Low charging at low RPM's, 10/09

[greybeard14]

My '91 4.0 decided to charge very little at low rpm's yesterday. The battery light comes on, then when tromped upon the revs will kick up a bit. It used to do this only in 30 below occasionally, I thought it would be sticky guts inside the alternator, now on a warm summer day??? What is the order of troubleshooting that I should do? Battery check up first? Best source of a new/rebuilt altenator?? I use the truck for work and don't want to strand myself or my client...

 

[greybeard14]

overwhelming response, thanks.

 

[Opera House]

About the only thing to check is the 12V power line at the regulator. Should be the yellow white wire. Some have reported bad contact with the 15A fuse. This is also the where the regulator monitors the battery voltage.

 

[cjpwalker]

Possible you have lost a diode in the alternator. I haven't done much w/ fords, but GM runs 3 diodes in the alternator (to change the AC that it is generating to DC). If one or two goes you get regular charge when revved up, but low at idle. Most parts stores can test it for you while it's still in the rig and will check your battery in the same test.

 

[jkexplorer]

I had a similar problem namely low charging at idle. Interestingly the car computer upon sensing this condition increased the idle speed to compensate for the low charge. Luckily the car was still under warranty. The dealer replaced the alternator and that took care of the problem.