thor68372rockon

Hey mrvedit, Sorry about that 1st post, was up 2 days straight. I have both gold shims from transGo HD2 kit in the 2-4 servo & the pin is still too short. I guess I will have to weld an 1/8th or so onto the tip of the pin, good time to try out my new Miller MaxStar 140 Welder. Do you know what would throw the check engine code P0751? Says it's the "A" solenoid stuck off. They are brand new. If not failing, What is telling "A" solenoid to stay off? Evidently PCM is telling Trans. to shift to OD, because it shifts in & out, but "A" Sol. is not staying on. Will the pin not being long enough to engage the Reverse Drum Band cause the "A" solenoid to power on & off? Any thoughts or experience with this? Thanks.

mrvedit

The " P0751 shift solenoid "A" performance" code means the PCM does not detect the correct RPM/Speed ratio after the commanded shift. Sure, it could be a bad/stuck/clogged solenoid, but I suspect a slipping band would give the same code.

I assume you replaced the band. Strange that the pin appears to be too short. Hopefully the band is properly anchored. The anchor pin is under the VB.
Kinda late now, but I check the band operation and servo end play before I installed the pump, partly to be sure the band is properly anchored and the pin properly engages into it.

thor68372rockon

Yes, replaced with the stock width Alto Red Eagle Band. yes, checked the play between the band & reverse drum after installation, stroked it with air pressure 5-6 times to make sure it was seated, then moved both ways with screwdriver to make sure it was free moving & not tight on drum, air checked all clutches to make sure they were seated good before taking measurement for friction clearance. I wonder if that is why the code set is because the band is still too loose on the drum & it slipped? I guess I might have to weld a little on the ole pin after all. What do you think about that theory of the code being set because of the band being a little loose, have you seen this happen before? I am a perfectionist, That is why this is driving me crazy tryin' to figure out why it is not performing correctly. This is the only trans. i've ever had this much trouble with, LOL! That d@mn servo. I had the pan off 3 times just to check valve body for smooth valve movement, used Bench Buddy on it & they floated very smooth in the bores. Even took down the boost valve to make sure i ground the land flat on the valve for the updated valve. Then remembered i used the one out of the kit which was already updated, sonnax or TransGo kit. But now i know it is not stuck in there, it's moving fine. Well tell me what ya think about the theory of the band slipping & the code set. If yes, i'll pull that little ba@tard again & pull the shim washers & add that much weld. Then add another 1/8 & install it to check where i'm at, & adjust from there. Appreciate your advice & patience, Thanks Again, Donny.

P.S. Could the VSS be dirty? You said the PCM does not see the correct speed, that made me think of this.

bbond105

The PCM does not see the correct speed because it is slipping, I’m sure the VSS is good or you would be having problems with the other gears also.

Yes your theory of the band slipping and setting the code is correct. There is something not right with your 2-4 servos. Going off the measurements that you are giving there is too much clearance. As stated before you need to be somewhere around .075"to .125" of servo travel. I would shoot for the low side of this number.

At this point I don’t see were you have anything to lose by welding the pin and adjust to reduce the servo travel to .075. The worse that can happen you may have to get another pin.

bbond105

Some very good info here.

http://www.trutechtrans.com/Topic-1-00-01.html

thor68372rockon

Thanks bbond105, Will procede then. Also thanks for the other info.,will have to read. Let you know how this all goes when done. Donny

GMCSIERRA5.3

another thing to check would be the 3rd acummalator check ball..sometimes during rebuilds it can fall out or some people like to put a plug on there..and that will cause 2nd and 4th gear to not function

mrvedit

As Mr Bond mentioned, the VSS is fine and the PCM is reading the actual speed of the car.
What I was try to say is that after the PCM commands the 3->4 shift, it expects to see more MPH/RPM (or RPM drop at the same MPH). If not it throws the P0751 code because the trans still seems to be in 3rd gear. So Yes, a slipping band can throw that code.

ferrellrooster

I read that you can not tune, or the computor will not tune, to change shift points, if you install a pinless accumulator piston. Is this true? I dont know for sure so I bought a Billet Accumulator pistons for 1-2 and 3-4 that had pin hole. It was longer and has the 2 rings on it. I only have a Diablo I-1000 and was able to change shift points. And I didnt have to do any boring when I installed them. Why/what did you have to bore/machine? I have a 4L70E and it fit perfectly. Thanks

bbond105

The Sonnax Pinless Accumulator Piston will not work with the TransGo HD2 Kit because of the way the TransGo kit instructions have you installing the springs and piston. The Pinless Piston will bottom out and that keeps the springs from doing their job. This is only in the 1-2 accumulator position. If the spring and piston are installed in the stock order the 1-2 accumulator work but not the way TransGo intended it to. For a stock rebuild without a TransGo HD2 Kit the Pinless Piston will work fine.
As for tuning shift points the PMC does not know or care what kind of piston you are using. So I would say that is false.

