Trans has developed intermittent problem. Originally had developed the common 1-2 Hard Shift after extended highway drive. Could shut it down for 5 minutes and it would return to shifting fine around town until next highway drive. Never set a code.

I replaced the Valve Body with a Reman unit with Sonnax Kit.

New problem is intermittent. Sometimes will not downshift to 1st gear when making a complete stop. It will then start in 3rd, just barely creeping away from stop as you apply gas (you realize even as you sit there that it hasn't shifted to 1st).

Will shift to 2nd gear in Manual D2 (as it is supposed to) but in Manual D1 will also shift to 2nd gear (it is supposed to apply & hold 1st gear in D1). Starting in 2nd gear, it will immediately shift to 3rd gear at any speed (like 10 or 20) if the selector is moved to D3 or D4. It will then shift normally from 3rd to 4th.

That tells me 1-2 shift solenoid valve is operating normally. But I think 2-3 shift solenoid is either not functioning or the valve is sticking.

Trans shop has tested on 2 occasions and of course it has never failed while testing. Owner/tech says it works so well, if it had been a rebuilt trans coming out of his shop, he would have been pleased with the performance. So no other apparent issues. It showed no codes and MIL has never come on.

Since solenoids are not likely to fail intermittently, I doubt it is a solenoid problem. But shop that sold me the reman valve body refuses to believe there could be something wrong with it. They point to electrical (like a loose/dirty connector) or possibly a check ball hanging up in the plate but IMO that ignores the specific symptom that occurs (failure to downshift from 3rd while stopping).

Recently went 1 month with no glitch, hundreds of stops, then last Fri. happened for an extended time thru many successive stops and even after shutting off for 15 mins. First time long enough to actually get a sense of what it was doing. Usually, it happens once then "pops" into 1st after a few secs on and off gas and off you go. This time, had to put it in D2 at multiple stops to launch in 2nd gear. Then sat off for 90 mins and started in 1st from D4 at start up. But has happened stuck in 3rd several times since.

Besides a sticking 2-3 Valve, any idea what else could cause this? Can the 2-3 Solenoid be functional, but weak?

Welcome AF. No year given here, but a full scan with a trans function scanner during a road test can confirm shift solenoid operation and shift points, you can also command the solenoids thru the scanner to eliminate an electrical problem causing this issue, I don't see how during the diagnostic process this was not done.

Thanks for the welcome. I believe this was done. Tech had a pretty sophisticated looking box that he hooked up and drove off with. He found no issues. He did this twice about 2 weeks apart. Says unless it happens while he has it hooked up, not much he can do. Prior to this past Fri., I could only kinda guess what it was doing, all I knew was that sometimes it would only creep from a stop. Sometimes it would "pop" and go after just getting off the gas, other times, I would shift down and up from D4 to D1 to R and back to D4 and by then it had popped into 1st. I could never quite tell what gear it was stuck in. Now I know it is 3rd and I can put it in D2 to get 2nd gear manually so at least I can get rolling.

It is a '97, PWM TCC solenoid control.

99.9% of the time, it has shifted flawlessly since I put the reman VB in. Been searching here, have seen at least one guy suggest the VB Sonnax repair is just a band-aid. That contradicts what GM has published. I don't doubt this trans has other wear but I don't think general wear is related to what is going on here.

There is obviously power to the Solenoids. And I'm pretty sure there is a good signal from the VCM to all Solenoids.

What causes the 2-3 Shift Valve to return to Off position when the 2-3 Solenoid is not energized? There is no spring like there is in the 1-2 Shift Valve bore.

If I understand the 2-3 Valve positions, when the Solenoid is OFF the Valve is fully retracted in the bore and the trans can be in 3rd or 4th gear. I assume that line pressure somehow pushes the 2-3 Valve back toward the Solenoid. And since it functions in 3rd & 4th, I assume there is no problem getting this Valve retracted when the Solenoid is OFF.

But when the Solenoid is energized, it is supposed to push this Valve away from the Solenoid. Is that correct?

That is where the problem exists. Does the Solenoid have to overcome the line pressure to push the Valve thru the Bore?

Assuming it does, if the Solenoid was weak, I can see that the Valve might get hung up. I just don't know if these Solenoids are prone to getting weak.

Let me know if I am misunderstanding the operation somehow and whether these Solenoids will get weak.

Any debris in the VB can cause these anamolies, did the trans do this before the reman VB, is the VB modified with the Sonnax sleeve for the TCC apply or was this done and the entire VB reman'ed? Usually the solenoids are on or off and what you feel maybe the trans in limp mode, which usually sets a trans code, a momentary loss of power to the trans can cause this,. The ign switch on 97 GM produced trucks were common failures with similar symptoms.

maxwedge, thanks for reply.

Ignition switch failure was suggested by the VB guy. The same circuit powered by the Ignition Switch thru the 10A Trans fuse also powers the PRND321 Shift Indicator Lamp thru a separate wire circuit from the fuse. No loss of Indicator Lamp when failure occurs.

