jnnking

I noticed this earlier but dismissed it. Then again yesterday, it happened again. I was out enjoying a little drive and noticed that when the revs go over about 4500, the lights (interior and exterior) dim by something like 50%. When the revs go back below 4500, they come back up to full power. It's a stock car and I've had not other troubles. What do I check?

seege

Slipping belts?

kkim

Voltage regulator?

User 4221

I'm with Kelly on the voltage regulator.

Take a simple voltmeter and measure the voltage at the battery when you are revving the engine.
With a healthy regulator you should see about 13.0V - 14.4V. Anything higher or lower indicates a charging system issue.

Battery voltage lower than 13V is usually an alternator charging issue. Higher than 14.4V is typically a regulator.

As an example, I have seen an '87 Carrera output 30V with a failed voltage regulator. Weird things would happen such as the radio turning off at higher RPMs, etc.

HTH,
Scott

jnnking

Thanks for the suggestions. I think I have a voltmeter, or I'll borrow one, and check it out. I did change the belts this fall and will double check them too.

jrgordonsenior

New belts are prone to stretching and subsequent slipping. I'm with Seege on this one. Check for belt play...
JRG

jnnking

Jr and Seege,
How does a slipping belt cause this? Not questioning, just trying to learn. I would assume it would cause a drop in the recharging ability of the alternator, but that the battery would have enough stored up to cover the short fall.

seege

I'm no mechanic and the voltmeter to the battery sounds like a better suggestion than mine. But dimming lights at higher revs was a symptom of a slipping alternator belt back in the days when most of us could work on our own cars, the dimming was usually accompanied by a squeel.

SundayDriver

My first guess would be the battery. As the revs go up, the demand on the electrical system goes up. Second guess is the charging system.

jrgordonsenior

If the belts have stretched and are now slipping, the alternator has not been charging the battery adequately. Now at those higher RPM's the draw on the battery is more than the alternator is providing and the battery simply isn't charged enough to handle the load.
Simple to check, if there's more than an inch of play in the belt is need to be tightened. You could then charge the battery by simply driving on the freeway for 15 minutes....
JRG

jnnking

As soon as it stops raining, I'll check it out. Thanks guys!

jnnking

Dumb follow-up question. Is the alternator behind the cooling fan? I thought i knew where it was, but I was thinking of the AC compressor.

jnor10

Yes; link to P-car.com. In the DIY section there is a alternator removal DIY that will give you "the big picture".

Jeff

jnnking

Here's the new info and my new question. I took the battery to an auto store, they tell me it's good. They test the electrical system with the car running and say it's 90% likely to be the alternator but need it off the car to check. I checked the voltage across the battery. It's 12 v when off, 11.7v at idle and 11.6 when reved to 4000 rpm. Is the voltage regulator separate from the alternator? Do I need to replace my alternator?

slant911

Alternator or Regulator sounds like it's toast. It should be pumping out around 13.6 to 14 volts. I am not 100% on this, but, I believe the regulator is internal to the alternator on these vehicles.