Mitsubishi Eclipse. Technical Information Manual (1994) - part 49

POWER TRAIN 

 Automatic Transaxle

Vehicle Speed Signal

The vehicle speed signal is sent as a direct input
from the TCM to the PCM. This system is called
electronic pinion. The output speed sensor signal

is sent to the TCM and used as the vehicle speed
signal. The 

 does not use a vehicle speed

sensor as in past transaxles. The TCM converts
the output speed sensor signal to an 8000 

Scan Tool (MUT-II) Interface
The TCM and other components have the ability
to interface (“talk” to each other) over the CCD bus.

The scan tool (MUT-II) connects to a data link con-

nector located in the passenger compartment. This

mile signal. The signal is then sent directly to the
PCM.
Using the transaxle output speed sensor for the
vehicle speed signal has resulted in reduced
manufacturing process time, cost savings, increased
reliability, and increased speedometer accuracy.

allows communication with the TCM on the CCD
bus. Refer to the 

 Transaxle Powertrain Diag-

nostic Procedures manual for the data link connect
location.

POWER TRAIN 

 Automatic Transaxle

TRANSMISSION CONTROL MODULE OPERATION

Introduction

What does it do and how does it know? You have

probably asked yourself this question more than
once as you pondered some transaxle problems.

This is really a very complex question. Just stop
and think about it for a minute. Think of all the
electronic logic features that have to be programmed

into this small control module so that it will always
know just what clutch to apply and release, at just

the right time, under all the different driving condi-
tions that could possibly take place.
A description of the 

 electronic functions can

be simplified if we look at them, one main logic
function at a time. We will use this “logic function”
approach to describe in simple terms how the TCM
makes some of these complex decisions. We will

TCM Routine and Logic Functions

Many of the control logic functions discussed in

this section rely on output information from a prior

logic function as their main input information. This
should be kept in mind when you are doing your
diagnosis. Most of these logic functions are recalcu-
lated and updated on a regular basis and in a specific

Start Routine

The first function the TCM performs, when the igni-
tion key is turned on, is the Start Routine. This

routine allows the TCM to check critical inputs and
memory circuits before it powers-up the system.
If everything is ok up to and including the switched

battery tests to the TCM and pressure switches,
then the TCM will power-up the transmission control

relay and finish the remaining checks as described
below.

The Start Routine is performed when the ighition
switch is turned to the OFF (column unlock) or RUN

positions and again when the ignition switch is re-

turned to the run position after engine cranking.

concentrate on the main logic functions and define
the primary input and output requirements. By doing
this we will be able to use simple block diagrams,
charts and graphs to illustrate how some of the
decisions are made. We will not cover electronically

how the actual logic circuits inside the TCM work.

The main logic functions, covered in this lesson,

include the Start Routine, Transmission Range and
Park Neutral Position Switch Logic, Shift Lever Posi-

tion Logic, Speed Sensor Logic, Transaxle Tempera-
ture, Shift Schedule Logic, Torque Converter EMCC

Logic, Shift Selection Logic, Shift Execution Logic,
Solenoid Switch Valve Control Logic, Clutch Apply
Status (Clutch Volume Tracking), Normal Shutdown
Routine, Fail-Safe Routine.

sequence. This sequence and the time interval that
it takes to complete these normal functions is re-

ferred to as a program loop. A program loop occurs
once every 7 milliseconds 

 seconds). Some

special checks or logic functions may be performed

more or less frequently.

NOTE

The ignition switch has been redesigned to allow
for power up of the TCM, BCM and electronic

 display. This occurs with the key in the

column 

 position, just before the ignition run

position.

Any invalid result sets the appropriate diagnostic
trouble code, and the system immediately goes into

limp-in. The TCM identifies the crank period by the
crank signal input provided at pin 8. When the crank
signal is present, the TCM is stopped and is 
ized after cranking to eliminate the possibility of
low cranking voltages or spikes confusing the Cen-

tral Processing Unit (CPU). The CPU is a small

microprocessor inside the TCM that contains thou-
sands of transistors and diodes on a chip of silicone
small enough to fit on the tip of your finger.

POWER TRAIN 

 Automatic Transaxle

Random Access Memory (RAM)
One of the start routine checks is to verify that the
data storage bits in each RAM location are 
ing properly. RAM is a form of memory that can

Read Only Memory (ROM)

A ROM check also occurs during the start routine.
The check is to verffy that all of the data stored

in the ROM is valid. ROM is a permanent memory

be written to and changed, as well as read from.
Code 17 is reported with a failure of RAM.

in the TCM and is used to store the programs needed

to run the system. Code 16 is reported with a failure

of ROM.

Transmission Control Relay and Driver Circuit Continuity Checks

Prior to power-up, the relay contacts should not
be passing current (relay open). Code 14 is reported

for each solenoid. Codes 41 through 44 are reported

with a failure of the transmission control relay 

with a failure of any one solenoid.

 (stuck closed).

After completion of the Start Routine, all of the 

All three pressure switch inputs should be in a 

essary control logic variables are reset prior to 

off condition. Code 20 is reported with a failure

vating the main control program. Some of the 

of any pressure switch (switch closed, indicating

ables that require resetting are clutch volume 

pressure when none exists).

ing values set to zero, initial speed values are 

With the above checks all valid, the TCM then closes

lated, various counters and timers are cleared, and

the relay and verifies that the relay contacts are

shift logic selection is set to neutral.

passing current. Code 15 is reported if this fails

For the first few seconds of main program execution

(relay contacts never close).

following start up, the pressure switch checks are

With the transmission control relay contacts closed,

not performed. This allows time for the transaxle

the TCM then confirms the driver circuit continuity

oil pump to develop pressure and purge the air

from the various hydraulic circuits.

