guess who p0751

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denalilex

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ok. i guess i spoke too soon.
p0751 is going to be the death of me.
but first i want to know why ??

everything has been smooth sailing until last night
i’m back in limp mode & throwing p0751
when i ran my codes originally after all the camshaft sensor & other repairs i mentioned - i had p0751, p0776, p2723 …. i cleared them, drove again & nothing came up for about the last however long it’s been since i posted - 2 weeks-ish. i drive it daily. 30-40 miles a day - 17-20 each way to work & back.

last night on my way back , i was coming off from a stop at a red light & the dreaded delayed acceleration reared it’s stupid ass head & then wouldn’t shift out of 1st. i pulled over , did the tcm reset thing with the pedals & key method , pulled my gear shifter down to see if it worked - it did, i was able to shift into M but it was immediately in M3 so i knew i was in limp mode. i tried to start the truck & it wouldn’t turn over so i had to jump it…. i know that if any 2 sensors are disconnected (for any reason be it electrically or another ) - it will result in a no start fail safe. after jumping it it was fine. shifted fine but CEL was lit & so before i went anywhere i plugged in my scanner & it’s the p0700 & p0751. i assume the other codes were stored from before repairs. cleared it again & it came back. it didn’t do the shift issue again & needed a jump but i also didn’t trickle charge my battery after having it disconnected for 2 weeks … so it’s on the trickle now & i’ll reply if it starts as normal or if it’s refusing to on a full battery. not a battery issue. my battery is new.

fluid full & clean
trans runs at normal temp


can anyone please help me figure this out. it’s been like over a year at this point & im loosin it


thanks.

sincerely,
from the grave of the 6l80 problems i thought i conquered.


*EDIT* fwiw - the shift solenoid stuck off as well as the pressure control solenoid stuck off codes i was getting on literally all of them have cleared & not returned in any fashion even be it as a surprise f u to me. (except this one) & i ran my theory about the cam sensor & cam mag & faulty harness by the guy who owns the transmission shop up the street & lets me use the lift after hours (so really he’s something more of an angel in human form) & if that was plausible or i’m crazy & my truck is just a bottomless pit of broken everything .. he also agreed & said that was a hell of a find to link together - so i didn’t post here in ignorance when i shared that fix - for any one who may be experiencing similar issues … just didn’t want to make it seem like that post was full of incorrect info that could potentially steer someone down a misguided path.*
 
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Geotrash

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ok. i guess i spoke too soon.
p0751 is going to be the death of me.
but first i want to know why ??

everything has been smooth sailing until last night
i’m back in limp mode & throwing p0751
when i ran my codes originally after all the camshaft sensor & other repairs i mentioned - i had p0751, p0776, p2723 …. i cleared them, drove again & nothing came up for about the last however long it’s been since i posted - 2 weeks-ish. i drive it daily. 30-40 miles a day - 17-20 each way to work & back.

last night on my way back , i was coming off from a stop at a red light & the dreaded delayed acceleration reared it’s stupid ass head & then wouldn’t shift out of 1st. i pulled over , did the tcm reset thing with the pedals & key method , pulled my gear shifter down to see if it worked - it did, i was able to shift into M but it was immediately in M3 so i knew i was in limp mode. i tried to start the truck & it wouldn’t turn over so i had to jump it…. i know that if any 2 sensors are disconnected (for any reason be it electrically or another ) - it will result in a no start fail safe. after jumping it it was fine. shifted fine but CEL was lit & so before i went anywhere i plugged in my scanner & it’s the p0700 & p0751. i assume the other codes were stored from before repairs. cleared it again & it came back. it didn’t do the shift issue again & needed a jump but i also didn’t trickle charge my battery after having it disconnected for 2 weeks … so it’s on the trickle now & i’ll reply if it starts as normal or if it’s refusing to on a full battery. not a battery issue. my battery is new.

fluid full & clean
trans runs at normal temp


can anyone please help me figure this out. it’s been like over a year at this point & im loosin it


thanks.

sincerely,
from the grave of the 6l80 problems i thought i conquered.


*EDIT* fwiw - the shift solenoid stuck off as well as the pressure control solenoid stuck off codes i was getting on literally all of them have cleared & not returned in any fashion even be it as a surprise f u to me. (except this one) & i ran my theory about the cam sensor & cam mag & faulty harness by the guy who owns the transmission shop up the street & lets me use the lift after hours (so really he’s something more of an angel in human form) & if that was plausible or i’m crazy & my truck is just a bottomless pit of broken everything .. he also agreed & said that was a hell of a find to link together - so i didn’t post here in ignorance when i shared that fix - for any one who may be experiencing similar issues … just didn’t want to make it seem like that post was full of incorrect info that could potentially steer someone down a misguided path.*
This makes no sense to me. I don’t understand why if the truck is in failsafe somehow that a jump will get it going when it’s own battery wouldn’t.

