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post #1 of (permalink) Old 08-17-2008, 04:19 PM Thread Starter
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Alternator Charging Woes

I am having charging issues with the OEM alternator/electrical system on my 1994 4.0L motor that is in my 85 CJ. My dash OEM volt gauge shows about 10.5V at engine idle. Increasing RPMs does not increase charging voltage. Using my digital volt meter, I get about 11 volts across the terminals. I pulled the alternator and went to my neighborhood AutoZone where I had them bench test the unit. AutoZone said the alternator was fine and in all operating parameters. I reinstalled the alternator and cleaned all the battery terminals, cables, and wiring connections to the alternator. Even used scotch brite pad on the nuts and connectors, put dielectric grease on my all alternator connections too. This did not help. BTW, I run a separate ground wire from the AC compressor bracket to the battery. What else should I check and do guys?

The markings on my alternator are;
5600 5685
IN 121000-3450 12V
Family 90HS Denso 70113

I am not sure what the stock amperage is on this OEM alternator, can anyone tell me?
If I need to replace this alternator, what would be an OEM replacement (Jeep/Dodge) that would provide greater amp output but be a easy swap that I could find in the junk yard? I don’t have the cash for a new mean green or similar type alternator.

Thanks for the support and help.

1985 CJ with a transplanted 94YJ 4.0L motor, T176 tranny, mainly stock otherwise. ><>

Last edited by DuaneD; 08-17-2008 at 08:06 PM.
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post #2 of (permalink) Old 08-17-2008, 08:15 PM
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The first guess is that it's not geting the "excite" voltage that it needs to bootstrap itself up. Sometimes if you really wing the revs up you can spin the alternator fast enough to make it start charging. Once is gets going it will keep on charging, even at idle, until you shut the engine off. See if you can do that and get it to put out 14 to 14.5 volts.

If you can, there's a discontinuity in the circuit between the ignition switch and the alternator, but if it's using a Denso alternator the circuit is probably different from the Delco units I know about. Someone else will have to help you track it down.

If really revving the engine doesn't do it, you need to try jumping from the battery to a terminal on the alternator while the engine is running, but on a Denso, I don't know where terminal that would be. If there are two wires, it will be the smaller one. If there are more or fewer than two, I'm lost.

EVERYTHING's easy for the guy who doesn't have to do it.
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post #3 of (permalink) Old 08-17-2008, 10:13 PM
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Idiot light?
If so - Key on, engine off - does the light come on? Should be "yes."

Engine Running - bulb on then? Should be "no."

I know that sounds like "busy work" and would be obvious, but -

Reason I asked is the excite current for the alternator comes through the light. If the light's burned out, it can't charge.

The bulb check also checks not only the bulb, but the excite wiring itself.
It runs from "ignition switched 12 volts," through the bulb, to the alternator (or if it has a separate regulator, to that.)

If the above bulb checks are fine, then - Engine Off, Key Off - measure right at the big alternator terminal - should be 12 volts. If not, check the wiring between that terminal and the batt +.

Autozone testers have tested lots of defective alternators as "good." Reason is most people buy their alternator without bringing it in to be tested first. When the new or rebuilt Alternator they just bought doesn't work properly, they bring it back. Before adjusting one for warrantee, Autozone requires it to be tested first.
Naturally all their trash will test OK so they don't have to exchange it. Their test will not usually detect a bad diode, or a bad diode trio. They all will test good. Verify that by taking that same alternator to an alternator shop so they can test it - you'll see.

Last edited by RRich; 08-18-2008 at 09:58 AM.
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post #4 of (permalink) Old 08-18-2008, 09:21 AM
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Has anyone ohmed out the rectifer pack? One of the diodes could be bad...
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