Duramax Squarebody “shift range inhibit”

[Sutton]

I bought a perfectly running and driving 2001 duramax with an Allison and dropped it into my squarebody using a dmax swap.com engine and transmission standalone. The transmission will not engage in reverse, and will only engage in drive for a short few seconds before disengaging.

The transmission has great line pressure and NO CODES.

I have been diagnosing for over a month and have no luck. I have had two different NSBU switches on it. I also had a scanner hooked up to it to read ISS, turbine, and OSS to make sure they’re reading properly.

I have checked the main harness connector to verify there is no fluid in it and that it is cleaned out. I have ohm tested all the solenoids. I have checked for proper voltage and grounds where necessary.

I have tried everything I can think of and have read every forum known to man on the issue to try and figure it out.

What do I try next??

 

[Sutton]

If I force the transmission into limp mode it will stay in gear but still has no reverse.

 

[Sutton]

And the last thing, if I jack the rear tires off the ground and put it into drive, it will stay in drive unless I touch the brake, which will cause it to disengage back into neutral.

If I put it in drive with the tires off the ground and touch the gas pedal before ever pushing the brake it will stay in drive when I push the brake. If it stays in drive and I give it a decent amount of rpm it will never shift but it will throw a code of P0731

 

[2004LB7]

I'm no Allison expert but I'm going to guess one of the solenoids are not moving freely. Someone else here should know which one

 

[Sutton]

Thank you for the quick response. I did provide power to the A and B solenoid and was able hear them clicking, but that doesn’t necessarily mean they were moving through the full stroke.

 

[2004LB7]

Or that they where fully returning to their closed position when power was removed

 

[Sutton]

Exactly. I’ll look forward to everyone’s help. Thank you again for your input so far!

 

[drhoyt]

Is it 4wd, on mine with the np205 it would limp right away if it's in low range

 

[Sutton]

Yes, it is a 4wd. It also has an np205. It does the same thing regardless of if the t case is in high, low, or neutral, including not going into reverse at all.

 

[JoshH]

What are you using to check line pressure and codes? It sounds similar to what I've seen them do when they are low on fluid. You need to get a scan tool and monitor pressure switch status when shifting R-N-D and see if things in the valve body are working properly.

 

[Sutton]

I have a 300 psi mechanical guage in the pressure port next to the spin on filter. When it is in drive and stays in drive with the rear tires off the ground I see over 200 psi when given throttle. I’ve pulled codes with a CTS3 module as well as a a generic code reader off Amazon.

I will try the scan tool pressure switch reading and see what I can find.

 

[JoshH]

I have a 300 psi mechanical guage in the pressure port next to the spin on filter. When it is in drive and stays in drive with the rear tires off the ground I see over 200 psi when given throttle. I’ve pulled codes with a CTS3 module as well as a a generic code reader off Amazon.

I will try the scan tool pressure switch reading and see what I can find.

I know Edge won't pull TCM codes. It's likely a cheap code reader won't either.

 

[Sutton]

I will have a scanner tomorrow morning and will see what more that can tell me. I will update soon.

 

[Sutton]

I had no luck being able to see the pressure switch with the scanner that I had. What scanners are you guys using to monitor the pressure switch status?

 

[Nate’s Dmax]

It sounds similar to what I've seen them do when they are low on fluid.

This is exactly my thoughts that came to me within the first few lines i read of your question. Do you have the fluid up to the correct mark on dipstick with engine running. The engine must be running When checking level.

 

[Sutton]

Yes, the fluid level is dead on. With the truck running, I remove the dipstick, wipe it off, put it back in for a second or two, and pull it back out. It is at the perfect level every time.