Hi all,
Seems like I got some TCU ( Transmission Control Unit) related problem myself (91 TR 6 cyl 3 plug TCU)

For the last couple of months, the car would intermittently clunk in reverse when I first started off from home and then be stuck in 3rd (I think) gear..
I would drive to the shop, and then by the time a started it up then, it would be driving OK again.. (Intermittent, maybe with heat?)

Anyhow, eventually she packed up altogether and remained in the limp home mode..
Having got the good clues from this forum, I realized it must be TCU type problems.

I connected a LED lamp to the diagnostic pin of the TCU .... Sometimes it would light if it felt like it and sometimes just flash a fast load of crap..
I pulled out the unit and checked connections, soldering and there was some corrosion under the larger electro cap like one of the members showed here in a photo..
Anyhow I cleaned everything up and carefully checked any affected tracks for continuity.

So I put it back in and of course it still didn't darn well work !

Now it was very hot in Adelaide last week and I was forced to drive around to a couple of places.
When I left the library, I thought I would see how the air conditioner worked..
Well yes it did work but the funny thing is, is that the LED on the TCU started pulsing nice and steady, like never before...
..And then when I started driving, she was changing gears like a bought one :wtf:

Since then, it has gone intermittent, but only ever works at all if the aircon is on..

I have been printing out some of the manual to try and get around it all and then I can do a few measurements .

Any thoughts on this matter would be most welcome :)

EDIT...Sorry 'bout original title ... It's TCU not TSU !

sorry to be so dumb / i have been under a lot of stress /what is a TSU

you seem to have a good knowlage of the internal workings of the TCU

if it isnt too expensive get a s/h Tcu just fit it to try

are any pins at the three plug- ins bent or broken

could you have a faulty /corroded pin connections in loom going from the TCU
to the electronics in the box or in the box itself

Again, I cant really help much except to say that if turning on the aircon makes it work, it suggests that there is a dodgy connection somewhere, and this dodgy connection is 'corrected' by having another 'circuit' in operation.
I think it needs a systematic check of all the plugs and sockets in the car; pain in the A I know; but sometimes a fault needs to be hunted down by checking. The observation that 'the car finds 3rd gear' suggests the transmission is in 'fail safe', or 'limp' mode
Hope this helps,