c9night

When I race my truck down the road I notice my battery light comes on around 4,000 rpm. Then I lose power until the truck shifts gear or I take my foot out of it. I checked the voltage and everything looks ok. When I rev it up it drops below 12 volts at the 4,000 rpm range. Battery test good and the alternator test good at idle. Does anyone know what the problem is?

DaughtryLc

If u take the alt. off u can run it to AutoZone, Napa, Advanced or what ever u have and they have a machine to run it up and test it.

My01SCrew

Could also be belt tension, check the belt and/or tensioner.

AFSOCSGT

^X2 sounds like belt slippage.

c9night

How can I check the belt tension. I push on it and it feels tight. I haven't found anything in my chiltons or on the Internet to tell me how.

akdoggie

The alternator is t/u. When you exceed a certain point the armature and the brushes loose conductivity and thus the light on at that RPM is usually the culprit. The Ford alts with the brushes and voltage regulator combo tend to cut into the armature deeply and will also cause the voltage drop, Take it in and have it tested. I'd bet you anything that its bad.

harleyguyusa1

I had the same issue with my 97. I procrastinated over the summer and instead of fixing it at that time I had the pleasure of installing the new alternator in the cold blowing snow. You can test it where it is. If you have a multi meter you can test at the alternator its self rather than at the battery. Try it at an idle and carefully try it with someone slowly raising the RPMs (stay cleaf of the fan). You won't have a load on the motor so try to turn on as many electrical accessories as possible to increase demand on the alternator. If it comes up ok at high RPMs try the same again at the battery. If it drops at the battery but not directly at the alternator you have bad cables. My best guess is you'll find the alternator bad.

c9night

I thought it was the alternator but I thought if it was bad it was bad. Never had one do this before. I will have to take it off and get it tested.

bipolardave

This happened to my truck.

I was test driving it before I bought it last month and the light came on around 3500 RPM.

The dealer at first tried to replace regulator/brush assembly but the commutator was too etched so he ended up replacing the entire alternator. Problem solved.

The voltage didn't drop out, though, with the new regulator. If there's no grooving on the commutator, you might get by with just new brushes and regulator. They're part of the same unit and swap out in no time.

c9night

Cool thanks