TCI T-Com, 4L/60e, selected gear problem

[rcotteleer]

We have a '96 impala SS with an LT1 and a 4L/60e. The engine controller is a FAST C-Com system and the trans is run by a TCI T-com. Both controllers are a few years old (maybe 5). The issue is that the "selected gear" field continually jumps around on the T-Com "dashboard", this jumping around actually knocked the sprag out of the trans. It will drive fine under part throttle where shifts are actuated by MPH, but causes issues under full throttle. We have swapped the trans and sent the controller back to have it checked out and it checked out fine.

Has anybody seen this before? Any ideas on what to look at next?

Thanks

Randy

[Steve Chryssos]

Randy,
Confirm: selected gear--not actual gear. The TCU reads selected gear from of the PRNDL switch. Your problem is mostly likely there. Not likely to be the TCU. I've never experienced this problem personally.
Here's a list of available service electrical parts for your transmission.
And here's some basics to double check:
-Make sure the trans case is grounded to the chassis.
-Replace the case wiring harness located inside the pan. Cheap and easy to swap, so might as well do it.
-To avoid another failure, test the car with the rear axle on jackstands--rear wheels two inches off the ground (front wheels chocked obviously)

[rcotteleer]

Thanks Steve, I will look into that stuff. To confirm, it is the "gear select" field on the dashboard. It jumps up and down. I do not know what the parameters are supposed to be, but the output seems incorrect going up to values like 163.

Thanks again, Randy

[Steve Chryssos]

Yeah gear select relates to the transmission lever position i.e. Floor shifter in "D". So driving down the road, it should look as follows:

Selected Gear 4 (regardless of what gear the transmission is in)
Actual Gear 2 (if the trans has shifted once)

So it is my understanding (I could be wrong), that Actual Gear is based on TPS ad VSS, while Selected Gear is based solely on lever position. Try pulling the lever down into D (3rd Gear) rather than OD (4th gear) and retest. Again, test with the rear wheels slightly off the ground and monitor the software dashboard screen. TPS...VSS...Actual Gear...Selected Gear.

Might as well recalibrate the TPS just in case the TCU is getting confused by a faulty engine load signal. But like I said, bad TPS or VSS should affect Actual Gear--not Selected.

I've never experienced this problem personally, so I hope that helps.

[rcotteleer]

Yes that's really odd, the numbers we see are 63 or 67 at idle and it increases with RPM up to 164 at 2400 RPM. We have had 2 different transmissions and a new harness (from TCI) on it and it seems to make no difference. It seems like it could be a noise issue, which is why your idea of checking the case grounding may be of value. We will try the other stuff you mentioned. Thanks again for all of your help

Randy

[Steve Chryssos]

Yes that's really odd, the numbers we see are 63 or 67 at idle and it increases with RPM up to 164 at 2400 RPM......
Randy

TPS percentage? 67% at idle? That'll scatter your transmission for sure. Idle or 2400rpm, the trans computer thinks your foot is thru the floor but the engine is providing little or no load. A good range is about 5% to 70% or so. Might as well replace that TPS sensor. If you are using the TCI remote TPS kit, replacement is a match for an old Tuned Port Iroc Z. About $80 at NAPA.

TPS can and should be tested with key on/engine off.