Alternator Question
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Joined: May 2009
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Alternator Question
I have a 2002 F350 4wd cab and chasis with the V10, auto trans. Last week the battery light on the dash came on. I used my volt meter and had 12.2V with the motor running and 12.2 off the back of the alternator. I 12.4 with the engine off. I replaced the alternator with a rebuilt and had the same reading. I took the truck to local Ford Dealer and they verified that the new alternator was no good. I got another alternator under warranty. I installed the 2nd alternator. It charged at 14.45 and but the dash light stayed on. Back to the Ford Dealer and the light would not come on for them. I took the truck back and pulled a trailer for 1 day. The next morning, pulling the same trailer in the rain, the dash light came back on. I took it back to the dealer. They tested the "alternator and complete wiring into the instrument cluster, removed cluster assembly from dash, ran resistance test on all charging circuit wires. All in spec. Check ground circuits. Remove plug from alternator with engine running, still charging at 14.14 volts. Diagnosis faulty alternator." They got a new alternator. I put it on and had 14.4V charging and no dash light. This morning, I hooked up to a different trailer and the dash light came back on. The alternator is still charging at 14.3. Any ideas? I can't think of anything in the trailer harness that would cause the light to come on. This truck has no hot wire in the trailer harness to charge the break away brake battery.
Post Fiend
Joined: May 2006
Posts: 10,007
Likes: 4
From: Central Florida
The way that light works, is it has 12v power from the somewhere in the cluster. Then it's connected the field wire I think it was on the alternator, when this wire is charged, there is 12v on both sides of the bulb and it doesn't light. I can't remember which pin on the plug it is, but I believe that the two outside ones need to have 12v and the middle is grounded with the key on.
Maybe the 12v supply to the cluster bulb is grounding out and causing the field to collapse or just show the battery light. I had one with the wrong voltage regulator and it would not charge at all. I wouldn't expect this to be the case.
Maybe the 12v supply to the cluster bulb is grounding out and causing the field to collapse or just show the battery light. I had one with the wrong voltage regulator and it would not charge at all. I wouldn't expect this to be the case.
