[ImperialCrown]

It is very difficult to offer any help or diagnosis with the information given here. An intermittent lock-up converter that isn't putting the TCM into 'limp-in' (2nd gear) may be considered a soft failure. The TCM may not be seeing any conditions out of the ordinary and is carrying on without lock-up.
It could be because it thinks that the transaxle is too cold for lock-up if the ATF temperature sensor is sending wrong information, for example. If the TCM sees a believable value, it won't set a fault code unless that value goes against what another sensor is saying.
Any history on this vehicle in the way of TCM updates or transaxle repairs?
Any stored fault codes would help here. The faults won't light the 'ck eng' or 'service engine soon' light and would need an OBD I scan tool with the Chrysler-specific connector cable to read them. The scan tool could also tell what the ATF temperature sensor was reading.
If the TCM has the finned housing, it can be reflashed with the latest and greatest software (released in 1996?) which is highly recommended. It addressed many problems on the spot. The non-finned TCM's are basically throwaway.

 

[Samuel Thompson]

Since the vehicle was purchased from a dealer 9mo. ago, It came with little paper work and records. The only maintenance item I have is the last oil change sticker, and the sticker seemed up to date, so I am guessing the previous owner took at least decent care of the vehicle. The only other small issue I have had with the trans is that it used to have a small leak at the radiator, but I adjusted the fitting and the leak seemed to stop. The transmission shifts extremely smoothly and as it should other than the lock-up. I have a basic obd1 tool and it had no trans. codes and no engine light, however, there is a 6 pinned finned connector under the dash that the other obd1 connector is not compatible with. When I bought the car I recall a code of some kind but I am unsure if it was trans or not. It has always had the lock up issue as long as I have had the car.

 

[Oklahoma Wolf]

Supposedly, the TCM gets its lockup info from an ambient temp sensor attached to the ECM. I can't seem to find any info on a separate sensor for that, so it may be part of the ECM itself. It would be the logical place to have it.

 

[ImperialCrown]

The 6-pin underdash connector would be the one to access the TCM. The one under the hood would be for PCM (engine).` If you Google the brand and model # of your OBD I tool, do other accessory cables appear to be available for it?
I thought that there was an internal ATF temp sensor that was part of another sensor to control shift schedule and the lock-up function (lock-up is only allowed after the fluid is warmed).
Does the dash temp gauge rise into the normal range while driving?

 

[Samuel Thompson]

I looked online for different cables, but I could not find any. My tester is the Innova 3120 diognostic scann tool. When I drive it the temperature always is maintained where it should be according to the gauge.

 

[Oklahoma Wolf]

I believe the only internal temp sensor is used to redirect flow through the cooler. At one time I believed that's what the extra little temp sensor behind the rad grille was for on the Imperial, but once I got my hands on wiring diagrams I learned that one was only for the AC. I do know there's nothing like that in the wiring diagrams for my Imperial.

Can't seem to find any indication at all of this ambient temp sensor supposedly used by the ECM for TC lockup, except for a passing mention here and there. There has to be something in the Imperial like that, because it does hold off on TC lockup in the winter, sometimes for as long as 15-20 minutes.

 

[KOG]

The engine temperature sensor is used for lockup. No lockup occurs until engine is up to normal temperature. This applies to most Chrysler OD automatics to my knowledge. I know my Cummins trucks have the same setup.

 

[Oklahoma Wolf]

That's what I always thought until Mitchell confused me with this ambient temp sensor stuff.

At any rate, this sensor should probably get checked. If the computer thinks the engine's always cold, that might explain why the MPG drop seems even greater than just not having the TC locked up. Wouldn't affect the temp guage in the dash if a 1993 3.3/3.8 is the same as my 1992 3.8, because there's a separate temp sender for it.

 

[ImperialCrown]

Agree. The cooling system appears to be working OK if the dash temp gauge reads normally.
There is a 2-wire sensor near the thermostat housing that supplies the PCM (not the dash gauge) with engine temperature information. A single Vt wire sensor is for the dash gauge.
The Innova tool might be able to read the PCM sensor supplied temperature to see if it reads a normal warm engine temperature that would then allow TCC (torque converter clutch) operation.
The brake switch for the cruise control would be another possibility for keeping the TCC from applying if it were stuck on or mis-adjusted. It is in the same switch assembly as the brake light switch, but is a separate switch than the rear brake lights. A 3rd switch in the same assembly grounds a wire to the PCM to tell it that the brakes are applied. Come to think of it, this may be the section that disengages the TCC. Does the cruise control work?

 

[Samuel Thompson]

The cruse works fine. I took the brake switch out and tested it. all values seemed to be ok.

 

[Samuel Thompson]

Yesterday, I checked the coolant temperature sensor, and the reading that I got at 100F was 3500 kohms, and after driving for a few minutes to get the engine to full temperature, it was reading 400 kohms. I think this is within reasonable operation area.

 

[ImperialCrown]

This was on the 2-prong coolant temperature sensor. Correct?
I remember that the DRB II scan tool had a line on the display for TCC operation status. It was either 'allowed' or 'not allowed'. Usually the answer for 'not allowed' was found on the PCM sensors or inputs/outputs screen. The big reasons for 'not allowed' would be an active fault code, coolant/ambient/ATF temperature, brake switch status, engine load or road speed.
Internal transaxle reasons might be an internal hydraulic leak or solenoid failure. A plugged filter screen to or from the solenoid would also interfere with TCC operation. So could a worn front pump shaft. reaction sealing rings or the t/converter itself. A pressure port is provided on the outside of the case for TCC diagnosis, if this isn't an electrical problem.