After needing a new gearbox, i got another one from the wreckers with no guarantees. The old gearbox was an F4A33-1-MNQ, the replacement is an F4A33-1-MNP and the TCU has not been changed.

F4A33-1-MNP6/9: 27 teeth splined driveshafts, used in V6 models
F4A33-1-MNQ3: 27 teeth splined driveshafts, special transaxle control unit, used in V6 models.

The problem is trying to get it in gear. On the first few tries, it will just stall and spin the wheels. Sometimes it will move, and the revs will be very low but it can get going and drive normally. Also, reverse doesn't usually work while drive gears are working.

Interestingly, when going from drive to neutral the revs can drop as low as 100 RPM then pick up again or stall.



It seems as though some kind of clutch is not working.
It might be strange if the problem is electronic, however i also switched the interior wiring loom from a 4 cyl Elite into the V6 Executive. Only slight changes to connectors in the engine fuse box were needed, as well as adding a few wires into the engine bay loom for the immobilizer, alarm and power windows.

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After needing a new gearbox, i got another one from the wreckers with no guarantees. The old gearbox was an F4A33-1-MNQ, the replacement is an F4A33-1-MNP and the TCU has not been changed.

F4A33-1-MNP6/9: 27 teeth splined driveshafts, used in V6 models
F4A33-1-MNQ3: 27 teeth splined driveshafts, special transaxle control unit, used in V6 models.

The above descriptions infer that your old transmission needed a special TCU. That TCU may or may not be compatible with the other box you now have installed. Without knowing for sure, I'd suggest the first step should be to try a TCU matching the box you put in. Maybe you can arrange that on a "loaner" basis so that you don't incur the expense of the TCU if it is not the issue.

Does the interior loom you swapped include the TCU wiring?

The looms do interconnect, but the interior loom does not have dedicated TCU wiring. I left the ECU/TCU wiring intact. The point where it connects to the interior loom is interesting. TCU wiring has a female plug with wires in every socket, even though a few are never used on either the Exec or the Elite. The male plug differs by "only" 3 wires, 2 are 4cyl only and 1 is V6 only, all are on different pins.

I figured it might be something else, there was a third center console plug. One the V6 never had. It is mainly used for cruise control, but i connected it anyway. 4 wires run the cruise actuator, one goes to overdrive, and the last 2 wires splice into the throttle position sensor. This did not fix it.

Topping up trans fluid didn't seem to help, although i overfilled it.

The gearbox shows more action with no TCU at all, it actually gets into gear and moves a bit (it just has no 'clutch').

Putting the other TCU in did make a difference. Rather than getting gears, it decided to make every gear neutral. Going back to the incorrect TCU, or even no TCU at all results in stalling in gear. That also happens with a 4 cyl ECU. This is so wierd.
And this clue may be the most interesting. Whenever shifting from gear to neutral, the engine will drop the revs below 500. If i try to stop it by giving throttle, as soon as i let go of the throttle it will remember that it has not yet tried to stall. After it either tries or succeeds in stalling, revs will go back to where they should be.

Thanks for your help, coldamus

Here is a video which may help identify the problem

http://youtu.be/rdPk2CaGb5k?t=11s

I had some error codes from the TCU

41 open shift control solenoid valve "A" circuit
47 open damper clutch control solenoid valve circuit (this would explain the stalling)

All engine bay plugs + TCU got a decent spray of contact cleaner after that.
The error codes disappeared, and i could get them back by disconnecting the TCU and reconnecting it while the battery was still connected.

The mixture of harnesses makes it all a bit difficult. However those error codes seem relevant to the symptoms you described. Were those codes read while the TCU that matches the box was in use or were they with the old TCU?

Personally, I would persevere with the TCU that matches the box and try to troubleshoot the box in that configuration.

According to the factory service manual, code 41 indicates damaged or disconnected wiring of the shift control solenoid "A" system.

Remedy:

Check the solenoid valve connector.
Check shift control solenoid valve "A" itself. (resistance should be approx 23 ohms at 20 deg. C.)
Check the shift control solenoid valve "A" drive circuit harness.


Code 47 indicates damaged or disconnected wiring of the lock up clutch control solenoid valve system.

Remedy:

Check the solenoid valve connector.
Check the lock up clutch control solenoid valve itself.
Check the lock up clutch control solenoid valve drive circuit harness.


If you have the factory service manual, page 23 -2 -16 describes how to check the lock up clutch control solenoid valve. It also says " If the lock up clutch is applied constantly, the engine will stall when drive or reverse is selected."

Unfortunately, i figured out how those error codes came up. They went away after clearing the codes and cleaning the connector. I reconnected the TCU while the battery was still connected, and that seems to have caused the codes. Those codes would explain everything! If only they stayed on.

The codes were retrieved with the old, incorrect TCU.

Testing with 12v to all 4 solenoid wires shows they all work. I have never seen 23 Ohms though, but it was warmed up. One of the solenoids was quite close to about 3 Ohms, yet it still clicks.

Unfortunately, i only had access to the right TCU for one day. It didn't seem to help at all, but could have been faulty. Trans had over 4ltrs of fluid at the time, yet showed near empty.

I tapped into the solenoid wires, then started the car. The shift control solenoid valve "A" was always on, while the lock up clutch was never on during a test through the gears. Then there was that solenoid that rapidly actuates whenever you shift into a gear, the pressure control valve solenoid. It would alternate between positive and negative (using a decent homemade LED test light). Shifting into reverse gave normal operation, actuating for just a few seconds. But in drive it would do that, then it would stop for a few seconds, before actuating indefinitely. But that last test was without the car running.



I cannot dismiss wiring problems, since without a TCU the car goes into gear, with the TCU all gears are neutral. Having another look at the plug shows a green wire on one end, and a yellow/black on the other. And there are other wires out of place even though the majority is fine. The plug is called "C-15"

I don't know what to suggest. Going back to completely original looms is probably too much work to contemplate when the problem may be with the tcu, the box itself or somewhere else such as the tps.

3 ohms is about right for both the lockup clutch control solenoid valve and the pressure control solenoid valve. The shift control solenoid valves "A" and "B" are 23 ohms.

I am definitely no expert on auto transmissions. Do you have the factory 2nd gen. service manual?

I'm not sure which one i have but it is massive (1364 pg). I downloaded it from this site somewhere, and it covers TR, TS, KR and KS.

To find the error codes, i used a 1993 Diamante Auto trans manual for the F4A33.

Right now, i am following every wiring diagram to find any changes at the C-15 connector between 4 and 6 Cyl

Thanks for your help and persistence.

I'm not sure which one i have but it is massive (1364 pg). I downloaded it from this site somewhere, and it covers TR, TS, KR and KS.

That's the one. To find things within it using Acrobat Reader, select "Edit", "Search" and choose "Use advanced search options". Tick "Whole words only", then type in a phrase such as "shift control solenoid" and click search. It will list all occurences and you can just click through them.

The gearbox is at fault. After disconnecting every gearbox solenoid wire except the pressure control solenoid, i found that shift control solenoid "B" must always have power.
If power is removed from shift solenoid "B", and the car is in gear, it will dump the clutch and stall. The other solenoids have no effect.