99 montana alternator good...but not charging battery? Help!

[tracehill]

Battery symbol light appeared on dash, After determining battery was good, I removed alternator and took it to have it bench tested at the parts store. It tested bad so I purchased a rebuilt alternator and had it bench tested before I left the store and it tested good. I installed it and it won't charge the battery. Tried a new battery...same thing. I tested the voltage at the battery and it reads 12.3 and the same with engine running. Engine runs until battery is drained. I removed the alternator and took it to a different store and they bench tested it good also! Does anyone have an idea?

[Tech II]

Sounds like the fuseable link to the alternator is open......Whenever you think you have an alternator that is not charging, always check the large red wire attached to the alternator first.....with the key out of the ignition, see if if the wire has power with a test light(test light should be as bright as when it is attached across the battery terminals).....if it doesn't light, the fuseable link is blown.....

[tracehill]

I checked that already, it has the same voltage at the alternator as it does at the battery. There is another connector on the alternator that has a 4 "pin" plug with only one wire attached, a small red one. I read on another site that this is a "field" wire that connects to the ecm and this controls the alternator. Does anyone know about this?

[Tech II]

Did you check with a test light or a voltmeter?

If with a test light and it was bright, and you checked the alt sensing fuse, and you know for sure the alternator tested ok, then yes it could be a PCM or the wiring in between....

[tracehill]

I used a voltmeter to test the voltage at the alternator. I've read on another site about the single smaller wire plugged into the alt. that controls the alternator via a module. Do you are anyone know if this module is the one under the dash and if it is some way it can be tested without specialized equipment?

[Tech II]

You don't want to test that red wire with a voltmeter.....could have one stand of the wire intact, and a voltmeter could show bat voltage......however, the current is compromised....so check it with a test light....check the sensing fuse?

There is no module.....it's a direct shot to the PCM.......

Have seen mouse nests at the PCM, eat wires there.....think it's located in the air box housing......

[tracehill]

Hi, I wasn't testing it on a strand or two of wire. I tested the voltage on the 13 mm nut that was attached tightly to the output terminal on the alternator. Thanks for your suggestions. Today I think I've found the problem. I attached a voltmeter to the battery with pigtail clamps and started the engine. I then moved the harness around that the small red wire runs thru I described earlier. Nothing changed until I made my way to the alternator and slightly pushed on the connector for this small wire. The volts jumped to over 14! I hope it will just be a bad connector and not a problem with the male "pin" it attaches to inside this rebuilt alt. I'll post a follow up. It was a good feeling though to see that voltmeter jump!

[tracehill]

This is an update to the above: The fix to this problem was the "pin" type connector at the alternator. The one with the four spaces but only has one red wire. The rebuilt alternator is working correctly now.

[Tech II]

Good job, finding your own problem....