How do I.......
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Posting Guru
Joined: Aug 2007
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From: Canton, IL
How do I.......
How do I test dual alternators to make sure both are working properly?
I don't seem to be having a problem, but my batts never seem to be fully charged, even after a long run. The Batts are only 8 months old, and are interstate's, so not thinking that is the problem.
I'll try to take some measurements, but the voltmeter in the truck indicates that the alts are working. Looking at the engine bay, the upper alt looks pretty new and has ford stickers on it. The lower one is VERY dirty and looks like it has been there since new. 242K miles now. Seems pretty impossible that the lower alt would survive that long that close to the roadway in a salt belt state.
any opinions? ways to test them individually without taking them off?
I don't seem to be having a problem, but my batts never seem to be fully charged, even after a long run. The Batts are only 8 months old, and are interstate's, so not thinking that is the problem.
I'll try to take some measurements, but the voltmeter in the truck indicates that the alts are working. Looking at the engine bay, the upper alt looks pretty new and has ford stickers on it. The lower one is VERY dirty and looks like it has been there since new. 242K miles now. Seems pretty impossible that the lower alt would survive that long that close to the roadway in a salt belt state.
any opinions? ways to test them individually without taking them off?
Junior User
Joined: Sep 2010
Posts: 92
Likes: 0
From: B.F.E., TX, USA
Not exactly sure how a diesel alternator system works (do they both keep the truck running or does one run accessories?) On a single alternator / battery setup like my old Honda, if you disconnected the ground on the battery the car would stay running if the alternator was good and die if it was bad. If your dual battery / alternator setup is similar, you could disconnect one alternator, start the truck and disconnect both battery grounds, and see if the truck dies... Again, I don't know for sure. Maybe someone can confirm?? Any diesel mechanics here?
Lead Driver
Joined: Sep 2007
Posts: 8,180
Likes: 669
From: Scio, OR
I think the first thing I would do is to put a digital VOM on one of the batteries. Check all of your battery cable ends for corrosion and broken wires, etc, to eliminate that as a potential problem.
If your battery voltage is about 13 to 13.5 at idle after running a few minutes, at least one alternator is charging properly. You could then unplug one alt at a time and see what happens. Just unplug the 2 wire connector. That will shut down the alt's regulator and stop the alt from making power.
If you unplug one alt and your battery drops below 13. You've probably isolated the other alt as a problem.
P.S. It is not unusual for new batteries to be bad. The only real way to tell is to remove them, charge them separately and load test them separately.
If your battery voltage is about 13 to 13.5 at idle after running a few minutes, at least one alternator is charging properly. You could then unplug one alt at a time and see what happens. Just unplug the 2 wire connector. That will shut down the alt's regulator and stop the alt from making power.
If you unplug one alt and your battery drops below 13. You've probably isolated the other alt as a problem.
P.S. It is not unusual for new batteries to be bad. The only real way to tell is to remove them, charge them separately and load test them separately.