Also, the 1-2 Shift Solenoid is powered from the same Ignition Switch circuit as the 2-3 Shift Solenoid. The 1-2 Shift Solenoid performs flawlessly, triggers 3-4 gear change in D4 even when coast downshift to 2nd & 1st fails.

So I have ruled out the Ignition Switch as source of failure.

I am very skeptical of debris being a problem. This is the only symptom I'm getting. If debris was an issue, why is it just hanging in 3rd gear? Never fails to shift up or down in any other situation. Only symptom occurs while coasting to a stop.

This problem did not occur until after Reman VB installed. See first post, basically Trans never was an issue except on rare long (2+ hours) highway drives. And on those occasions, the trans did not always exhibit violent/hard 1-2 shift after exiting the highway, but when it did, I would pull over, shut the engine off for a few mins. and then drive on with normal shifting.

The trans had just a light film of black on the pan when I dropped it. Fluid was not burnt. No signs of any significant deterioration.

The Reman unit was NOT my original, came complete with Solenoids and I sent my complete unit back as a core.

Have owned vehicle since new, so know the complete history.

The Sonnax TCC sleeve was installed, the unit was then pressure tested and solenoids were electrically tested. I assume solenoids were not new unless they were found defective in the test. Just presumably good used solenoids.

It was advertised as "completely remanufactured" with "tested solenoids" but I'm not sure what else was new except for the Sonnax TCC Regulator Valve.

I am reasonably certain that the 2-3 Solenoid is energized ON when this failure occurs. You can actually feel the "pop" when it does drop into 1st gear after being stuck in 3rd at the stop. I don't think it is logical to believe that the Solenoid is suddenly powered, I believe it is powered the whole time, and finally the Solenoid kicks the valve thru the bore.

Nobody has any comment on the possibility that a Solenoid could become weak? Or whether it is overcoming line pressure to move the Valve when the Solenoid is energized?

I don't see any Seals/O-rings on the Valve itself, it looks like there are O-rings on the actuator of the Solenoid, perhaps if they are damaged or missing, leakage past them might impair the Solenoid from pushing on the Valve causing an intermittent "stuck" valve?

I'm thinking I will just get a new Solenoid, drop the pan, pull the Valve & Shuttle out to see if there is any sign of sediment or unusual wear on it, put it back together with the new Solenoid and cross my fingers unless somebody has a better idea?

The solenoids are always powered as as you know, the ground side is pulse width modulated. I would give this scenerio to the VB reman source and insist on a replacement, why should you go through this " torture" if the issue is traceable to having started with the reman VB installation.

Just to close the loop on this. I decided to drop the pan, pull the VB back out. I then pulled out the Spacer Plate which meant pulling the Accumulator Cover off. I did not do this the first time, believed that Spacer Plate Gaskets and Spacer Plate Check Ball holes were okay.

On inspection, saw that some Spacer Plate holes were wallowed out. Purchased and installed new ball seats for the Spacer Plate. I would have just replaced the Spacer Plate but the trans parts shop was out of stock and suggested the hardened seat repair kit instead, told me lots of rebuilders used them.

Old Spacer Plate gaskets were hard/brittle. No telling if they contributed to the problem. I installed new TCC and Shift Solenoid Screens on the Spacer Plate, they are cheap so no point cleaning the old.

For good measure, I replaced both Shift Solenoids. I now understand that the Shift Solenoids don't actually push on the respective Valve Trains, they simply block off an orifice when energized and fluid pressure then moves the Valve. Likely my Solenoids were truly okay but probably because of the worn Spacer Plate Ball Seats and/or the hard/brittle gaskets, fluid pressure would intermittently not build high enough to move the 2-3 Shift Valve on coast to stop, preventing downshift from 3rd.

It has only been a few days since the repair, but seems fine now, downshifts on coast to stop every time so far. It was an intermittent problem, so will be awhile before I am supremely confident, but I think I am now good.

VB guy had originally suggested not replacing the Spacer Plate gaskets if they weren't torn on removal of VB. I had not noticed any tears, but because I didn't pull the Spacer Plate, I did not realize some of the Checkball Holes were peened. And as hard as the old gaskets were, they might not have resealed so good on reassembly either. For anybody contemplating the VB repair for the TCC Regulator problem (my original problem), I recommend that you take the extra step of repairing/replacing the Spacer Plate and putting new gaskets on while you are in there.

On reassembly, be sure to put the Accumulator Pin and Spring back up in the Piston (hold with Petro Jelly) and be sure the Case Checkball is also in place. I'm embarrassed to say that I had it all back together and then found the Pin in my Catch Container, had to disassemble all over again, and then almost didn't see that the Case Checkball had dropped to the floor. Be careful when reinstalling the Acc. Cover that your Spacer Plate Gaskets stay aligned (use Petro Jelly to hold them to the Plate, gaskets are marked, C to the Case, V to the Valve Body).

For a guy who had never even changed fluid before on an auto trans, I think I did okay diagnosing my problem and getting it put back together. I learned a lot about how this trans works in the process. I really appreciate the help and advice given here.

Agreed, well done on your delving into this, wouldn't let go huh, I am impressed.

thanks for closing the loop

1 year old thread!?, Lets let this one go.