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Transmission Control Unit/TCU

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

TCU ID

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.

1st Generation / 1G
Name Model
MD750092 90-92 AWD 2.0L Turbo
MD738998 91-92 FWD 1.8L Non Turbo
MD750093 91-92 FWD 2.0L Turbo
MD750525 91-92 FWD 2.0L Turbo
MD754075 93-94 FWD 2.0L Turbo
MD754076 93-94 AWD 2.0L Turbo
MD754077 93 FWD 1.8L Non Turbo
MD754079 93 FWD 2.0L Turbo
MD756916 94 FWD 1.8L Non Turbo
MD756917 94 FWD 2.0L Non Turbo
2nd Generation / 2G
Name Model
MD756600 95 FWD 2.0L Turbo
MD756601 95 AWD 2.0L Turbo
MD759736 96 FWD 2.0L Turbo
MD759737 96 AWD 2.0L Turbo
MD762042 97-99 FWD 2.0L Turbo
MD762043 97-99 AWD 2.0L Turbo
MR483685 97-99 FWD 2.0L Turbo
MR483686 97-99 AWD 2.0L Turbo

Limp Mode

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.

TCU Codes

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.

# Code What to check to resolve this issue
0No Pulses or constant 12VTCU is bad
11Excessive Large TPS (Throttle Position Sensor) outputCheck TPS Connector, Adjust TPS, Check TPS, Check accelerator switch (Code 24)
12Excessive Small TPS output
13Defective or improperly adjusted TPS
14TPS adjusted improperly
15Open circuited low-oil-temp sensor <1992 and 1993>Check Oil Temp Sensor Connector, Check Oil Temp sensor
16Short Circuited High -oil-temp-sensor <1992 only>
17open circuited high temp sensor or short circuited low oil-temp-sensor <1992 only>
21Open-Circuited Kickdown ServoCheck Kickdown servo switch connector and Kickdown switch
22Short-circuited Kickdown Servo
23Open-Circuited Ignition pulse pickup cableCheck Ignition Pulse Signal Line.
24Open-Circuited or improperly adjusted accelerator switch <1992 Only>Check & Adjust Accelerator Switch & Connector
31Open-Circuited pulse generator ACheck Pulse Generator A or B, Check vehicle reed switch (chattering)
32Open-Circuited pulse generator B
41Open-Circuited shift control solenoid valve ACheck Solenoid Valve connector, Check Shift control solenoid valve A
42Short -Circuited shift control solenoid valve A
43Open-Circuited shift control solenoid valve BCheck Solenoid Valve connector, Check Shift control solenoid valve B
44Short-circuited shift control solenoid valve B
45Open-Circuited shift control Pressure Control Solenoid valveCheck Solenoid Valve connector, Check Pressure Control Solenoid Valve
46Short-circuited shift control Pressure Control Solenoid valve
47Open-Circuited torque converter clutch solenoidCheck Solenoid Valve connector, Check Torque Converter Clutch Solenoid
48Short-circuited torque converter clutch solenoid
49Defective torque converter clutch system Check TCC Hydraulic Unit, Check TCC Solenoid, Replace TCU
511st Gear Incorrect RatioCheck Pulse Generator Connectors A & B and the generators, Rear Clutch Slipping
522nd Gear Incorrect RatioSame as 51 except also Kickdown Brake Slipping
533rd Gear Incorrect RatioSame as 51 except also Front Clutch Slipping
544th Gear Incorrect RatioCheck Pulse Generator Connectors A & B and the generators, End Clutch Slipping, Kickdown Brake Slipping
61Short or Open-Circuited torque reduction request or execution signal lineCheck Torque Reduction Request Signal Line, Check Torque Reduction Execution Signal Line
62Open-circuited torque reduction request lineCheck Torque Reduction Request Signal Line
63Short-circuited torque reduction execution lineCheck Torque Reduction Execution Signal Line
81Open-circuited Pulse Generator ASee code 31
82Open-circuited Pulse Generator BSee code 32
83Open or Short-circuited Shift Control Solenoid valve ASee code 41, 42
84Open or Short-circuited Shift Control Solenoid valve BSee code 43, 44
85Open or Short-circuited Pressure Control Solenoid valveSee code 45, 46
86Incorrect gear ratioSee code 51, 52, 53, 54
tcu.txt · Last modified: 2019/06/01 05:30 (external edit)