Need help with 1989 B2

[BroncoMike8789]

The volt gage drops below 12v and the headlights dim when I come to a stop but the volts go back to normal when I start driving. I checked the alternator its good and my battery is good. Any ideas whats going on? Thanks

 

[alwaysFlOoReD]

How did you check the alternator?
A newish battery should have 12.8V in the morning before starting the truck. Couple years old will be 12.5V. On its last legs is 12.2V...time to replace.
Alternator should be putting out about 14.5V on start up. It should go down to about 13.2 after a few minutes.
It sounds like one of the diodes is dead. There are 3 diodes to convert AC to DC in the alternator. If 1 is dead the other 2 will charge the battery at higher rpm but won't keep up with lower rpm.

 

[JerryC]

In addition to AFs comment, check the battery cables, clean the battery terminals, and check the alternator belt for wear and proper tension.

 

[BroncoMike8789]

How did you check the alternator?
A newish battery should have 12.8V in the morning before starting the truck. Couple years old will be 12.5V. On its last legs is 12.2V...time to replace.
Alternator should be putting out about 14.5V on start up. It should go down to about 13.2 after a few minutes.
It sounds like one of the diodes is dead. There are 3 diodes to convert AC to DC in the alternator. If 1 is dead the other 2 will charge the battery at higher rpm but won't keep up with lower rpm.

I just pulled the positive cable off battery while engine was running. Thanks for advice I will use a volt meter and see what the volts are. I will let you know what I found.

 

[alwaysFlOoReD]

You shouldn't pull the cable off while the truck is running. That can damage the alternator.

 

[wildbill23c]

Had this same issue with my 88 Bronco 2. The poor alternator was only running on 2 of its 3 fields, so it would struggle at idle and most times wasn't charging. I installed a new alternator problem solved.

 

[BroncoMike8789]

The alternator was bad so I put in a new alternator and now the B2 seems to run fine.