a voltmeter on it after checking the battery to make sure it was ok and when he revved the engine, the voltmeter did not change. Was still staying solid at 14-15. They want to pull the alternator to test it on their machine so I have to do that sometime soon. But…I noticed something on my drives to work the past two days. When driving to work in the morning, it did not seem as if the battery light was coming on as much as when driving home. We are still having some cold mornings in the Northeast so when driving in the morning, I’ll have the heat on low and the headlights on. Yesterday when I was driving home, the light came on within seconds of hitting 74 mph on the highway. It kept coming on and off very frequently, but…when I turned on the heater and headlights, the battery light shut off. I turned the heater and headlights off and within 10-15 seconds, the light came back on. Am I right in saying the extra power draw is helping hide the fact that the voltage regulator is pushing too many volts to the battery?

I’m thinking the problem may be linked to heat soak in the voltage regulator. Today’s cars use a small electronic module as the voltage regulator. They are prone to strange failures when they get too hot. Back in the 70’s and 80’s, they used transistorized regulators. Before then, electro-mechanical regulators were the norm.

But back to the electronic regulator. It needs to be tested when the engine is nice and hot, like during your afternoon commute. Drive it to the shop and have them re-test the alternator with as few accessories on as possible. A lighter amp load on the system seems to make the regulator fail and turn on the light. Call them ahead of time to make sure they are ready, because you don’t want it to cool down any.

Well, I wanted to finalize this forum, just so everyone knows the solution. I tok the car down to the shop that rebuilt the alternator in Feb 2011. They ran it through their testing procs and found that it was a faulty voltage regulator. That part was something they replaced last year so they ended up giving me a free replacement and just charged me $20 for labor costs. Works like a charm now.