If the manual valve is being correctly slid into its ‘Reverse’ position by the rooster comb then hydraulic pressure is being sent directly to the Reverse Clutch and the Low & Reverse Clutch to provide drive in reverse.

https://i260.photobucket.com/albums/...psgi6fyi2l.jpg

The only thing that can stand in the way of this is the Reverse Lockout Valve, which can be activated if the controller senses that the driver is trying to select Reverse while the vehicle is still moving forwards.

This is achieved by using the torque converter lock-up clutch solenoid (normally de-energised in Reverse) to activate the Lockout Valve to prevent hydraulic pressure being sent to the two clutches.

If the electrics are disconnected at the 20-pin connector then the TCC solenoid cannot be energised. Similarly, the three shift solenoids (1-2, 2-3 & 4-5) are not energised in Reverse. Hence, this can’t be an electrical or electronic issue.

Phil

P.S. To be able to start the engine with the 20-pin plug disconnected, a piece of bent wire would need to be fashioned that connected together pin sockets 2, 4, 11 & 16.

Thank you very much Phil for your response.

As I said I have tried unplugging the connector and car is not going anywhere in reverse. Also engine is not turning freely, it is being loaded similarly as if I was appliing throttle while standing on brake. Also I am certain that manual valve is working as intended.


Is there forward clutch that can possibly be seized causing the car to move in neutral and preventing it to reverse but at the same time not preventing transmission to work correctly in forward drive?

It is not possible to move car by pushing it when reverse is engaged and engine is running.

My thoughts, exactly. A seized forward clutch.

Phil

Hello, yesterday I have removed and disassembled the transmission. Diagnosis was 100% correct. Forward clutch was completelly burned. All of the material was gone. Friction discs have welded toghether causing transmission to lock up in reverse an drive forward in neutral. None of other friction discs are burned and are worn by 0.05mm or less.

Now I am wondering what could have caused forward clutch to burn up and friction weld itself. I really dont want just to replace forward clutch discs and have this happen again.

Any ideas on what should I verify before assembling the transmission regarding forward clutch?

Only other thing that I found wrong with the transmission were broken accumulator springs.
Thank you guys very much.

Have you been doing heavy towing work with the car? If it’s the first gear clutch that had seized it could be just heavy use or age.

Could be a few oil changes were missed Or just general bad luck and natural wear. The GM transmissions were always weaker than the ZF units, it’s one of the reasons they were replaced on later models.

Personally I’d check all the other clutch packs, friction and plates, and replace as nessicary.

Or if you have the money I’d get a full rebuild kit and change all the friction plates and all seals while your there. If you have the transmission apart then why not do a full rebuild and you know you will have a fully restored transmission in the car.

I’d empty and clean out as many oil galleries and the tourque converter as possible to remove any risk of material from The siezed clutch pack getting back into your newly built transmission!

Do the GM boxes have a cooler and thermostat? Some ZF boxes have a thermostat that can sometimes break and never let the trans come up to temp or worse not allow the trans to cool!

I’d check with one of the GM guys if there is a internal thermostat as you could ensure it’s working correctly as maybe the transmission overheated and burnt it’s way through the disks?

I have bought the car with damaged transmission. He told me that he drove it normally and all of the sudden he had no reverse gear.

I have checked other all other clutch packs and they dont need to be replaced. Wear is minimal.
Only the forward clutch is completelly obliterated. Maybe somebody neutral dropped it or whatever.

I will do my best trying to figure why it got burned, but I am not seeing any other damage, pressure was good etc.

Is

How many miles does the car have? Maybe the oil change was just missed or litterly never changed. Once ATF fluid gets contaminated or overheated it looses a lot of it’s required properties and 1,2 and may 3rd gear take the highest forces normally so it would make sense that 1st would be the first to burn itself up.

I would just ensure that the plates can move easily in the clutch pack assembly, check for any cracks, deformation or burrs. Anything that could mechanically stop or restrict the clutches from releasing. Other than that I’d ensure the mecatronics unit was clear of debris paying close attention to the 1/2 gallerys.

Also you mentioned that you couldn’t hear the solenoids all working? I’d ensure they are all functioning correctly before refitting? If that’s possible!

I would rebuild the pump for the trans. The regulator valve could be worn and is not maintaining the pressure needed to prevent the clutch pack from burning out.

Here's a video from one of my favorite Youtuber explaining this problem.

https://www.youtube.com/watch?v=_QVpTPrDKrg

You may find this useful. It’s the repair manual for the GM 5L40-e

https://at-manuals.com/manuals/gm-5l40e-5l50e/