Archived Generator light on

[PKnight]

Have 97 Ram 2500 4x4 with Turbo and 130000 miles. My gen light came on and there was no charge on the Amp meter. I went to NAPA and bought a new alternator and now my gen light comes on showing too much charge. It pegs at 18 volts while driving even with the lights and everything else on. Anyone have an answer. The truck has been trouble free until today with only the fuel sending unit and the starter contacts being replaced. P Knight, Olympia, Wa

 

[B.G. Smith]

Sorry I can't be of more help but here are some ideas. The alternator output is controlled by the PCM. The PCM uses a feed from the batteries to determine how much to tell the alternator to charge. I have read that the CPS (crank position sensor) sends a signal to the PCM when the engine starts to activate the alternator. In a dwg. that I have of a 98, one side of the field winding in the alternator goes to the PCM and the other side goes to the auto shutdown relay. For a good alternator to charge too much the field would have to be either shorted or not be seeing battery voltage. Have you recently done any repairs/changes to any of these components??? bg

 

[PKnight]

Thanks, BG for the information. I have a battery charger on over night but have not started it up to see if that helps. No, no other work has been done on the truck since the starter contacts were replaced a year ago. Guess I will have to drive my 55 Plymouth to the Habitat for Humanity building site today. Maybe this evening I will have time to read my service manual. Thanks again, Red Ram

 

[PKnight]

Re: Gen light on. I charged my batteries overnight just in case they were not fully charged. Started truck this morning and everything appeared normal for 2-3 minutes, charging at 14-16v, then the amp meter pegged at 18v again. This is what happened yesterday after installing the new alternator. Any suggestions?

 

[AKAMAC]

Make sure the chassis ground and engine ground are solid. If they are, just for grins, pull the neg. terminals and turn on the key for a couple seconds. Hook terminals back up and try it again.

 

[PKnight]

Gen light on (Now Off)

Got it working! After reading posts from B. G. Smith and AKAMAC I finally found time to read my service manual. I tested the battry temperature sensor which was ok, tried AKAMAC's suggestion an then did the Charging system diagnostic trouble shooting procedure. It gave me two codes. One was "charging system voltage high", which was obvious since it was pegged at 18v, and the other was #41, meaning that the generator field was not switching properly. It states with this code an open or shorted condition is detected in the generator field circuit. BG nailed it! I took the rebuilt alternator back to NAPA and exchanged it for another one and now everything is working fine. Back on the road again! Thanks to all who helped.