Odd transmission quirk

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Post  rmk57 on August 8th 2019, 12:11 pm

C6 trans with Transgo shift kit and I installed an R-code servo awhile back. While driving at a light cruise, maybe 20 mph the transmission would start to brake and slow the car down to a complete stop, then release.
My foot isnt on the brake pedal and linelock works as it should. It did it twice when the trans was cold and never happened again. It wouldnt lock the transmission and skid the wheels, but more like a light brake pedal application. Any ideas?

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Post  BBFTorino on August 8th 2019, 4:29 pm

I dont have the answer as to the fix, but my C6 did the same thing a long lime ago, except it locked the wheels! I did not have time to diagnose it myself, so I took it to a a friend's trans shop. He said it was a problem in the valvebody where it was applying reverse while moving forward.
Scary event! Good thing I was not racing at the time.

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Post  Gregaust on August 9th 2019, 5:09 am

Interesting , So trans was in 3rd gear when it faulted ?
Shifter is adjusted spot on ? Reason I ask is there is a position on a C6 on a 2-3 downshift that can cause both gears to be applied and bind

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Post  rmk57 on August 9th 2019, 10:35 am

The shifter is a B&M Pro stick. The shifter left in D was shifting very erratic so I discovered the modulator line had popped off. When the problem first occurred I was using the shifter manually trying to figure out what the heck was wrong with the trans. After hooking the line back up it was back to normal but that doesnt account for the trans slowly locking up would it? My first thought was the cable had slipped or backed off but everything is tight and secure. The only issue I've had with the shifter is the trigger /lever hole worn oblong and unable to upshift into 3rd and the park detent is a tad off being a universal shifter.

Randy

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Post  Gregaust on August 10th 2019, 1:46 am

Modulator line off will only affect the shifts plus trans will default to full line pressure.
For the trans to lock up has to be a possible internal leak in the valvebody

Was the R servo a Sonnax ? If so di you machine it at all? I've found every one I have fitted needed to outer piston groove machined. Could be possible that is hanging on ??

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Post  rmk57 on August 10th 2019, 12:27 pm

The servo is a Superior brand and I didn't have to machine anything. I installed it and used low air pressure to check if it's working the way it should. As far as the valve body goes, it could be something internal. I put the shift kit in 2-3 years ago along with the servo and it's been working fine until last week. I'm going to be pulling the engine/ trans soon and see if I can see anything obvious. Is there anyway to bench test a a valvebody?

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Post  pmrphil on August 10th 2019, 1:43 pm

The inability to upshift into 3rd leads me to believe the shifter may be at fault - leaving it in "D" might just not be all the way into the detent. Try putting it into "D" (from low) (with the engine off) and see if the cable pin is free, or if there is some bias in the lever.

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Post  rmk57 on August 10th 2019, 2:58 pm

The cable pin is free in 1, 2 , D, N, R. The only real issue is park. I think the detent in a gm trans from reverse to park has a longer throw than a Ford and from what Iv'e heard you can mess up the cable or the valvebody by forcing it into the park shifter detent. I put in a stop bolt to prevent over extending the cable. I'm going to try road testing and see how it goes.

Randy

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Post  Gregaust on August 11th 2019, 1:02 am

rmk57 wrote:The servo is a Superior brand and I didn't have to machine anything. I installed it and used low air pressure to check if it's working the way it should. As far as the valve body goes, it could be something internal. I put the shift kit in 2-3 years ago along with the servo and it's been working fine until last week. I'm going to be pulling the engine/ trans soon and see if I can see anything obvious. Is there anyway to bench test a a valvebody?

Ok got that covered .. As for valvebody , a strip clean and ensure all valves are free and fall under their own weight in their bores

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