Alternator?

[KallyI]

While driving today, my tach reading dropped to 0, but would jump intermittently. At the same time, the voltage reading dropped to about 9 volts, and stayed there. I'm assuming that it's the alternator that's Fubar, since it happened to me 2 years ago as well, but I thought I'd just touch base with all the gurus and see if it might be something else that I'm missing.

[gerry1]

I've changed out an alternator for similar symtoms. Come to find out that is was a bad cable between the batteries. The problems would come and go. Checking voltages at idle would look OK. But after driving a bit, it would start acting up again. I found it by loading the electrical system at idle (A/C on, high beams, brake lights, ect.) I read a large voltage difference between the positve terminals of the batteries. Replaced the crossover cable and the problems were gone.

[Robyn]

The GM system on these diesels uses the left battery to dump the alternator into.
The problem is many times the ground cable on the left battery. If they have some time on them replace both ground cables. The problem is the eye at the engine end, they corode in the crimp and then quit working due to high resistance.
My 94 did this and it was the reason I got the truck. The former owner had had the truck to many shops and everyone told him it was a bad computer.
HMMMMMMMMMMMM low volt readings, wierd running and such. Replaced the grounds and Pooooof all fixed.
Guy practically gave me the truck just to be shed of the problems.

[signgrafix]

Yep, it is all true. Mine was doing this and the alternator soon went out. Replaced the alternator, problem stopped. One of my next projects is to replace all the battery cables with Welding cables.

[CareyWeber]

My factory alternator lasted to about 100K miles I bought a lifetime replacement and promptly replaced it about once a year for about four years. I finally replace all the battery cables (to include the cable from the alternator to the battery) and after that my alternator has lasted.

Since I owned the truck I have cleaned the cables every fall, but the cables coroded up inside the cable.

I used 1 gauge welding cable for mine.

Carey

[KallyI]

Thanks for all your replies. I purchased the alternator at Napa 2 years ago in August, so it was still under warranty. We can't get a lifetime warranty from Napa here in Canada. They gave me a replacement, which has a 3 year warranty, and all seems good now. The voltage does fluctuate more than the previous alternators did, so I'll keep an eye on it, but I do have my tach reading back :) . Does anyone have any idea why these trucks of ours seem to eat alternators? The original alternator failed and was rebuilt in 2001, at about 75,000 miles, then it failed again in 2004, with about 130,000 miles, to be replaced with a Napa unit, and now with about 150,000 miles. That just seems excessive to me.

[signgrafix]

Thanks for all your replies. I purchased the alternator at Napa 2 years ago in August, so it was still under warranty. We can't get a lifetime warranty from Napa here in Canada. They gave me a replacement, which has a 3 year warranty, and all seems good now. The voltage does fluctuate more than the previous alternators did, so I'll keep an eye on it, but I do have my tach reading back :) . Does anyone have any idea why these trucks of ours seem to eat alternators? The original alternator failed and was rebuilt in 2001, at about 75,000 miles, then it failed again in 2004, with about 130,000 miles, to be replaced with a Napa unit, and now with about 150,000 miles. That just seems excessive to me.

Yep, that is an interesting question as far as alternator life. I wonder if it is underhood heat? Our trucks are kind of compact with not much open air flow area.