bbond105

Any updates thor68372rockon?

mrvedit

All exactly correct.
There is no way the type of accumulator piston (or anything in the trans) will affect shift points as that is completely controlled by software in the PCM and adjusted by the "tune".
I suspect whoever posted the incorrect info (somewhere on the net) didn't know that WOT shifts do not occur until -both- the tuned MPH -and- RPM are reached. Some people adjust only the MPH table or only the RPM table, which results in no change; they might then blame the no-change on something else they did.

thor68372rockon

Hey bbond105, Sorry for the long wait. Been trying different things with the Transmission. Welded the 2-4 pin, put it in, seemed fine, it was too tight, burned band & drum. So I got a used drum & put all my goodies on it. Put new band in it, ordered new 2-4 pin, new teflon seals, put 2-4 back to stock & Put Trans. back in & still no 4th. Took it down to a Trans. guy who has a Snap-On Solace. We drove it around & it showed D3 while in D4. Also showed the HI LO HI readings but I don't remember them & he did not have the right cable to connect to a printer to give me a print out. Any way, I changed the PCM, the TPS (Dorman Repair Kit), the ignition Switch, Adjusted the Shifter Cable by the Transmission. Still no Joy. So we are going to hook it up to a shift box & command the shifts manually at a cost of $85.00, OUCH!! If he has no answers for me after that I think I am going to pull the trans. again & check the 4 Teflons on the Front Pump & make sure I did not knick one of them. While I'm in there might add that Forward Housing Update Kit, to make it so it doesn't explode. LOL! Anyways, that's where I'm at right now. Will keep ya informed as things progress. Thanks Again Guys.

mrvedit

Wow, sorry to hear of your troubles. Everything you have done sounds "logical"; too bad you got the servo pin too long.
If a scanner shows that the PCM never commands 4th gear, I wouldn't pull the trans, but instead figure out why that is.
I don't remember the details, but there are various codes, both trans and engine, and other conditions which will prevent the trans from shifting into 4th. I'm assuring the VSS is good and your speedometer reading correctly and steady. You might try replacing the VSS anyway.
If anyone knows the reasons that prevent the PCM from commanding 4th gear, please post them here.

thor68372rockon

Hey mrvedit, Before I say anything else, I installed the sonnax 4th Super Hold wrong. I read & re-read the directions & still installed it wrong. I did not install the small gold washer over the 4th piston shaft to emulate the stock 4th apply piston raised area. I thought the washer was supposed to be cut in half & used as a gauge for the apply pin rounded end shaping. I cut another washer instead though & did not ruin the one supplied with the sonnax. I am going to install it correctly this time & see if it has any effect. OK, now here is what I was going to tell ya but I had to get that out of the way first so we are all on the same page. The noid lights I jumpered to the wiring coming out of the PCM wiring with back probe pins showed (or, Lit Up) when 4th was supposed to happen. so that tells me that it was commanded but somewhere in the valve body or 2-4 servo, was not letting it happen?? As you said though, some codes will stop a 4th shift from happening. I seen the list, I have "AllData" & it shows the codes & what has to happen for them to set & if it sets what actions the PCM takes, & one is to not let 4th happen. P0751 was the code that kept setting (1 to 2 times in a test drive) & I kept thinking it was the solenoid or the 1-2 shift circuit. Now I think it set because of the Band slipping, as it will set if the PCM senses a slip. By the way I got the sonnax info from a "Clinebarger" here on "LS1TECH". Glad i stumbled on to it or I would have never thought to even check that. The directions by Sonnax are good, but not all that clear as to the assembly, I think they could use some improvement. If they wrote them like Transgo writes theirs it would leave no room for error, even a moron could do it, like me. LOL!! Thanks again for help & I will keep you informed of my progress.

mrvedit

Sounds like you are making progress in at least understanding the problem.
If you test lights indicate 4th is commanded, yet it does not shift, then it does indicate a problem in the trans regardless of any codes.
All the threads starting with "No 4th gear" came down to this:
1. Band is worn and servo has too much play
2. 4th piston installed wrong or with bad seals.
3. VB problem. There is a 3-4 shift valve and a combo of the 3-4 Relay and 4-3 Sequence valves. Make sure they are not stuck. In one or two threads the OP simply replace the VB to solve their problems after double checking the 4th piston and end play.

JWacker

OP, did you ever find your problem? I am going through the same thing. Its in an 04 tahoe 4x4 and I am trying to avoid welding on the pin and burning the band up like you did. I really dont want to pull this trans again. Amazing how much that transfer case complicates things.