cam1688

OK so I drove to my Girlfriends house the other day and when I got in my truck (4.0 1996 Cherokee sport) there was a check gauges light on and the voltage gauge was at 0. The truck made it home and I took out the alternator and got it tested and it was perfectly fine. Now, when I turn the truck on, the voltage meter goes halfway to 14 (14 is where its sopposed to be) and after about ten seconds it goes back down to zero. The fan/timing belt is not broken and as far as I can tell not slipping. I had to buy a new harmonic balancer before but that seems to be working good as well. Help would be greatly appreciated, seeing as this is my only car right now.

5-90

How did you test your alternator? Bear in mind that most "tests" only test voltage output, and don't actually test current - and current is the critical spec.

You can get 14VDC @ 1.5ADC on most benches, and that will "pass." But, it's not enough to run a good flashlight - much less running your ignition, fuel delivery, engine management, and what-have-you.

This is why I suggest you take your alternator do a dedicated shop - the benches they have can usually wring an alternator out quite well.

And, if you search for my writeup on primary electrical testing here (and on several other fori,) you'll get qualitative, but non-quantitative, and still effective load test which is at least an order of magnitude more reliable than the chain-store benches (I used to work on those - they can give you a fairly accurate measure of starter no-load draw, but only test an alternator at 1-3ADC output, they really only check voltage.)

DFlintstone

My old 90, with everything on (drawing current, blower, brights), holds/reads up near 14 Volts with a $5 DVM from Harbor freight on the battery. If it didn't I'd check the belt, then wires and connections. 12.7 is a full battery resting, (at room temp), so if it holds steady over that, it's carrying that load.

Like 5-90 says, just having voltage, doesn't say it can do work/hold voltage under load.

cam1688

I got it tested at Autozone or Advanced Auto Parts and the guy there told me that my belt wasn't tight enough. The belt seems tight though, only having about a half an inch of movement when I pull in it. Also my dad was the one who drove and got it tested for me, he just said that the guy said it was good, so I don't really know exactly what current it was putting out.

5-90

1) Mistake in taking it to Autozone/Advanced - reasons given.

2) If you don't have a Krikit II belt tension gage (available at NAPA for a nominal fee,) the general test is that you should just be able to rotate the belt 90* through its longest span - for RENIX, that's usually the vertical between the aircon compressor and the alternator. Grip it between your thumb and two fingers, and try to twist it a quarter-turn. You should just be able to make it: too easy, and the belt is loose and you'll lose output (and probably get it squealing.) Too tight, and you'll end up smoking bearings in things (shafts must run on two bearings to run true - close to even odds which one you'll smoke, but the front bearings are usually a shade more heavily loaded, which means the fronts are just barely more likely to go first.)

I tell you - once I saw the innards of those benches, I decided that I'd not get alternators tested at chain stores anymore. The rollaround Bear ARBST unit (the red one on the black cart) is marginally better - but still won't tell you anywhere near everything you're going to want to know.

But, just because it passed on the bench doesn't mean the damned thing's any good. And the Bear ARBST usually only tests to about engine maintenance current plus 20-30A - while this is more useful, it's not useful enough.

kjc

The GOOD thing about autozone/o'reillys/advance type stores is you can just buy one and throw it on. If it doesn't fix your problem, return it. I know others will chime in about less than ideal quality parts, but they're CHEAP (the parts, not the chimers) and as far as alternators go.. parts is parts! Get a new one cheap, and use the money you saved to buy a little belt tension checker

5-90

Yah - but the best warranty is the one you don't need.

Which is why I go to a local rewind shop, instead of buying from a chain store. Which is why I send people to my rewind shop.

Which, ultimately, is why I end up getting better parts than the chain store, for better prices than the chain store.