99 K3500 diesel shift problems and DTC P0751

Hi, my 99 k3500 dually with a 6.5 diesel started shifting harshly a while back and then corrected itself. 1-2 shift could either be normal or snap your neck. It finally set a dtc of p0751 and now it doesn't shift to 2nd at all. I replaced the 1-2 shift solenoid on the transmission and that did not correct the problem. What I do know is that when in drive I can manually shift to second and it will go. Under power in 2nd it will slip which indicates low fluid pressure or bad clutches. There was not enough debris in the bottom of the pan to indicate any clutch damage. The pan was clean. I believe the 1-2 shift solenoid is not being activated and that is why it slips.
I traced the shift solenoid circuit from the transmission to the pcm and there isn't any damaged wiring. The solenoids are powered by fuse 20 in the fuse box and if it is pulled I don't have pndrl or tach operation so I know the fuse and circuit is good. I also know power from fuse 20 is feeding the solenoids as 3-4 and od is still working and they get their power from the same wire as the 1-2 solenoid.
Has anyone had any issues with the pcm failing and not allowing the 1-2 upshift? My thoughts were to splice into the wire between the pcm and the transmission and ground it while driving in 1st to see if it does shift to second. I know the pcm grounds the circuit so I shouldn't damage anything if I do.
If there are any tranny guys out there please respond.
BTW, the truck has 97k miles on it with very little issues. Thanks for any response...

P0751 possible causes: Low trans fluid level, dirty fluid, bad 1-2 shift solenoid valve, 1-2 shift solenoid harness or connectors, 1-2 shift solenoid circuit open or shorted. A buddy told me the harness connector wiring gets brittle from age and heat and becomes problematic or intermittent. Also since the problem comes and goes you can try some Lucas transmission treatment. He's said theres been some success with that cleaning up the valve body and solenoids. HTH

with this code both A/B solenoids are replaced/ they work together . the transmission if slipping will delay rpm change which will throw this code. clutch materal at the pan bottom would indicate if the cluches are wearing and slipping.

how long you owned this ? if for 12 years how often do you replace the fluid and filter ?

if you never changed the fluid / filter wear issues and contaminated components are very possible.

when you last serviced this on your valve replacement what fluid did you use ? dexron III is not a GM approved fluid any more. [2006]

I bought this truck new. It got a fluid change at 50k miles and was due another shortly. It has 97k miles on it right now. Although it did pull a 5th wheel travel trailer early in life its been relegated to pulling a 19' bass boat for the last 7-8 years. It also gets about 5k miles per year put on it. It sets quite a bit.
There wasn't any clutch material in the bottom of the pan. It had the normal black/grey film that is always in pans when the fluid is changed. I traced wiring back to the pcm. It is in the C3 connector and is pin 4. It has voltage at the connector so I know the shift solenoid is powered. I tried to drive the truck and ground the wire to engage the solenoid and it made no difference. The truck shifts from 1st to 3rd with no slipping. I'm now thinking there is a possible stuck valve. I am not a fan of miracle in a can but I may try the Trans go product. I am not convinced this has any major internal problems. The fluid was fairly clean and not burned when I dropped the pan to change the shift valve.
This valve looks like it just closes a bypass passage to apply fluid pressure to the clutch pack. I checked the old valve that I replaced and it does seem to work properly so I doubt it was a valve problem. I hate to keep dropping the pan trying different things but I also am not looking forward to dropping the transmission out and replacing/rebuilding it.

I bought this truck new. It got a fluid change at 50k miles and was due another shortly. It has 97k miles on it right now. Although it did pull a 5th wheel travel trailer early in life its been relegated to pulling a 19' bass boat for the last 7-8 years. It also gets about 5k miles per year put on it. It sets quite a bit.
There wasn't any clutch material in the bottom of the pan. It had the normal black/grey film that is always in pans when the fluid is changed. I traced wiring back to the pcm. It is in the C3 connector and is pin 4. It has voltage at the connector so I know the shift solenoid is powered. I tried to drive the truck and ground the wire to engage the solenoid and it made no difference. The truck shifts from 1st to 3rd with no slipping. I'm now thinking there is a possible stuck valve. I am not a fan of miracle in a can but I may try the Trans go product. I am not convinced this has any major internal problems. The fluid was fairly clean and not burned when I dropped the pan to change the shift valve.
This valve looks like it just closes a bypass passage to apply fluid pressure to the clutch pack. I checked the old valve that I replaced and it does seem to work properly so I doubt it was a valve problem. I hate to keep dropping the pan trying different things but I also am not looking forward to dropping the transmission out and replacing/rebuilding it.


well like I stated before the a & b valves are the same and they work together. they cost 25.oo ea. replacing both is what is done .

valves may look good the resistance of the coils must be measured should be around 2o ohms. buy another valve and compare the old valve resistance to the new ones.

sounds like electrical issue since you have done transmission servicing at a fair interval. I say this because towing needs more frequent fluid servicing especially if you used dexron III. I have found this fluid is turning to acid after 25,ooo mi of use on our trucks . towing just adds to it ..

all it takes is a slight delay and this code is thrown.

I dropped the pan a second time and checked the shift solenoids again. Both were in tolerance but I changed them anyway. I also checked the valves in the valve body behind the solenoid and both moved freely so I doubt they were the problem.
I broke the wire at the pcm and used a momentary switch to open the circuit while driving. Breaking the circuit while in 4th or overdrive would cause a downshift to third. Completing the circuit would cause an upshift so I am sure the solenoids are working properly. I still don't have second unless I physically pull the lever into second. If I take off with the lever in 1st and then shift to second it will go. Once in second if I shift it into third it will over rev and then go into third.
I dumped a bottle of Lucas into the transmission to see if there will be any improvement. I am not confident it will help. If not I see a replacement in the near future. I'm not looking forward to pulling and replacing that beast.

I dropped the pan a second time and checked the shift solenoids again. Both were in tolerance but I changed them anyway. I also checked the valves in the valve body behind the solenoid and both moved freely so I doubt they were the problem.
I broke the wire at the pcm and used a momentary switch to open the circuit while driving. Breaking the circuit while in 4th or overdrive would cause a downshift to third. Completing the circuit would cause an upshift so I am sure the solenoids are working properly. I still don't have second unless I physically pull the lever into second. If I take off with the lever in 1st and then shift to second it will go. Once in second if I shift it into third it will over rev and then go into third.
I dumped a bottle of Lucas into the transmission to see if there will be any improvement. I am not confident it will help. If not I see a replacement in the near future. I'm not looking forward to pulling and replacing that beast.

with your testing the solenoids must be not the problem. the next item would be the servo . then the most common failure would be the sunshell has cracked. very common .