Transmission woes continue. I need real expertise

Bought the car a 1990 with 90k miles – Transmission A seemed to work fine. Did not notice any issues with it, seemed to shift well and didn’t have issues with slipping. No Transmission lights.

But the engine (Engine A) was bad. Corroded head AND corroded block.

Installed an engine from a 1991. The engineA and transA came out together. All intake and exhaust and controls from the 1990 were retained and put on the 1991 engine (Engine B). With no modifications other than new fluid and filter for transmission, the package was put back in the car.

EngineB starts right up, idles great, no codes. But car now doesn’t drive well. In both reverse and drive, more throttle has to be given to move it, its slipping. But I can get it to shift into all gears, and even get to lockup speed and the torque converter engages.

I realized that I’d mistakenly refilled transA with Merc V not Dex3. I also had problems getting the right amount. I had overfilled it by quite a lot. I tried draining the Merc and replacing with Dex3. Putting what was an exact amount of Dex3 back in didn’t seem to make a difference at all – still had a slipping issue.

At this point I turned to experimenting mode. Overfilled it would work – kinda – would be drivable after a long warm up and some jerky overrevs. It would stall until everything warmed up. At the correct dipstick level it would slip a lot and not be driveable. At a lower fluid level, no movement at all in R or D. I’ve gone through GALLONS of Dex3 at this point.

One thought was that I had damaged the torque converter or the pump. I didn’t remember refilling the TC it after the engine swap. Perhaps it ran dry and was damaged in someway. I thought that perhaps it was the torque converter that had its tabs sheared – perhaps it wasn’t engaging the pump enough to build pressure. I should point out that I still had NO codes, no engine codes, no trans warning light. I would smell burning fluid. Shifts smoothly up and down the gears after warming.

For the first time, I dropped transA. I sourced a new TC (TC-A2). Inspected the seals and found them intact. I removed the pan and did some light poking around in the valve body testing the solenoids, not finding anything obvious there. There has never been particles or grit built up anywhere.

So I put it all back together with new TC. Results: Some improvement but not much, same basic issue with the car slipping. The engine would idle lower (new TC-B) and stall. I made some adjustments at the throttle to raise the RPM a bit. But I would often stall at lights, having to shift to N and revving to keep running. By this time at minimum, I’m thinking that the A clutches are slipping, the pump isn’t working right, and the transA needs to be rebuilt or replaced.

Summary – Engine B, Trans A, TorqueConverter-B, all intake and controls from Engine A.

Via eBay, I was able to source a working low mileage trans (TransB) from a 1994. Supposedly there are differences with the between a 90-92 and 93-94 transmission but I could not find any explanation of what exactly. To me it was worth a risk of some kind of conflict. Remember – every control component from EngineA (ECM, TCM, TPS, CPS) everything WAS functional and installed on Engine B. No check lights.

Yesterday, I completed a transmission swap using TransB and keeping the matched TC (I’ll call TC-B1). I checked it over before installing it. Made adjustments to shift/gear selector. Refilled with 5 quarts, started and ran the car while on jackstands. Shifted up and down and it seemed to engage in R/D. Checked level and it was spot on for the Cold level. I’ve definitely not overfilled it this time. Dex3 only in the system now.

Dropped the car off the stands. Tried to drive. Will back up in reverse with high revs, but will not go forward now. Engine idles fine.

91Engine-B, 94Trans-B, 94TC-B1, all engine and trans controls still from 90Engine-A.

I’m stuck as to what to do now. I essentially have the same symptoms – slipping – that I had after the original engine swap, despite now replacing both the TC and the transmission itself. I DO have a TCM for a 93-94 so I’m considering that maybe it’s the TCM from EngineA that has gone bad and trying swapping that out.

I’m also (belatedly, perhaps) considering there could be blockage of the cooler/cooler lines that would prevent pressure build up. But considering I’ve dropped the trans twice now and the lines have been EXCELLENT at dropping fluid in my face while doing the job, blockage seems highly unlikely.

Any experienced advice is welcome. Reminder, I have NO engine or trans lights. No codes or blinks to read, nothing. At this point I wish I had some errors to point to.

Tl;DR – After an engine swap and two transmissions and two torque converters and gallons of fluid, transmission still slips.

Tagging @Robin_O_Connor @Bryan_N @abercanadian @Grooveman for visability.

I take it this is the 4hp24 trans?
It sounds like its the A drum burnt out, this is due to the seals into the trans from memory. I went through three trans on a car that I purchased to do a charity run, that was the A drum failing each time. I know your pain, OI went through <> 60 litres of ATF before we got the problem sorted. The car came off the hoist about 1.5 hrs before the the evening meal prior to departure the next day.

I assumed this on TransA after trying TC-A2. Why then would I get same slip on TransB? TWO transmissions with A bands worn out?
I’m going to try a cooler line flush - it’s the cheapest, low labor thing to do next.

Yep as I said I went through three. Within a space of 2 days trying to get the car ready for a 1500 klm trip.

Basic question - How are you checking your trans fluid levels?

At this point every way imaginable. But in general after a good warm up of the engine and if possible a drive of some sort.
With this most current transmission drop, the TC is definitely full, cooler is drained, pan has been drained and sat overnight. I put in 5 quarts.
After engine warm up 10 min and some R-N-D shifting, the level is at the MIN mark.

Ok. Level is fine then.

Yep. Next step is this TCM swap I suppose …