C4 Issues

[carprojects 10]

I posted this on network 54. Reposting here....

 

It was suggested to check the modulator. I have checked the vacuum to the modulator. It's good all the way to the manifold. It was also suggested to check the function of the modulator, not sure how to do that.

 

I rebuilt my c4 with the badshoe video and all went well. I replaced the gaskets and seals. It ran and shifted great for a long time. Recently the shifting behavior seems to be very sensitive to temperature. When the engine is cold, the shift between 1 and 2 and hard and happens at high rpm. It takes a couple of miles before it will shift into 3rd gear (seems like when it warms up a bit). When the engine is up to temperature, the shift behavior changes to be very smooth and quite early. The shift between 1 and 2 happens early, sometimes early in acceleration. The shift between 2 and 3 happens smooth but also can be what seems premature. Just a few weeks ago I drained it and replaced the pan seal that I had overtightened. It made no difference and the fluid looked good. I even drained the torque converter and primed it with a fresh quart (pumped it in through drain hole). Sorry for the run on note, just not sure where to go from here. Could it be a torque converter going bad? I didn't do anything to it during the rebuild.

 

Thanks, David

[DRASTiK 22]

I'm no tranny expert, but your problems seem very similar to an issue I had with an aftermarket C4 from TCI a few years ago. The issue for me was in the valve body and replacing it resolved the issue.

 

Did you use RTV on the pan gasket? I've seen situations in the past where an excessive amount was used to compensate for a bent pan flange and some of the excess squeezed off into the pan and got sucked up and caught in some rather unfortunate places in the valve body.

[carprojects 10]

During the rebuild I went through the valve body. I guess it's possible that one of the pistons is sticking. I will have to check into it. I hate draining it.... such a mess. It was also suggested to dig out the modulator pin and make sure it's moving freely, still have to drain...

[StephenC 10]

Had this ame problem on my C4 found out that moister got into the transmission vac. line somehow and it was causing it to run poor in cold starts. Also want to check the vac. modular on the trans itself make sure its good you can also adjust the modular in the trans. There should be a small screw inside the hole were you vac line goes. These do go bad often and if you get moister in the line they go bad even quicker.