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.
|1st Generation / 1G|
|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|
|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 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.
|#||Code||What to check to resolve this issue|
|0||No Pulses or constant 12V||TCU is bad|
|11||Excessive Large TPS (Throttle Position Sensor) output||Check TPS Connector, Adjust TPS, Check TPS, Check accelerator switch (Code 24)|
|12||Excessive Small TPS output|
|13||Defective or improperly adjusted TPS|
|14||TPS adjusted improperly|
|15||Open circuited low-oil-temp sensor <1992 and 1993>||Check Oil Temp Sensor Connector, Check Oil Temp sensor|
|16||Short Circuited High -oil-temp-sensor <1992 only>|
|17||open circuited high temp sensor or short circuited low oil-temp-sensor <1992 only>|
|21||Open-Circuited Kickdown Servo||Check Kickdown servo switch connector and Kickdown switch|
|22||Short-circuited Kickdown Servo|
|23||Open-Circuited Ignition pulse pickup cable||Check Ignition Pulse Signal Line.|
|24||Open-Circuited or improperly adjusted accelerator switch <1992 Only>||Check & Adjust Accelerator Switch & Connector|
|31||Open-Circuited pulse generator A||Check Pulse Generator A or B, Check vehicle reed switch (chattering)|
|32||Open-Circuited pulse generator B|
|41||Open-Circuited shift control solenoid valve A||Check Solenoid Valve connector, Check Shift control solenoid valve A|
|42||Short -Circuited shift control solenoid valve A|
|43||Open-Circuited shift control solenoid valve B||Check Solenoid Valve connector, Check Shift control solenoid valve B|
|44||Short-circuited shift control solenoid valve B|
|45||Open-Circuited shift control Pressure Control Solenoid valve||Check Solenoid Valve connector, Check Pressure Control Solenoid Valve|
|46||Short-circuited shift control Pressure Control Solenoid valve|
|47||Open-Circuited torque converter clutch solenoid||Check Solenoid Valve connector, Check Torque Converter Clutch Solenoid|
|48||Short-circuited torque converter clutch solenoid|
|49||Defective torque converter clutch system||Check TCC Hydraulic Unit, Check TCC Solenoid, Replace TCU|
|51||1st Gear Incorrect Ratio||Check Pulse Generator Connectors A & B and the generators, Rear Clutch Slipping|
|52||2nd Gear Incorrect Ratio||Same as 51 except also Kickdown Brake Slipping|
|53||3rd Gear Incorrect Ratio||Same as 51 except also Front Clutch Slipping|
|54||4th Gear Incorrect Ratio||Check Pulse Generator Connectors A & B and the generators, End Clutch Slipping, Kickdown Brake Slipping|
|61||Short or Open-Circuited torque reduction request or execution signal line||Check Torque Reduction Request Signal Line, Check Torque Reduction Execution Signal Line|
|62||Open-circuited torque reduction request line||Check Torque Reduction Request Signal Line|
|63||Short-circuited torque reduction execution line||Check Torque Reduction Execution Signal Line|
|81||Open-circuited Pulse Generator A||See code 31|
|82||Open-circuited Pulse Generator B||See code 32|
|83||Open or Short-circuited Shift Control Solenoid valve A||See code 41, 42|
|84||Open or Short-circuited Shift Control Solenoid valve B||See code 43, 44|
|85||Open or Short-circuited Pressure Control Solenoid valve||See code 45, 46|
|86||Incorrect gear ratio||See code 51, 52, 53, 54|