Worked my butt of trying to figure this one out. New converter,installed when we replaced the motor.New lockuo solenoid.New speed sensor,TBS, ECT,tried unplugging the cruse control,checke OD switch,swapped ECM.
I cut the wire from the ECM that controls the lockup solenoid. Drive down the road and apply current to the solenoid,and I have lockup.
Hook a ohm meter up with the conveter wires off the ecm, and you see Voltage applied.But after hitting crusing speed the voltage drops out,unlocking the converter.

 

I assume you already checked for codes. My first thought would be a bad speed sensor. That would generally throw a code.

Make sure your TPS is adjusted properly. If it is not, your TPS may be sending an "idle" signal to the ECM as you let of the accelerator when you reach cruise speed. This of course would unlock the converter.

Make sure your brake signal is correct. Check voltage between terminals Tt and E1 of the DLC. With brake pedal depressed it should read battery voltage. With pedal released it should read 0 volts.

 

Yep, check the codes, if you have a bad temp sensor it will prevent lockup. I had an ECM that was throwing a bad temp sensor code (checked and replaced, no change). Turned out to be the ECM capacitors were leaking (common once vehicles start getting past 15yrs or so. Sent off the ECM to a shop who fixed it for $180 with a 3year warranty.

 

We changed the brake swithch also.No codes on the trans or engine side.I'll try a spare ECM. 91 diesel, you mean the trans temp sensor?

 

no, it was the engine temp sensor. If it's bad (turns out mine wasn't but the ECU/ECM thought it was) the ECU won't let the trans go into overdrive or lock up the converter until the engine 'heats up' enough.

 

quick and easy fix for this is just wiring up a floor switch with a relay to lock up the TC. That's what I did, not to fix the problem (the ECU fixed that) but because the diesel is annoying to drive with the gasser TC that toyota put in it. I can lock up the TC in any gear except 1st if needed. You just have to make sure to unlock it coming up to a stop.

 

How did you wire yours up? That was the next plan.