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Discussion Starter · #1 · (Edited)
Hello All, this is my first post here and would like to say thanks right off the bat for any help anyone provides

Vehicle is 2008 GMC sierra 2500HD durmax 6.6 turbo with Allisson 1000 6 speed transmission
So the vehicle is my dads but I am helping him and hes not as tech savy on the computer so I am posting this for him. To start off I will say my dad has worked on his own vehicles since he was a teenager so fairly competent but his not a licensed mechanic so no expert.

-From the start originally what happened was he blew a hole in the cooling line, he never had any codes and didn't think he drove around very long before noticing it leaking while stopped, had it towed home and ordered and replaced both lines. After he finished he filled it up and then upon starting and trying to drive it, he started receiving multiple codes he didn't write them all down and I wasn't there at the time but EDIT p0762 (Problem with shift solenoid C), but this code only once never came back and then other popped up 0766 Shift solenoid "D" stuck off and cant remember code number but one that was for Shift solenoid "B" stuck on, during this time it seemed to drive fine in 1,2 ,3 then going into fourth it would pop the multiple codes

-After he looked online alot of people pointed to that most likely dirt got in the system when he replaced the cooling lines and shift "D" solenoid was sticking or not working, so he dropped the pan pulled it apart , the body that the solenoids and valves are installed in (I'm not super technical on property name) , pulled it all apart piece by piece cleaned everything ensured all the valves were sliding it and out properly started checking all the solenoids had property resistance , applied voltage made sure they functioning and found that all seemed fine but the TCC solenoid it was not actuating and resistance was out of spec, so figured must be a problem with it and bought a new one, also tested the other solenoids with an airline while actuating them to confirm they were opening and closing against pressure, At this point he felt everything was free and clear he reassembled

- After he filled the system and started warming it up to check hot level while trying to shift to Drive or if it was still in park it would immediately start popping P0877 (Transmission fluid pressure sensor/switch "D" circuit low) and only this code and this would put the truck into inhibit/limp mode within a few seconds of being in Drive, if you clear the code in neutral and put it in reserve it seems to work fine but if you go to drive you hear no noise from transmission that it went into gear and 877 code pops something if you go to park and it sits for a few seconds it will pop that 877 code again, so basically anything but Neutral position or reverse pops 0877 code, at this point i started getting involved and felt like best option might be to take it to a shop who can put in on a tech 2 scanner and actually see some data versus just seeing the codes which we did (it was towed to shop as couldnt drive it) when they put it on their scanners they said nothing shows any abnormalities and all the data looked good even with the code present, the only thing they could recommend was to check the internal inlet filter to see if it was plugged.... unfortunately they were short staffed and too busy to do anymore checks on it so we had it towed back to my dads house to try inspecting that

- Once towed back to this house we again drained the system and dropped the pan, this time when he dropped it the internal filter fell down with the pan and we thought oh well maybe that is the problem and the filter seal was no good and no holding it properly, so we bought new filter and installed and felt nice and snug this time filled with new fluid to cold and began warming up and immediately we are still getting 0877 code we even warmed it all the way up to 147deg F and checking level and we were just a low of the Hot center but in the hash marks. which I figured was pretty close since we were still a little off ideal temp of 160deg F, what I did notice physically is that from neutral to reverse it seems smooth and we dont see any code, if put in park it shifts to park smooth but if left it park for more than few seconds gives p0877 code also if you shift out of park back into reverse it seems very rough and hard on the transmission, if you shift to Drive from neutral it pops the code after a few seconds but even before that you cant hear or feel the transmission do anything as if its not even trying to shift into Drive... every time you clear the code while in neutral you can hear a solenoid or two activating

-At this point the only thing we are thinking to try is pull the pan again and go back and double check the PSM sensors I believe there is 4 of them on that module, even though electronically they are showing fine I'm wondering if they are not changing state when seeing pressure? (When he had this all apart he did look at the PSM but didn't think he seen anything alarming on sensor body and appeared to look normal) I thought i read on a forum somewhere should be able to use a pencil and lightly press each one to see if it changes state simulating it seeing pressure. We will also check all the valves again but since he has that that module apart twice and each time ensured all valves are sliding in and out easy seems unlikely something is sticking? He only used rags and I think brake cleaner never any emery cloth or said paper and all valves orings looked good and felt they had a good tight seal so I wouldn't think we are leaking by any solenoid valves but at this point I'm open to any suggestions.

Could internal pump not be putting out enough pressure?
Does it not require the same pressure for Park and Drive as it does for Reverse? Or why wouldn't reverse also through the 0877 code?
Or we off basis and should be looking at something else?

Any help would be much appreciated and welcomed
 

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Discussion Starter · #3 ·
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Thanks Ron, I had seen that before but took a better look this time, I'm surprised it says it should set when truck is in park, neutral or forward, yet in neutral its fine..... if I'm understanding it correctly if PS4 remains on or in closed position ( I'm assuming meaning it's seeing pressure) then it trips this, if that is accurate and that sensor really is seeing pressure and not faulty then whatever feeds that sensor must be leaking by

Gives us acouple things to specifically look at, unfortunately we don't have any of those break out boxes or connectors to meter anything while running, I did break down and tries to find a cheap scanner that can view this information to assist troubleshooting but we will see how good the units I bought are it will arrive tomorrow
 

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Have you replaced the fluid pressure switch.?
 

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Discussion Starter · #5 ·
Hey guys just to bring you an update.

To ottowood no we didn't replace the pressure switch but did pull it out and tested all 4 sensors and the tested fine, although reverse seemed to be a little morr stiff in pushing on it then the other 3 but I noticed that sensor membrane does look bigger and slightly different than the other 3......

What we ended up finding was "C" valve was actually stuck/ hung up inside the valve body and not reseating all the way back inside, can't positively say if when he put it back together last time he double checked that or if it happened on its own sliding back to far and binding... after we pulled it out and cleaned and put it back In we tested it a dozen times and every time it moved fine....

Put everything back together and now qe are back to his original problem he had the first time after replacing the cooling lines.... transmisions shifts fine 1 thru 3 but won't go into 4th , if you are driving in automatic after and as it tries to go into 4th it pops code p776 (solenoid B stuck off) ... if you start out in manual and shift up to 4th it pops p0734 (something about incorrect gear ratio)

A few forums people said clutch but also people found lots of shavings in pan when they dropped it, which he did not the very first time he dropped it, and this was the same thing happening right after he originally replaced cooling lines but I believe was different codes

Not sure where to go next replace B solenoid? When we had it apart we double checked all valves and everything appeared to be moving free after we found problem on C
 

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Those pressure switches are known to fail and cause problems.. I would replace it.
 

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Discussion Starter · #7 ·
We did order a new pressure switch the other day which will be here Monday or Tuesday so we can always try it. I also ordered a Xtool scanner that should be here today hoping to see if it would show us some more diagnostics while actually driving.

Otherwise we were going to try and bring it back to the shop tomorrow to have them put in on scanner and see if anything is unusual during the 3 to 4th shift.
 
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