Hey Guys,
I have a problem with the Allison automatic in my 2003 GMC 2500HD. After it goes into 3rd it stays there. Sometimes it works, sometimes it don't. I've already had the shift solenoid changed and it's still giving me problems. The service guys gave me the following info and service codes:
SHIFT SOLENOID has LOW VOLTAGE or NO POWER
P700
PO562
PO841
PO871
PO872
Somebody please help me!

Need info on your truck.

What info would you need?

It has a 6.6 Duramax diesel engine, with 60,000 miles

When this happens, what is the shift indicator doing? Does it sometimes show the wrong gear or no gear at all? If so, it is a defective PRNDL switch (neutral safety switch) bolted to the side of the trans. The new design is brown, the old design is black and is suceptable to water intrusion which causes a brain cramp for the tcm. It can set a wide variety of codes. If this the problem, you need to take it to the dealer, this part is covered under the emissions warranty up to 80k miles. If they tell you that its not, tell them to look in the policy and procedure manual.

There is a post somewhere on this forum that says it is covered for 100K miles.

Thanks, GOODRNCH, for the info. I had the dealer replace the switch, and thought that was it. However, the transmission is still doing it's "thing", and not wanting to go into gear. I had to turn off and restart the engine for almost 15 minutes tonight before I could get it into drive. Any other ideas?

Check all grounds, possible ign switch not getting good power to TCM.. Possible PSM also.

I dont see where a shift sol could be at fault. Fluid level?

I will do some more investigation at work tommarrow and see if I can get any more info. Like Dmaxallitech said, we have had quite a few problems with the psm's that are bolted to the valve body, but not as much as the PRNDL switch. If yours was the old design, (black switch, new one is tan) it was a good thing to get the new one installed.