O/T '90 Jeep charging system

I've tried changing the alternator with a known good one. It works the same way with both of them: charges fine with nothing on. Start firing up lights, heater fan, etc. and the gauge gradually drops.

The owner 'thinks' the battery is pretty new. Haven't thrown a load tester on it yet.

I'm wondering if it's a ground wire problem?

Any suggestions are welcomed; I appreciate your assistance.

Thanks,
Anthony

1, you don't have more laod than alternator capacity.

2, battery is good

3, truck has a analog volt meter gauge that is operational when ignition is on.

What I would do is make a jumper wire of 10ga and jump from alternator charge stud to battery post. Then look at your volt meter.. if it increases when you hook up this wire, then you have an insuficience int he charge wire.. or a bad connection.

Also wouldn't hurt to know what the volt reading is at the back of the al, vs the battery.. if there is a big discrepency, then I'd deffinately look at the charging system wire harness.

Soundguy

2. Check how many volts the alternator is putting out.

3. Full-field the alternator and see how many amps it's capable of putting out.

4. Then check the amperage draw of the accessories, one at a time.

Somewhere in the above, you'll find your answer.

Can I take amp-draw reading at the accessories' fuses in the panel under the dash, or will that not be accurate?

Appreciate it,
Anthony

You can test the amperage draw at the fuse panel, but if you're using a standard volt-ohm meter, they usually have an internal 10-amp fuse. So if your headlights are drawing 25 amps, your meter MIGHT register it just before the fuse blows, or it might not. You really need something like a VAT-40 [or whatever Snap-On calls their digital equivalent] to properly test the system.

Once you determine whether the alternator is the problem, then--if you also bring the battery with you-- they can test it as well, and see if the battery is the problem.

If the battery tests OK, and the alternator tests OK, then you know the problem is elsewhere...and you can start checking current draws.

I'm not trying to be agrumentative; I just want to make sure I've got it fixed right.

Thanks again,
Anthony

Gerald J.