Weird TC Lockup Problem

[longbedder]

I've got a transmission issue on the '94 Burb. At 68 MPH, the torque converter cycles rapidly in and out of lockup (about 2 seconds on, 2 seconds off, back and forth). It's kind of unnerving and actually jolts the truck pretty good - I'm sure it's also not good for the trans.

I did a run and captured the ALDL data, and it appears that the lock/unlock fluctuation is being commanded by the ECM, but I can't for the life of me figure out why. The jolting is a result of the TCC duty cycle going from about 90% to 0% and back instantaneously (versus a gradual dropoff/rise).

The TPS voltage does not fluctuate when this happens (I thought maybe I had a bad spot in the TPS). The problem is very speed-dependent.

Any ideas out there?

 

[longbedder]

Anybody?

 

[DeviousK5]

Does the scanner your using have the ability to control duty cycle, try commanding it and hold that steady speed. Brake switch input at time of failure?

 

[longbedder]

Does the scanner your using have the ability to control duty cycle, try commanding it and hold that steady speed. Brake switch input at time of failure?

No, my software (TTS Datamaster) does not have the ability to control TCC duty cycle. I verified that the brake switch is in a steady off state during the problem (and I also verified that the software does correctly capture brake switch status).

 

[DeviousK5]

I'm not familiar with that software personally, but your gonna have to get a hold of a scanner that can perform functional tests, such as control of the duty cycle drive it with it commanded at max(usually 99%) and watch the pid of TCC slip to determine if the converter is at fault for excessive slippage . If it doesn't act up while commanded, then the problem is something the pcm is commanding. It could just as well be the solenoid though.