All, I have a strange problem with my 92 F-150 Supercab 5.0L. The rectifier inside the alternator shorted out, so I replaced the whole alternator (75Amp). Since I replaced it my amp light on the dash stays on all the time. I tested the new alternator and it is charging the battery, and the voltmeter fluctuates when I rev the motor, or turn on the lights. Everything seems to be working fine, except for the amp light. Any ideas would be appreciated, since my temporary fix was the old piece of electrical tape over the light trick.

 

Try unplugging the Baattery for 10 minutes to "reset" the computer. If that does not work i don't know

Chris

 

Are you absolutely sure your '92 requires a 75 amp alternator? Incorrect amp output will cause your amp light to stay on.

My '93 has a 5.0 EFI and uses a 95 amp alternator with the voltage regulator mounted to the back of it. Just for kicks, check it out. If that IS the problem, THEN after you replace it with the correct alternator, you will need to disconnect your battery for a few minutes to reset the computer.

 

Thanks for the ideas. I had the battery disconnected for a day when the rectifier shorted out, it was drawing 5 Amps, and killed the battery. So I took it out to charge it before changing the alternator. But it couldn't hurt to try it again. The old alternator had 75A stamped on the casing. I don't know if it was ever replaced before I got the truck. I was looking at the schematic, and I noticed there is a resistor shown across the amp light. I am thinking that the resistor may need to be a specific size depending on the amps the alternator puts out. I don't know if this can be causing it, or where it's located, but I assume its in the dash.

 

The Charge light is turned on by the alternator. It does this by grounding the LG/R wire when it thinks there is a charging problem. Apparently the alternator you got is charging but the circuit built into it to turn the light on is not working properly. The computer has nothing to do with turning on the charge light. The best thing you could do is to take the alernator back and get it replaced under warranty.