1997 LC Transmission Shifts Correctly in Drive on Lift, Stuck in O/D on Ground

[RFC]

This is really weirding me out. Bought the truck with knowledge that it was stuck in O/D when selector was in drive. Shifts normally if you manually go from L->2->D. Getting a P0753, shift solenoid A. Put it on a lift and tested the harness and it all ECM to transmission connector voltage sequences correct for the 1/2,2/3,3/OD shifts. AND in fact on the lift it shifted fine. Went through the gears at appropriate speeds with the tires off the ground. Went ahead and dropped the pan, tested the solenoid resistances (good) but went ahead and replaced them anyway.

BTW I have a snap-on scanner and was able to watch the shifts occur automatically in drive as well as control the shifting through the scanner. Could also see TPS%, vehicle speed, IAT and ECT and they looked fine. Also the P0753 went away.

Put the truck on the ground, shifted into drive (still shows 1st gear on the scanner) however as soon as gave the truck some gas and it inched forward all voltage went to zero from the ECM to solenoids and truck was defaulted to overdrive (as it is supposed to with both solenoids off). I tested the voltage at the ECM and at the harness connection. No difference. Could no longer control the shifts with the scanner as well.

Put it back on the lift and everything ok again. Voltage from the ECM went on and off in the correct sequence for shifting and in fact it shifted fine again.

Anybody have thoughts on this?

 

[RFC]

I probably should have been more specific than "any thoughts on this".

Is there any sensor or signal that tells the ECM to lock in O/D other than the speed sensor and TPS? Is there some line pressure deal or limp mode triggered by the little amount of gas I gave it to move forward? Is there something that can get disconnected simply by having the wheels on the ground and not suspended?

Also as another data point, it did not appear that shifting to 4 wheel low or locking the diffs made ant difference.

 

[landcrshr]

I'll hazard a guess and say maybe the ECM is detecting a short circuit and going into a safe mode (no output to the solenoids).

Did you try this test?

 

[RFC]

Yes we did. In fact we actually ran a second set of wires from the ECM to the transmission connector to bypass the harness just in case. Again the weird thing is that on our lift with the wheels hanging it shifts normally and the ECM provides the 13v to the solenoids in the proper sequence. I can watch it go up to an indicated 40 mph on the lift and it shifts right through the gears. As soon as it on the ground and inched forward in drive ECM voltage shuts down and O/D defaults.

 

[Whileemptyheadsdream]

I'll hazard a guess and say maybe the ECM is detecting a short circuit and going into a safe mode (no output to the solenoids).

Did you try this test?

View attachment 1021533

X2 whenever I change position and function works,Its most likely a short. Good Luck!

 

[landcrshr]

....we actually ran a second set of wires from the ECM to the transmission connector....

Isn't there also a length of harness between the connector and the solenoids?

A short may occur for only a very brief time. Like during the brief time when your motor torques to one side from putting it in gear. That may not occur when your wheels are in the air.

Just a thought.

 

[dallen341]

I believe the transmission control wiring is also part of the engine harness bundle that goes right by the egr tube. I'd guess something may be shorting right there when the engine torques under load.

 

[RFC]

There is wire in the pan from the connector to the solenoids but everything looked ok and continuity was good.

 

[RFC]

I bypassed the harness so as keep it away from the egr tube and also bypass the middle harness connector that is near the starter. One thing I did not do which is probably stupid was clip the original wires. If they are shorted, I guess the circuit is still shorted since the bypass wires are hooked to them. I'll fix that this weekend.

 

[RFC]

One other data point is that on my scan tool the VSS miles per hour reading fluctuates all over the place (goes up to 150mph) where as the actual vehicle speed reading on the scanner is constant and the same as the speedometer. Does the VSS send any signal to the ECM for shifting?

 

[Tools R Us]

One other data point is that on my scan tool the VSS miles per hour reading fluctuates all over the place (goes up to 150mph) where as the actual vehicle speed reading on the scanner is constant and the same as the speedometer. Does the VSS send any signal to the ECM for shifting?

IIRC, there are two, the VSS on the transfer is for the speedo, etc, the one on the trans gives input to the ECU for shifting.

 

[RFC]

IIRC, there are two, the VSS on the transfer is for the speedo, etc, the one on the trans gives input to the ECU for shifting.

Lucky I have another 80 (96 LX450) to swap parts around before I have to buy anything. I am going in to the shop tomorrow and start trying a few things.

 

[RFC]

I bypassed the harness so as keep it away from the egr tube and also bypass the middle harness connector that is near the starter. One thing I did not do which is probably stupid was clip the original wires. If they are shorted, I guess the circuit is still shorted since the bypass wires are hooked to them. I'll fix that this weekend.

SUCCESS!! Clipped the original harness wires for the solenoids and just let the bypass wires carry the voltage. Now shifts properly. Doh!!