my truck is a 2000 f250 with v10 motor with 145k. my alternator died on me going down the road. so i got a new one from a local guy that has a good reputation that rebuilds them with parts made in usa, or so he says. seems like a very knowledgable guy on starters and alternators. after putting new alternator in my volts would jump intermittenly from 14.3 to 16.5 or so with truck running at the battery. so i took that alternator back and got another one and have the same problem. so i had battery tested and had a bad cell in it, so i replaced the battery with a gold from advance auto. now the volts are constantly jumping and i cannot figure out a source. when the volts increase all the lights on the truck brighten and then dim when volts go back down. also my battery light comes when the volts get too high, it seemed like about 16 volts would kick the battery light on. i checked the wiring harnes behind the alternator and it looks fine. i checked and cleaned grounds but did not test the ohms on the grounds. any suggestions would be greatly appreciated.

 

Welcome to FTE...

It still sounds like a voltage reference issue. If you connect a jumper cable from the body of the alternator to the negative post on the battery and things settle down, you've found the problem. Ground issues comprise probably 70% of all electrical issues.

 

OK I will try that tomorrow thanks. If that does fix it do you know if that tells which ground is bad?

 

No, not really, but I would suspect that there is a ground wire in the harness behind the alternator that is bad or a connector that's not quite kosher..

 

Okay thanks again. I will check that out tonight. I really appreciate it

 

Okay I will check that tomorrow. Thanks again I really appreciate it.

 

I tried grounded alt. To battery and no change. Possibly I got 2 bad voltage regulators in both new alternators? Any other thoughts?

 

Very odd... It may well be possible that you got two duds.. I would talk with the guy that is rebuilding these things and get his take on it. Is it possible for you to get the truck to him so he can observe first-hand?

Another thing I do is to clip my meter leads to the battery and watch the display as I move the wiring harness around a bit. If there's a bad connection, it will many times show up in this manner.

 

So I took the alternator back to the local shop and got one from Auto zone yesterday. Same problem. I had my dad watch the gauge and I wiggled all connections. I even unplugged the single wire which is the battery light wire. I hooked up my volt meter to battery and jumps from 14.3 to 16.5 or 17 at the most. I have ran out of ideas other than maybe plug is bad or chaffed wire somewhere but cannot find it. Any other ideas? Thanks for input.

 

I would not rely solely on the dash volt guage. Put a real volt meter on the battery and see what that's telling you. The odds of getting three bad alternators in a row are really slim.
I'm not sure if that the voltage problem you're seeing is the alternator itself or if the voltage regulator and rectification circuitry is at fault here. Either way, getting confirmation of your guage is something that needs to be done..
Sorry I don't have a precise answer or solution here - just common automotive tech...

 

i put volt meter on battery when it starting doing this to make sure it wasnt just the gauge at fault. the reading i posted are with a volt meter at the battery. i would like to test the alternator plug and make sure it is good somehow but i dont know how. i am going to try and do some research today and figure out how to do that and i can go from there.

 

i replaced the plug on back of alternator for $15 and fixed it. it hold steady at 14.3 volts but will flutter a little higher and go back down every now and then. i dont know why it still does it a little bit but its fixed for now. thanks for all your help.

 

SWEET

They all fluctuate a little... Good job!

 

Which clip is it? I have this same problem going on. I am guessing I can just order that part online?