P0756 & p0877

[24vchevy]

Hey need some help please, 2007 LBZ.

Limp mode, shift inhibited, rpm limited, stuck in one gear, has reverse if you coax it.

Have replaced the internal wiring hardness, the pressure switch assembly. Shift solenoid #2. Have tried another TCM. New filters oil level good.

Put a pressure gauge on it well over 250.

Pulled the valvbody, it all checked out, even had it tested by a transmission shop on a test stand.

All the solenoids click when tested with a scan tool.

I am at my wits end.

Thanks in advance

 

[Carston]

I’m having this exact same problem. I know it’s been a while but did you ever get this figured out?

 

[Ron Nielson]

Any codes?

 

[Carston]

Yeah I have the same two codes that he did. There were handfuls of memory codes but the only active ones are P0756 and P0877

 

[Ron Nielson]

See attachments

 

[Chevy1925]

I’m having this exact same problem. I know it’s been a while but did you ever get this figured out?

you need to give a run down as to what you have done to the truck, if you did anything to the trans prior and what initiated this issue

 

[Carston]

you need to give a run down as to what you have done to the truck, if you did anything to the trans prior and what initiated this issue

I bought it non-running with a bunch of issues. It quit for some reason and was used as a parts truck for a while. I’ve been throwing parts back at it and now it runs and drives but won’t shift. I replaced the nsbu switch in it and changed both transmission filters but it did not fix the problem. I know it is still able to go into every gear because it picks a different forward gear to be stuck in every time it drives. Reverse works occasionally with lots of persuasion. The transmission is actually in an international truck but the best place for allison transmission advice is a chevy forum. I did replace the ECU in the truck (which contains the tcm). I have wondered if the new computer is what is causing the problem but I have no way of knowing if this problem was happening with the old ECU too. Sometimes the transmission will not respond to the shifter at all. The PRND321 display on the dash displays the correct gear for about the first 10 seconds of messing with it. Afterwards, it displays the wrong gear or no gear at all.

 

[Chevy1925]

whats the make model and year? it is a allison 1000?

 

[Carston]

Make:International
Model:3200
Yes it’s an allison 1000

 

[Mike L.]

Never seen an Allison 1000 without its own separate TCM.

 

[Carston]

Never seen an Allison 1000 without its own separate TCM.

I could be wrong, but because it is in a ford/international system, I think the ECU is responsible for the transmission and powertrain. I will look into it and see if i can find any information on it having a TCM when I have some research time.

 

[mopar3]

I agree with Mike every truck I've seen with a Allison has a stand alone TCM. I've seen them mounted on the crossmember by the back of the transmission. Mounted under the hood on a Freightliner.

 

[Carston]

I crawled under there and it looks like you guys are right. Found a stand alone module connecting to the transmission. So the ECU theory is out. Any other ideas?

 

[mopar3]

Follow the diag steps in post 5. You need to test and inspect the wiring. You already replaced the pressure switch.