I am having the same issue on my 99 2500 and my alternator isn't charging. The PCM controls the O/D shift solenoid and charging. Check the O/D relay in the underhood power distribution box and see if the relay is being powered up from the PCM. If not, bypass the relay temporarily and then see if O/D works. If it works, the relay isn't being energized from the PCM. The previous owner already changed both batteries, the alternator and the PCM and it's still not working. I'm trying to find out what reference sensor signals the PCM HAS to receive before it sends a signal to energize the O/D relay. I've asked on a few foruums and can't seem to get an answer. If the PCM isn't getting the proper sensor references, it won't send out a signal to the shift solenoid. So I can't condemn the PCM until I know it's getting all the required information.

 

have same problem just came from the shad tree found two wires out of the plug on the side of the trans . dont have a new plug in thank tjhis might be it ?

 

My 2000 2500 3/4 ton 4 wheel drive diesel won't shift into od either. But if I put my truck in neutral then back into gear it'll go into od. Checked battery terminals and they are ok. Any ideas?

 

Getting any trans codes??

 

No there's no codes. No check engine light. I've checked the fluids the battery terminals and the relay for the transmission. I have no clue what is causing this issue. I'm suspecting it's a sensor but I'm not really sure.

 

Check your shift linkage.....

Does the lite for your O/D off button work?

 

Yes it does. I'm completely puzzled by this. I spoke to the previous owner (my stepdad) who owned the truck for 10 years and he said a few years ago he had the sensors in the transmission replaced at a very reputible transmission shop he local. He said he also replaced a sensor that has to do with the brake.

 

I also checked the shift linkage and it seems good

 

Did the trans stop going into overdrive coincidentally at the same time as the electronics were replaced?

 

No they were replaced 5 years ago.