I’ve had brand new batteries fail within weeks, so either a weak battery (dead cell) or battery connectivity issue would be my first steps. Basics first, always. And bad battery cables on these can cause all kinds of erratic behavior.

I had to go back and find your previous thread to get the context because there are so many threads on here it’s impossible for me to track each person’s diagnosis history by memory. So now you’re getting the transmission codes but not the cam actuator codes, right? Anything from the freeze frame data?
 

NickTransmissions

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ok. i guess i spoke too soon.
p0751 is going to be the death of me.
but first i want to know why ??
Valve body - likely worn clutch select valves and/or valve springs.

Generally speaking, worn valve trains will often trigger solenoid codes in the 6L family of transmissions to set but will persist or re-set after solenoid replacement. Limp mode may result as well if there's a break in the comm link between the solenoid(s) and ECM.

You can remove the valve body assembly, install hydraulic correction kits or replace with an OEM valve body from GM.

If you overhaul yours, you'll need to do a fast adaptation using a two-way scan tool that can do it (most snap on scanners, TECH2, etc)

Other causes:

Clogged filter, stuck check balls/worn check balls, worn clutch boost or regulator valves (valve body)

What year is your vehicle?
 
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denalilex

denalilex

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This makes no sense to me. I don’t understand why if the truck is in failsafe somehow that a jump will get it going when it’s own battery wouldn’t.

I’ve had brand new batteries fail within weeks, so either a weak battery (dead cell) or battery connectivity issue would be my first steps. Basics first, always. And bad battery cables on these can cause all kinds of erratic behavior.

I had to go back and find your previous thread to get the context because there are so many threads on here it’s impossible for me to track each person’s diagnosis history by memory. So now you’re getting the transmission codes but not the cam actuator codes, right? Anything from the freeze frame data?
crap it’s long again sorry


hey thanks for doing the leg work & getting back to me - sorry i’m so bad at this.

i’ve got some freeze frame on my phone from the codes - i’ll post them here to see if you can see something that i haven’t learned to dicifer (idk how to spell that lol )

i figured out my battery issue - it was actually the wiring harness STILL but it was the PD side. apparently a gorilla was under my truck with a hammer & pair of snipers & snapped off all the tabs to my electrical connectors. so where i fixed the ones from the cam & cam act wiring harness with the new bracket - i found the PD side kept sliding out & then realized the tab was also missing there. i zip tied the two 5 pin male & female together & it’s completely alleviated the power issue.


the P0751 - i think i’m figuring it out. i have replaced the checkballs , spacer plate, filter, gasket , fluid, lucas trans fix, like 3-4 months ago now? the last post i mentioned doing it. everything torqued exactly to spec.

my amatuerness learned after the fact that you need to - with these 6Ls especially - measure EXACTLY how much fluid you take out & replace with EXACTLY the same - i’m learning it’s exact - down the the very teaspoon. literally. i’ve had it parked & only taking it on test runs with a bottle of dex vi & when the p0751 pops i’ve been putting literally a teaspoon of fluid in. each time my shifts get noticeably smoother, & the code is taking a considerable amount of time longer to come up. yesterday it didn’t come up at all until i pulled into my driveway. i drive it about 40 miles on the highway to see if it would show up.

this being vastly different from everytime i came off from a stop.

the thing is - i check my fluid on exactly level ground - after the trans has been running, idling & is within 160-200 - mine usually operates hovering between 160-170, or lower when it’s colder out which i’ve read is normal? the line on the dipstick - to me - looked high, but taking some out was absolutely not the answer because then i wasn’t moving when in drive. so i put back 6/8ths of what i took out, still wasn’t moving. put back what i took out entirely & had delayed accel & have been teaspooning it since & this is the first time i’ve been seeing considerable improvement in the trans overall. it’s never shifted so smoothly - my 1-2 shift issue has completely disappeared altogether & i’ve literally never felt - or rather, not felt - my shifts on any shift.

the p0751 shows up & i know it when it does when i feel the slightest slightest slightest delay into first from braking very randomly & now only happened once at the end of the trip when i got back.

sorry for this long ass post. i’m trying to include the relevant info.

the fluid add has drastically changed - all but completely eradicated- my trans issues. i was thinking to get the valve bore spring kit from sonnax but do you think & i know it’s impossible to say for sure - that that’s still a wise choice given what i’ve said here?

oh & also yes so my caliper mounting bolt came off yesterday :’) while driving so that was a lot of fun. i guess most people don’t have to drive around with a hydraulic jack in the back but my paranoia was on my side yesterday lol. that was probably one of the scarier things i ever have experienced. this is the 2nd time it’s happened to me with bolts that have been torqued to spec & that i’ve gotten from my local GM parts dept of a dealership for a brake job. so they got loctited & my next brake job will probably be more difficult.

i’ll attach the freeze frame from yesterday
 
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denalilex

denalilex

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Valve body - likely worn clutch select valves and/or valve springs.

