Im unsure if these readings are good. I would guess I shouldn't be getting any resistance but im no elictrical guru by any means. In my area another pcm is not obtainable. I have been looking for some time with no luck. I dont want to just throw parts at it. The parts store where I bought the alternator will not replace it (what goods a lifetime warranty right) since it tested good on their bench.

 

You found the problem. The bottom terminal goes to the PCM, and is partially shorted to ground. That explains the bottom terminal reading of less than 12v when it was running away and the PCM should have let it go to supply voltage. The alternator field is shorted to ground, and that is what is causing the runaway.

Take it off and verify that the field is still shorted to ground. Then take that alternator, your meter, and a printout of this thread to the store and demand that the alternator be replaced.

A poor alternative would be to open it up and figure out where it's shorted but if it's in the armature itself, you won't be able to repair it, and anywhere else it would take a specialized insulator to fix it. Then it would also probably void the alleged warranty.

BTW, you've come a long ways from "Bubba says the ECM is bad, where do I get one." to "I don't want to throw parts at it." You have been a good student.

hope it helps
John

 

How do I verify its still shorted to ground once I have taken it off. Thank you so much Mr. Martin for all youve done.

 

Measure from the field terminals to the ground terminal on the case.

 

Both field terminals off the truck completely read 1.86 ohms to the ground trminal

 

I think you might have fried your ohmmeter, or the short is right in the middle of the field winding on the armature.

Either way.......Go Getum.

 

Take your ohm meter with you and measure the new one to verify it isn't also bad,

BTW, it should NOT read zero ohms, it should read infinity ( nothing )

Zero ohms would indicate a direct short.....

I also applaud both your tenacity and your willingness to try and learn.

Stick around, we need folks like you!

 

Well I fought tooth n nail and finally arose the victor. New alternator installed and voltage is holding steady at 13.5ish volts (dasho-meter). Only thing now is I still have codes showing up. 41 which I had before but now new one popped up. Code 46. Too high? Its not fluctuating anymore. Are these just codes stuck in the pcm that need to be cleared out. If so will unhooking the battery do the trick or do I have to have an obd1 scanner

 

Rub it in a bit at the auto parts store.

On the questions, I'll tag team to Pat...

Gotta do the sound for the church annual meeting. It's usually a lot of fun.

 

I just wanna say thank all of yall so much. I appreciate yall shearing all of yalls knowledge about this. Without yalls help ida just thrown parts at him after loosing my mind. Thank yall again so much. Hopefully its just stored codes that need to be cleared.

 

I did some digging and found that I have to have the codes erased by a scan tool or they will clear up after 50 starts. Hopefully its all ironed out. I'll keep yall posted on his status. Thank yall again so much. I couldn't have done this without yalls help. Now I can focus on my nv4500 swap.

 

J_Martin Thanks for posting in this thread you taught a 20 year automotive tech a thing or 2 about these trucks! thanks again and glad Vader 93 got this truck back up and running corectly.

 

Actually I missed a clear shot at post 29. Fault code 41 is shorted or open generator field. Should have had him probe for that right away.

The reason it didn't throw a 46 right away is the over voltage has to hold 1 volt or more over max for 30 seconds or more. Until he got his high acceptance rate battery sizzling, it held the voltage down.

Anyway, it's fun teaching when you have a genuine learner to work with.

 

Well I dont give up easily especially when I have good teachers. Thank yall so much again

 

If you disconnect the battery for 15 minutes, all codes will be reset. Upon reconnection code 12 (Battery disconnect) will set and hold for 50 starts.

That clears the codes so you can watch for more trouble.