P0751 & p0843

[duramax0001]

I would greatly appreciate some help with a transmission problem.

I am sporadically getting code P0751 while driving. The code is also thrown whenever the truck idles for more than 10 minutes. It is always accompanied by a limp mode of some kind. I am stuck in what feels like 4th.

P0843 will occasionally accompany the P0751.

What I have tried so far:
New fluid
New internal and external filters
New pressure switch manifold
New internal wiring harness (noticed fluid buildup in connector on back of transmission)

Stock 2009 GMC 4x4

 

[Mike L.]

Check wire #17 at TCM and follow it all the way to trans.

 

[duramax0001]

I would really appreciate some more suggestions.


I am still having the same problem. I have tried your suggestion of tracing the #17 wire on several occasions. I can't find any problems with the wire. I have also checked continuity on it with positive results. I did replace the TCM about a 18 months ago due to a loss of communication issue. It solved the problem, but this current issue started a couple months later. Could they be connected?


The issue is astoundingly consistent. I drive it to work everyday with zero issues, but if I ever let it idle for more than about 5 minutes, it throws the code and goes into limp. Also throws the code if driven long distances in excess of an hour.



Should I drop the valve body and clean it? Could flushing the fluid and replacing it help?


Thanks

 

[Mike L.]

Change pressure switch manifold and internal harness. Is harness leaking at the connector?

 

[duramax0001]

It was indeed leaking at the connector. I have changed the internal harness and replaced the pressure switch manifold. The leak is fixed. Strangely enough, the problem improved when I did those things, but did not go away. Before that, the code would throw far more often. Now it is just on long idles or extended trips.

 

[Mike L.]

Time to look at the valve body.

 

[duramax0001]

Thank you for the help. Replacing the valve body fixed my issue. Unfortunately I don't have the knowledge to properly diagnose the valve body, but swapping in a rebuilt one did the trick.

 

[duramax0001]

It makes me sick to say it, but the same problem came back. After swapping the valve body, it ran great for weeks. I even idled it for 30 min on two different occasions. I was sure it was fixed, but while towing about 10k it just happened again. P0751 and limp mode, when I came to a stop at a light.

I hate to keep asking, but is there anything I can try?

 

[Mike L.]

Shift valve must be sticking.

 

[duramax0001]

I don't understand. Wouldn't a new valve body along with new solenoids rule out any problem with the valve itself?

 

[Mike L.]

I don't understand. Wouldn't a new valve body along with new solenoids rule out any problem with the valve itself?

Not if it has debris in it.

 

[duramax0001]

OK. I have changed the fluid twice. Neither time showed any foreign material. Could it be something I cant pickup with the naked eye? Possibly clutch material? What else can I try?

Also, if it is debris wouldn't we expect to see valves affected indiscriminately? Both the old valve body and the new one are only showing failure on the same valve.

Thanks again for the help.

 

[duramax0001]

Are there any further steps I can take to get rid of any possible debris? Is this a sign of a larger problem?

 

[torqued2dmax]

Are there any further steps I can take to get rid of any possible debris? Is this a sign of a larger problem?

Pressure flush at a reputable allison shop !

 

[duramax0001]

Pressure flush at a reputable allison shop !

Ok, I will give that a try. Thanks