The transmission control unit or TCU is what controls everything about your transmission and the way it acts.

There are 2 ways to identify OEM Mitsubishi TCUs. One is by part number and the other is by plug. You of course need a TCU that matches the plug that the harness has. But you also need the right TCU for your transmission to prevent problems and so the transmission acts correctly.

1991-early 1992 use a three plug TCU and late 1992 – 1994 use a 2 plug TCU. Something to consider is FWD, AWD, and engine sizes. Some TCUs from differ engines or from a different drivetrain will plug in and work but the TCUs shift maps vary from each model. Say you have a FWD TCU in a AWD car. The TCU shift maps were calibrated for only powering 2 wheels so the transmission will have higher load and cause the transmission to shift earlier.

So basically it is best to get a TCU that is for the model it was made for. One thing to note is 90 Automatic TCUs have a 2 plug TCU but the plugs are separated and spaced out.

Limp mode is essentially operation of the transmission as if there is no TCU. This will leave you with Park, Reverse, Neutral, and Drive. Drive will only be 3rd gear. There will be no 1st or second gear. Shifts intro Drive or Reverse from Neutral or Park will be harsh as well since the TCU is not controlling line pressure for shifts. The transmission is still able to operate in P, R, N, D because of the physical shifter still being there.

There is really one root cause to limp mode and that is failing electronics. To elaborate, one is failing solenoids in which the TCU effectively gives up trying to control the transmission. The other being the caps are bad in the TCU much like how caps in DSM ECUs fail. The same thing happens in the TCU. It could be one or both of these causing your transmission to go into limp mode.

If your having problems and you pull the TCU and take it apart and look inside, 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 with key 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.

Use a multi-meter and 2 wires with alligator clips.

Hook up to pins and Set the mulitmeter for 12 V. Turn the key all the way but do not crank (the power windows work in this position). Watch the pulses of voltage. Write down L for long and S for short. The TCU will cycle the codes over and over. So you can double check them if needed. If you get constant short pulses, the TCU has no codes and all is well.

Once you write it down, figure L = 10 and S = 1. EX: L-L-S-S=22. GO to the chart below and it will tell you what the code means.

The left is the 1G Diagnostic plug. The right is the 2G. Red is you hook up the Positive lead of the Meter and Gray is Negative. If it does not work or read, try reversing the leads.

1G hook POS to #6 and NEG to #12.»»»»»»» 2G POs to #6 and NEG to #4.