Hi guys I was working on my truck having a cooling issue and had the alternator off to get to the thermostat, I was putting the alternator back on and touched it to the subframe and it arced off, due to not removing battery cable before starting work. Only for a brief second though. I got it all back together belt back on and started the truck and the battery volt had dropped to around 12. I didn’t notice this until the next day while I was driving and my gauge dropped to nothing and the check gauges light came on. I got home and tried to start it back up, only to find it wouldn’t start due to the battery being dead. With that being said I replaced the alternator off of a scrap truck that I knew had a good alternator. Still nothing, apon more research I found the 140 amp fuse for it was blown, replaced that and still didn’t charge. Battery was only a year old but I replaced with a new one, still doesn’t charge. Cleaned all grounds and terminals with a wire brush, holds charge for longer but still dies after a day or two. Any ideas of what could be the issue some things are pointing towards the ECM voltage regulator being bad but I don’t want to drop the 300 dollars till I knock out some smaller stuff it could be any help would be great thanks

 

I need to verify wiring but you should be able to put battery voltage to the field wiring a see if it will charge, this would help confirm weather or not the voltage regulator is working, I know its a 2 wire connector so im not sure which wire it is for sure.

 

Thanks for the response if you could explain more the field wiring that would be helpful, it’s seeming to me it’s either the regulator going bad or a poor connection because if I hook a reader up it’ll jump up and down about half a volt while I’m driving and if the battery is full I can go about 2 days of commuting on it without needing lights

 

It should charge north of 13.5 volts, even at idle, should be north of 14 while driving down the road.

Two wires for the field winds, Dark Green wire should be 12 volt feed from the PCM. Dark Blue feeds back to the PCM. If you can apply power on the dark green wire, and it starts charging, bad regulator. If there already IS 12 volts on that wire, could be a bad alt. Since you changed it though, and it still didn't charge, likely smoked the regulator in the PCM....

You should ALWAYS disconnect the battery when working under the hood. You just learned why.

 

Thanks for the response again I’ll give that a go tomorrow but it’s seems like the pcm any good brands?

 

Check your (gen) fuse in the PDC, Bet it's blown.

 

I did replace the 140 amp alternator fuse in the PDC unless you are referring to a different fuse

 

This does sound like the PCM regulator is bad. Does your truck have a factory security system installed? If not, I believe that year truck is plug and play for the PCM as long as you match up the PCM's part number. You can find good used PCMs via car-part.com and other sources.

 

Pretty sure the 98 PCM needs to have the vin programmed.....

 

Wasn't sure if the VIN re-program was required unless the factory security system was installed in '98. My '96 is certainly plug and play when it comes to PCM swaps...