Hello,

I have an '89 formula with a 305 and T5 trans. Last night I was teaching my brother how to drive a stick in a parking lot. He stalled out and when he tried to start it the starter sounded sluggish, and eventually caught. The voltage gauge on the car was at about 10, and eventually crept up to about 12, then when I turned the headlights off it went up a little more.

When I got home the voltage on the battery was 13.3v with the engine was off. When I turned the car and let it idle, then the battery was at 13.7v, and there was 13.8v at the alternator. The voltage gauge in the dash however showed less than 12v.

This voltage seems a little low to me I thought normal voltage is about 14.4 at the alternator and close to that on the battery.

Is the voltage gauge in the dash connect directly at the battery or after? Also any suggestions about what is going on?

 

I did a little more research with the voltmeter and found that at idle it is still about 13.5v, and it increases to above 14v around 2,800 rpm. If I turn the lights on the voltage consistantly drops a little more than 1.5v. So at idle with the lights on the voltage starts to drop very close to 12v, and will probably drop below. The voltage at the battery is still very close to that at the alternator. The voltage gauge is a little bit off, but not as much as I thought at first.

 

It sounds like your alternator is weak. Even at idle, it should take more than just the headlights being on to pull it down to 12V.

I'd get it checked...if it is weak, it may only be a few days or miles before it goes out all the way.

 

I wouldn't be surprised if you turned the heater on high along with the headlights on, that the voltage would drop below 12V at idle.

Alternator rebuild kits that I've used are around $20, changed my idle voltage from around 12 to 14.

Voltmeter in GM vehicles typically "reads" the voltage at the instrument panel via the printed circuit, so all the connections between it and the battery result in some "drop" in measured voltage. About .5V-1V less at the fuse panel from the battery in the cases I've tested.

Doesn't really matter where the guage reads, as long as you know what is "normal" on the gauge and when (if it's working right) it gets out of that normal range, that you notice.

 

What is your voltage at idle with and without the headlights on? How many amps does the stock alternator put out?

 

After I pulled the alternator and rebuilt it (died while I was driving it, voltmeter worked as intended, noticed it was out of "normal" operating range) the alternator puts out 14.4V at idle.

It's a 78 amp unit, but a cobbled together setup, so not likely to be the same rating as yours.