I have a 1968 Coupe with a 289. During startup the alternator belt would squeal and then go away after a small rev-up. I recently installed a new set of gauges that have the voltmeter. I noticed that at idle the voltmeter read a little over 12V. Any revving of the engine at all would send it to 13.5V which is where I understand it should always be. In reverse the voltage drops to about 11.5V. I replaced the voltage regulator, the battery (it was due anyway), the alternator and the belt. Also, the headlight switch is new. I am still having the same problems. Oh and when the headlights are on the voltage stays below 12V at idle and is around 12.5 to 13V when the engine is revving. Any ideas on what the problem could be?

Oh, and when I replaced the alternator I got it tight by leveraging a breaker bar in the left hand while tightening with the right. Any chance lack of adequate tightness is the cause?

 

If you running a stock alternator, I think your numbers sound alright. Have you checked them with a digital multimeter? That will give you a more accurate reading.

Having the headlights on will cause a drop in voltage. If you want you can get a headlight relay setup. You can buy a kit or make your own. This will take the load off of your switch and make the lights brighter.

Its hard to say if you have it tight enough or too tight. How much slack is there in the belt?

 

Using a prybar is not a good idea. You want the belt tight enough so that you can't grab the alt fins and spin them. If you get it much tighter than that, it's bad on the bearings. What is your idle speed at? It may be a bit low.

 

Just looked at the headlight relay and I should do that. Is there any tip as to why the voltmeter drops to 12V at idle? I am relatively new to owning a classic mustang (1 year). Is this normal? I was under the impression from driving newer cars that they keep the voltage constant regardless of rpm, headlights, etc. Is this attainable with the mustang?

Thanks for the info so far!

 

The voltage will drop at idle. It does on the newer cars too, but its not as noticable because they are higher output alternators. My 95 F150 has lower votage at idle too. Then turn on the lights, wipers, AC, stereo, etc and the voltage drops more.

 

I have to disagree with that. A battery should maintain its voltage even at idle. The alternator will put out as many amps as it needs to (to its limit) to hold it there.

 

Should I switch to a high output one-wire alternator? Right now I'm running a stock replacement to the original alternator from Kragen.

 

I disagree. I have seen it happen many times where a car especially an older one starts to accelerate and the headlights get brighter.

Every car I have driven has less voltage at idle if a lot of accessories are on.

As far as my truck goes, the same question comes up often on the Ford truck forum I go on. Everyone there basically agrees that its normal. I know that all of our cars have done this.

I am running a 1 wire 100 amp alternator on mine. It helps alot. The voltage doesn't drop much if any with the new alternator. It did though with the old stock one before. All of the lights are also brighter. I get 14 volts when the car is running. The old alternator only put out about 12 or so.

 

I haven't measured the voltage in mine lately, (maybe I'll do that this weekend) but if the voltage drops below 12 at idle...you have an issue. It could be too low idle, belt slip, dirty cables...but, there is a problem. I run a stock alternator, run a pretty good stereo, have my idle set to 650, and my headlights are just as bright at idle as they are on the freeway.

People keep trying to cover up minor problems by just putting more power to it. Why waste the money?