Transmission Control Systems: Description and Operation

In 1990, some MMC vehicles will have a computer controlled automatic transaxle called the 4A/T ELC. The advantages of this system are many with smoother shifts and built-in diagnostics at the top of the list.


How The On-Board Diagnostics Work.


The 4A/T - ELC transaxle ECU monitors critical input and output circuits relating to the control of the transaxle. Some of these circuits are tested continuously and others are checked only during normal driving conditions.


If the 4A/T - ELC senses a problem in the system, a fault code will be stored in the ECU's memory. Each monitored circuit has its own designated fault code. Any stored fault code will remain in memory until erased by the technician.


FAULT CODES

Fault codes are two digit numbers that identify which circuit is malfunctioning. In most cases they do not pinpoint a specific component as defective. It is important to keep in mind that fault codes can be set for hydraulic and mechanical reasons as well as electrical problems.


Hard Fault Codes


Any fault code that comes back a short period of time after it is erased is called a hard fault. This means the defect is there every time the controller checks that circuit.


Soft Fault Codes


A majority of intermittent failures and the soft faults that result are caused by wiring or connector problems. Detects that come and go like this are the most difficult to diagnose. Intermittent failures must be looked for under the specific conditions which they are reported to occur.


4A/T - ELC FAIL-SAFE MODE


The 4A/T - ELC ECU continuously checks for electrical and internal transaxle problems. When a problem is sensed, the ECU will store a fault code, and if certain fault codes are generated for a fourth time, it will cause the transaxle to be in a fail-safe mode. While in this mode electrical power is taken away from the transaxle. When this happens, the only transaxle ranges that will function are:


A Neutral and Park

B. Reverse

C. Third Gear


No upshifts or downshifts are allowed. The position of the manual valve alone allows the three that are available. Although engine performance is seriously degraded while in this mode, it allows the owner to drive the vehicle in for service.


The DRBII

The DRBII must be used to diagnose problems in the 4A/T - ELC system. The tester is connected to the MMC 12-way diagnostic connector located under the dash.


The following is a brief description of DRBII capabilities:


Diagnostic Test - This capability allows the technician to read fault codes stored in the 4A/T - ELC ECU's memory.


Circuit Actuation Test - This capability allows the technician to turn on and off (toggle) certain 4A/T - ELC components. This ability allows easier and more accurate testing of those circuits.

Switch Test - This capability allows the technician to view various changing switch positions as seen by the ECU.


Sensor Test - This cabability allows the technician to monitor 4A/T -ELC sensors on the DRBII display.




NOTE: In 1990, the Plymouth Laser, Eagle Talon, Eagle Summit, and the Plymouth and Dodge Colt two wheel drive vehicles may be equipped with a Mitsubishi Four Speed Automatic Transaxle with overdrive. The 113 horsepower, 1.6 liter will have the KM 176 version. The Laser's more powerful 2.0 liter engine will come equipped with the KM 175 which is essentially the same as the KM 176 but has extra clutch disks to handle the increased power of this engine.