So driving home last night, from Flagstaff to Phoenix, its about 150 miles or so. When I got into town my battery light on the dash would come on and the tach stopped working. Then it would go off and tach would operate normal. Then it happened again, so I pulled off the freeway and stopped in a parking lot and the light went out. The vehicle seemed to be running fine, the headlights or dashlights wern't dimming or anything and the truck seemed to be running fine. So I took the surface streets home to be safe and noticed the battery light/tach problem appeared anytime I got above 30 mph. So with everything running smooth except for that light and tach I was thinking it was maybe a short or something and hoping its not a bad alternator. Anyway I read that one way to test the alternator is to pull the positive battery cable while its running and if it the engine dies its a bad alternator, or you could test it with a voltmeter. I also read pulling the cable could fry some electrical components so I wont do that, so Im waiting for my wife to get home so I can go get a voltmeter to check it.

So my question is what should the volts be, around 13-14 Im guessing, or is there a possibility it is something else. Any suggestions?

 

13.5 minimum to 14.5V

 

OK so tested the battery with a voltmeter and battery is at 12.75, with engine running it gets to 13.8, at the alternator it a steady 14. When revved to 3000 rpm it drops about 1 volt and the light comes on and tach stops working, but as soon as you go back to idle it recovers.

 

So when you "rev it to 3000rpm" it drops from 13.8 to 12.8? I'd go to a parts store or tire shop and have them do a free on-the-vehicle battery, starting and charging system test.(shops still do this free, right?) If your charging voltage drops, I'd say the alt is crap. Are you testing at the battery or alt when you see the drop? What is the drop at both? Have you checked your grounds?

 

A wise decision to not remove the battery cable while the engine is running. The alternator can produce voltage spike and the battery acts as an Acumalator to absorb those spike before they fry a computer.

 

Also check the routing of your serpentine belt. It is possible to install it incorrectly. Then is slips and the RMP's go to zero and no output from alternator.


Derek V

 

So I took it to the auto parts store to get it tested and the battery was fine, When started the alt was at 14v. Then as he had me rev it a bit the volts dropped to 13 then 12 then bounced around for a bit. So he said the alt is on its way out. Before turning the engine off the alt started to make a rattling sound. So our guess is the bearings are going bad. So I know I need a new alt, question is do I replace it or rebuild it. If I rebuild it any suggestion on were to get a rebuild kit.

 

I did read that the brushes in the alternator are considered a replacement part and that doing a rebuild on your existing alternator is much cheaper than replacing it. I have not had to do this yet, so I do not have any first hand experience with it.

I would avoid cheap aftermarket replacements from the typical Auto Parts chain type stores and look for one of the old fashioned automotive electrical shops that rebuild starters and alternators if there are any in your area. Or even look into a good used alternator from a large parts recycler.

Even better, try giving Will Tillery a call (Roverguy.com). I have bought several items from Will, most recently a headlight switch for $55 shipped. Will gives you a good price on good used original parts and always includes shipping in his quote and no sales tax or other add-ons like some places do.

Paul Grant is another possible source.

 

Hey fellas, another fast and free way to see if your alternator is at very minimum working is to take a piece of metal........screw driver whatever......and hold it to the back of your alternator, if the rear of the alternator is magnatized then its atleast doing something. Of coarse its not a volt meter but if your in the woods and have no idea if your alternator is working, this method will atleast tell you its doing something.