NEED HELP Started with P0621 now no alternator charge

[Gerbil21]

OK well as stated in the title the alternator does not charge at all, it started Monday afternoon on my lunch break after stopping to get food and coming back to the truck, the battery light was on and it gave the message battery not charging.

At first I checked all cables and wires and no difference, then I pulled the PCM fuses and still nothing, then I tried starting again this time I gave it a little Rev and then the alternator came back to life and started charging. Now the alternator doesn't charge at all yesterday I had to Rev it a lot more to get it to do anything

http://www.duramaxdiesels.com/forum/showthread.php?t=34711
According to what I found the PCM is supposed to give no more than one volt with key on engine off but I've gotten 5 volts, I've even tried the pin directly on the PCM and got 5 volts. But for some reason now I seem to be getting 1.3 volts.

Yesterday I took it to autozone and they tested the alternator and it came out fine when the engine was running good but their machine froze and I wasn't able to test it with the engine on and not charging.

I got a new PCM but it needs a security relearn and when I tried to do it the voltage of the battery drops fast for whatever reason.

I need all the help I can get with this one, it's not making any sense to me.


I'm going to try changing the alternator too, I know it won't make a difference but it's not like it'll hurt and there all spare parts either way

 

[Gerbil21]

Well it seems like everything's working again "knock on wood"

While trying to do the security relearn with the other PCM, the voltage was dropping way too fast and got down to around 9.5 volts before 10 minutes.

So i went on and painfully changed the alternator in the cold and dealt with the stupid design , but after I replaced the alternator it fired right up and went straight to charging.

I drove it around for a while and I've noticed a nice difference, it seems to accelerate nicer and drive a lot more smoother as well

So far so good

 

[xavierny25]

Knock on wood but mine is still the factory original going on 8yrs and it's still doing its thing.

 

[Mooseman]

Wouldn't be a bad idea to have the battery tested even if all seems to be good. I've seen bad batteries kill alternators. I just exchanged a 6 month old battery that seemed good but froze in -20c.

 

[littleblazer]

Glad you figured it out, the vette was doing the same thing, it wouldn't charge until you were well past 3000 rpms and then it would act fine. The voltage regulator died and needed the high revs to excite it. Also had a much better driving feel after replacing. And I agree with having the battery tested as well, but I will also say a failing alternator will kill a battery in a few days as well. (Another symptom with the vette) But glad you got it all worked out.

 

[Gerbil21]

Well so far everything is still good and mpg seems much better too, I was getting 6 before now I'm getting 8 (still sucks) I also went and pulled the AC fuse which seemed to get me a extra mpg as well so well see.

And yea I'm going to take it back to advance auto and have it checked to make sure the battry is good today.

 

[Mooseman]

I also went and pulled the AC fuse which seemed to get me a extra mpg as well so well see.

Probably that is because if you use defrost and the outdoor temperature is above freezing, it automatically engages the A/C compressor to remove moisture from the air. Most of the time, it's not really needed. It cannot be disabled unless you pull the fuse or relay.

 

[xavierny25]

Probably that is because if you use defrost and the outdoor temperature is above freezing, it automatically engages the A/C compressor to remove moisture from the air. Most of the time, it's not really needed. It cannot be disabled unless you pull the fuse or relay.

I got a question regarding this why is it every time I hit the auto button weather it's cold or hot out the ac/snowflake light comes on? I've always just set it to 74f in the cabin.

 

[Mooseman]

I guess that's how auto works. If it needs A/C, it will engage it. You can turn on A/C in -20c but it will not engage because it's too cold. That's also why I don't use auto so much. I don't know what it's doing on auto. I'll leave it on auto but I will select the vents (like defrost and floor) and set the temperature. If I need a lot of fan, I then manually set the fan to high because it NEVER goes to full blast otherwise, even on defrost.