What is the
TCU?
The
TCU is your
Transmission Control Unit. It controls how your car shifts.
What is Limp Mode?
Simply put, limp mode is the lack of a TCU. If you have a working 1G automatic right now and unplug your TCU, you will be left with Park, Reverse, Neutral, and Drive. Drive will only operate in 3rd gear, and your shifts into Drive and Reverse will be hard and unpleasant. Thus, limp mode.
Limp mode is possible because your shifter is still connected to your transmission. So when you move to Reverse or Drive, you are manually making that change. It’s the electronics, controlled by the TCU, that allow smooth shifting in and out of gears.
Limp mode can be one of two things (or sometimes both). Failing electronics, such as solenoids can cause the TCU to give up in trying to operate your computer. Also, just like the ECU, the TCU suffers from a prone-to-leaking capacitor (more on that later) that can cause it to go out.
Testing your TCU
This information can be found at:
TCU Codes
If you don’t get anything (no pulses or anything) or you get a constant 12v then your TCU is most likely dead. To remove the TCU, remove both the passenger center-console kick panel and driver’s side center-console kick panel. The TCU lays flat behind the ECU.
Remove the two 12mm bolts on the upper portion of the mount and the one 12mm bolt on the bottom (towards the rear of the car). Unplug the wires, pull out from the driver side.
Remove the 4 screws and check it out. If it’s had problems, you’ll notice it right away as the 47uf capacitor has probably leaked all over the place. One issue is the corrosion can destroy traces and sometimes short the power regulator that’s right next to the cap (the green transistor). To test, measure the voltage (while TCU is plugged into the car and on) on the center pin. BE SURE NOT TO SHORT THE MIDDLE AND SIDE PINS TOGETHER, THIS WILL FRY YOUR ECU. (Thanks to steve for the diagnostics help)
If you get more than 5v (like 12v) then the power regulator is shot and it killed the chip on your TCU. Get a new one. Otherwise, go ahead and replace the cap (NTE part number VHT47M50 for a high-temp model) and clean up the board.
Replacing your TCU
There are 2 things to consider in replacing your TCU. The primary is the harness connection. 1990 vehicles use a funky two plug connection:
1991 through early 1992 use a three plug connection and late 1992 – 1994 use a 2 plug connection. (see Additional information below)
The other thing to look at is FWD or AWD and engine size. Yes a 3plug 1991 TCU from a FWD car will fit and work in your early 1992 AWD car. But the TCU’s shift maps are designed for a transmission only moving 2 tires. The transmission will see more load on the tranny and shift early.
So in short, it’s best to get a TCU that matches your current one perfectly. Here are some model numbers to help you out:
MD738998 91-92 FWD, 1.8L
MD750092- 90-92 AWD
MD750093 91-92 FWD, w/turbo
MD750525 91-92 FWD, 2.0L, w/o turbo
MD754075 93-94 FWD, w/turbo
MD754076 93-94 AWD
MD754077 93 FWD, 1.8L
MD754079 93 FWD, 2.0L, w/o turbo
MD756600 95 FWD, w/turbo
MD756601 95 AWD
MD756916 94 FWD, 1.8L
MD756917 94 FWD, 2.0L, w/o turbo
MD759736 96 FWD, 2.0L, w/turbo
MD759737 96 AWD
MD762042 97-99 FWD, 2.0L, w/turbo
MD762043 97-99 AWD
MR483685 97-99 FWD, 2.0L, w/turbo
MR483686 97-99 AWD
(Thanks to GDNF2ET )
Additional Information
Here are the pin-outs for the 2plug and 3plug connections
