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Alternator problem...... I think

68EFIvert

68EFIvert

Well-Known Member
I have been driving my Mustang as a DD for the last week as some body work was done on my normal DD. I went to the the Post Office yesterday to drop off the paperwork for a Passport and the car would not start. When I turned on the key the fuel pump would not even come on. I waited a few minutes and the pump would turn on but did not have enough juice to crank. I figured that it was my battery but was not sure.

I called AAA and they came over to give me a jump. It took a few attempts but it finally turned over and started right up. I drove the car home and put on my battery charger. It seemed to charge fine and the charger turned off automatically.

Fast forward to this morning. My DD won't be done until this afternoon so I figure with a freshly charged battery I should be good to go. When I start driving down the road I kept a close eye on the volt gauge. It was running about 1.5 volts while driving under 2500 rpm but when I reached over 2500 then the volts jumped to 14. I turned my car around and went back home and decided to carpool with the wife today.

When I got back to the garage I hooked up my battery charger which is also an alternator and battery checker back up. The battery was showing 85% and the alternator was only putting out 75% of what a stock alternator should. My alternator is a 130 amp 3g unit so it should have been showing about 150% of a stock alternator. I purhcased a new alternator today from Summit and will see how it goes.
 
Don't 3G alternators require a higher rpm to "kick-start" them into charging the battery (typically 2000-2200 rpm)?

At idle speeds, they simply do not put out enough current to keep a battery fully charged.
 
I am not sure about that but I do know that I have never seen that type of movement in the volt gauge. It has always gone straight to 14+/- volts as soon as the car is started. Last year I had a problem where I forgot to hook up the alternator to the EFI wiring and it never turned the alternator on causing the volts to drop rapidly.

I considered the gauge could be going bad but the headlights would dim with the lower volt reading and get to normal levels with full voltage. I am running a headlight relays with my setup if that matters.
 
"Midlife" said:
Don't 3G alternators require a higher rpm to "kick-start" them into charging the battery (typically 2000-2200 rpm)?
Click to expand...

You may be thinking of the 1 wire alternator. They require rpms above 1500-2000 to turn on the alternator. The 3G should have an exciter wire to turn it on no matter what rpm.. ( I think)
 
"6t6red" said:
You may be thinking of the 1 wire alternator. They require rpms above 1500-2000 to turn on the alternator. The 3G should have an exciter wire to turn it on no matter what rpm.. ( I think)
Click to expand...

+1 That's my understanding.
 
I was talking with a car friend of my and he said it sounded like a diode in the alternator went out. He said there were 3 diodes and the other two were trying to keep up but just couldn't. He told me that a diode going out would cause the battery to drain down. I have had a slow battery drain problem so this may make sense. He said that when the rpm get up then the good diodes would pull the load but not under low rpm. I went home last night and the batter had been discharged to 50%. It really sounds like the alternator was the problem but I'll find out tomorrow when the new chrome alternator from Summit arrives.
 
He told me that a diode going out would cause the battery to drain down. I have had a slow battery drain problem so this may make sense. He said that when the rpm get up then the good diodes would pull the load but not under low rpm. I went home last night and the batter had been discharged to 50%.
Click to expand...


I'm thinking you may have two seperate problems. The "bad diodes" in the alternator is certainly a possibility, but I don't believe this issue will cause the battery to drain while the car is turned off.


After you get your new alternator installed.... if you still experience a slow battery drain.... from my experience this is usually the result a "professionally installed" audio system. Where "professional" is usually equal to "they get paid to do it" and it looks/sounds nice, but not much more. Some of the wiring I've seen from professional audio shops is down right dangerous!
 
Since I installed my own stereo I guess I am now a "professional". If I do still have a drain I will look at stereo wiring and also my power window wiring. The body shop who assembled my car for me put the windows in and I don't think the installation looked very clean. I also have a problem with my interior lights. They have not worked for months so I have a short in the system somewhere.
 
I put on the new alternator today and it looks like it has solved my problem. It goes straight to 14 volts at idle.
 
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