Generally speaking, worn valve trains will often trigger solenoid codes in the 6L family of transmissions to set but will persist or re-set after solenoid replacement. Limp mode may result as well if there's a break in the comm link between the solenoid(s) and ECM.

You can remove the valve body assembly, install hydraulic correction kits or replace with an OEM valve body from GM.

If you overhaul yours, you'll need to do a fast adaptation using a two-way scan tool that can do it (most snap on scanners, TECH2, etc)

Other causes:

Clogged filter, stuck check balls/worn check balls, worn clutch boost or regulator valves (valve body)

What year is your vehicle?
hi sorry for my late reply

thanks for getting back to me!!!

so if you ask anyone here you’ll notice i’ve become the 6l80 girl lol… i have been studying this trans like a madwoman.

i do know that the clutch select valve springs / bore plugs / of the like can be an issue here. i have been considering getting the sonnax clutch valve select spring kit .

i just semi overhauled the VB… new spacer plate , new checkballs , new fluid, filter, gasket, all bolts torqued to spec & some new ones that i snapped on the first go thx to HF click torque wrench :’).

i just posted above to @Geotrash some of what i’ve figured out ….

my fluid mystery is baffling me. but adding some is seeming to fix the issues. the p0751 comes up much longer in between test drives when i add a teaspoon - literally - of fluid. my shifts very noticeably improve.

i have an 07 yukon denali 6.2 v8…. gen 1 6L80… so the problem child.

i’m gonna post some freeze frame data i pulled yesterday. you will know better what they say than i do.

being as how the shifts are improving as they are with the fluid - i assume it’s probably a wise choice to go ahead with the sonnax clutch spring kit anyway? couldn’t hurt. that’s in the plans for next week. even with the improvements i think cause the trans is old as it is with 300k on it.

nothing concerning when i dropped the pan 4ish months ago. no chunks of metal. normal shavings on the magnet. fluid was like black but the trans had also not been serviced & i can’t say it ever was before i took ownership of the truck. i took over & replaced everything fluid wise & have been rebuilding it ever since. slowly.
 

NickTransmissions

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@denalilex - except from your post:

the P0751 - i think i’m figuring it out. i have replaced the checkballs , spacer plate, filter, gasket , fluid, lucas trans fix, like 3-4 months ago now? the last post i mentioned doing it. everything torqued exactly to spec.

Did you:

- install a valve body correction kit (Sonnax zip kit or transgo reprogramming kit?
- replace the clutch select valves and springs?

Not sure what your vehicle model year is but GM changed those clutch select valve springs as well as revised the spacer plate design in late 2008; the previous springs were wearing out and triggering p0751 codes...

Did you follow the torque sequence for both sets of bolts (top and bottom of the assembly) in addition to torquing all fasteners to the correct tension (inch lbs)?

EDIT: just saw your above reply - install the full zip kit. If you have the sonnax vacuum tester, test all clutch circuits (valve trains).

Then do the fast adapt relearn once you reinstall everything.
 
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denalilex

denalilex

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@denalilex - except from your post:

the P0751 - i think i’m figuring it out. i have replaced the checkballs , spacer plate, filter, gasket , fluid, lucas trans fix, like 3-4 months ago now? the last post i mentioned doing it. everything torqued exactly to spec.

Did you:

- install a valve body correction kit (Sonnax zip kit or transgo reprogramming kit?
- replace the clutch select valves and springs?

Not sure what your vehicle model year is but GM changed those clutch select valve springs as well as revised the spacer plate design in late 2008; the previous springs were wearing out and triggering p0751 codes...

Did you follow the torque sequence for both sets of bolts (top and bottom of the assembly) in addition to torquing all fasteners to the correct tension (inch lbs)?
yes sorry i was responding to you lol you beat me to it.

here’s the screenshot of the freeze frame

i was getting back to you with my year & model. i did not do the clutch select kit or springs. from your posts i’m grasping it would still be a wise choice. i have the spring kit in my cart on ebay
 

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NickTransmissions

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yes sorry i was responding to you lol you beat me to it.

here’s the screenshot of the freeze frame

i was getting back to you with my year & model. i did not do the clutch select kit or springs. from your posts i’m grasping it would still be a wise choice. i have the spring kit in my cart on ebay
Yep, do them and the compensator feed regulator valve kit from Sonnax (bumpy, jerky shifts are the result when that valve wears out).

If you ever end up overhauling it, swap a second design stator/pump cover in place of yours as the first design sucks - sealing rings wont seal even when new due to the design of the stator sealing ring grooves.

That's in addition to all the other updates needed on an early unit.

Lastly, replace your parking rod assembly if/when you work on the valve body again - the rivet on the end likes to fail and the "bullet" separates from the rod itself and you cant put the vehicle in park anymore. GM didnt fully update the design until 2018 so i always replace when overhauling these units.

You will need a special tool, Kent Moore DT-48550 (collet-style roll pin puller) or an equivalent tool.

IMG_7402.jpeg
IMG_7401.jpeg
 
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