Park/Neutral Position and Transmission Range Switch Logic

This logic function reads the switch positions of
the transmission range and park neutral position
switches on the manual lever (rooster comb) to de-
termine the driver-selected operating mode.
The primary input for shift lever position to the TCM
comes from the transmission range and park neutral

position switches. The logic in the TCM for these
switches reads the four bit code determined by the
switch positions. When closed, each switch provides
ground for the particular circuit it is in (TI, T3, 

T42). The status of these switches is based on

shift lever, manual valve and rooster comb positions
which determines the logic output code.

Reverse, N   Neutral, D   Drive, 2   Second, and
L   Low. There are also some transition codes that

occur between switch positions that are recognized

by the TCM. They are 

 T2, and T3. An I code

also exists as an invalid code.
These logic output codes are then used by the Shift

Lever Position (SLP) logic in the TCM.

NOTE

The scan tool (MUT-II) does not recognize the differ-
ence between transition and invalid codes. A 
on the display screen, of the scan tool (MUT-II),

is used to represent these codes when they are
present. They can occur between shift lever posi-

tions. 

 Transaxle Electronics

The output signal may be any of the following, de-

pending on the logic output code. 

 Park,   

POWER TRAIN 

 Automatic Transaxle

T42

T41

TO1

TO3

P

CL

CL 

 

 CL CL

N

CL CL 

D

 

 CL

2

 CL 

L CL

 

 

 OP

CL 

 

T2 OP

 

 

Shift Lever Position output becomes the 
input to most of the other functions.

The R, P, N, 

 and 

 

 codes are always

accepted by the SLP logic as being valid and are
always converted directly to R, N or 

 SLP codes.

This is done because these codes can only occur
when the corresponding Reverse, Neutral or Drive

manual valve porting is established. This improves

the response time necessary for garage shifts (Re-
verse 

Neutral   

SLP output with 2, L, T2 and   codes is based

 current SLP position as well as current pres-

sure switch and speed ratio data. The pressure
switch and speed ratio data are monitored to identify
which hydraulic mode of operation exists (reverse,

neutral or drive) so that the appropriate selection
is made for Shift Lever Position.

NOTE

To help clarify the SLP table, keep in mind that

SLP logic only uses N because transmission control

On some vehicles it may be possible to cause pres-
sure switch or speed check diagnostic trouble codes

Speed Sensor Logic

Speed and acceleration calculations provide critical
input information to the logic functions within the
TCM. Speed sensor logic determines gear selection,
shift timing and shift quality. This information is used
as input to the shift schedule, shift logic selection
and shift logic execution functions, programmed into
the TCM.
There are four important input values that must

be calculated; engine speed, input speed, input ac-
celeration, and output speed.
The TCM receives a direct engine speed signal
from the crankshaft position sensor or distributor
depending on the vehicle and engine used. Engine
speed is calculated and updated with each sensor
pulse. The TCM also receives an engine speed
signal from the PCM over the CCD bus. Immediate
direct input of engine speed is needed for proper
transaxle operation. It cannot be provided by the
engine speed signal from the PCM over the bus.

However, both “engine run” signals allow the TCM

Shift Lever Position (SLP) Logic
The primary function of SLP logic is to provide safe, continuous,
but limited operation of the transaxle with the presence of
an invalid or transition input code. The SLP logic function
screens the input codes from the switches and provides an
SLP output signal to control actual gear selection.

Inputs include the codes indicated by the transmission range

and park neutral position switches, current shift lever position,
pressure switch indications, and speed ratio data from the
input and output speed sensors.

module logic is identical in Park and Neutral (same
hydraulic porting).

NOTE

An incorrect input (for example, a defective switch)
of one of these positions could, with sufficient time,

result in either a pressure switch or speed check

fault, when in fact the real cause is a transmission

range or park neutral position switch error. This

type of fault can occur, for example, when the manual
valve porting is in reverse but the code is indicating
“D”. In this case, the low/reverse pressure switch
data and input speed data will not agree with the
expected results for Shift Lever Position   

NOTE
It is extremely important that accurate shift lever
position data be available to the TCM. The accuracy

of any diagnostic trouble code found in memory

is doubtful unless the Shift Lever Test, performed
on the scan tool (MUT-II), passes without fail.

by holding the shift lever mid-way between 

 and

Neutral.

to distinguish between a failure of the crankshaft
position sensor or distributor pulse input through
the circuits versus an actual engine stall or key
on engine off condition.

Input speed data from the input speed sensor is

calculated by the TCM. The data is based on the
number of teeth and time between each tooth since
the last main program loop. Input acceleration is
calculated from the change in torque converter input
speed over time.
Output shaft speed data from the output speed sen-
sor is calculated based on the number of teeth and
the time between each tooth since the last main

program loop.
Speed information is also used to verify that steady
state in-gear operation is normal. Output shaft speed

times current gear ratio is continually checked
against input speed in order to identify the occur-

rence of an input or output speed sensor failure
or a clutch that is slipping.