my 91 7.3 idi with a recently rebuilt tranny (professionally rebuilt) just stopped locking the torque converter. Does anyone know how to test the lockup electrical system??? Any help is appreciated.

 

l'il ol diesel,
Welcome to the IDI diesel forum.

I am not much on automatic trannies, but someone with one will be along shortly.

About all I can say is touching the brakes unlocks the torque converter, so are the brake lights on?

 

several things can cause it , after checking the brake light switch
you really need to scan it for codes
89-91 processors do not have the ability to flash the o/d light when a code is stored

 

Is it bang-shifting? This would indicate the computer has gone into failsafe mode.

 

My 90 F-250 did the same thing, I tested the TPS (FIPL) and it was ok, then replaced it and cured the problem. Mine was shifting between torque lockup and unlock. Even though the switch showed the correct voltage.

 

the fipl is the most common problem , your test passed it because your meter didnt have a high enough sampling rate
we have had for years what is called a noisy fipl issue
thats when at a cruise condition the vibration etc causes a worn spot in the potentiometer to emit a wide voltage pattern and there is also a code for a noisy fipl

 

I quote from my documentation:

DIESEL FIPL SENSOR INFO

DATE 1992

REASON
Early Black FIPL has High Failure Rate

RECOMMENDATION
Always replace the black colored early FIPL or any colored FIPL having 60 thousand or more miles with the late gray colored FIPL.

PART NUMBER
F2TZ-9B989-C