Have a 99 F150 4.2 V6 with 130,000
Problem: Idles at 850 rpms and lights are deem, As soon as rpm's are at about 1000, headlights get bright again. I changed the alternator and battery in Sept 07, and today got a load test on both which indicate good. Truck seems to running fine, but when coming to stops headlights go deem and when starting off and rpms are up, they turn to normal. What could be the problem? Also the battery guage in the cab is a constantly in the middle range. Please guide me folks to the problem so I can save some money. This site has helped me before. Thanks in advance......

 

I'm thinking it's the battery or alternator, but you said they tested good under a load. Hmm??? It's got to be something keeping the current from moving like it's supposed too. Yeah, obviously. Isn't the starter wire part of the circuit that leads from the battery to the alternator? Maybe check the wires on your starter for corrosion. I've heard from a few people say that their starter wires had a greenish looking corrosion on them, and it caused problems with the electrical system.

 

Thats also normal to a certain extent.

 

Thanks I will check the starter wire and let you know, man I can't wait till we get warmer weather....cold out there....Bye

Well I checked from what I could get my eyes on and hands on, the wires appear to be OK and tight.

 

It could also be that they wires themselves are bad and the may need to be replaced. If you want to really know though you may want to go see a mechanic.