Test # 13 - Checking the Computer
TEST # 13CHECKING THE COMPUTER
Checking the Computer System
The purpose of the following tests is to allow the Professional Transmission Technician to locate the general area of a computer system malfunction. For a more complete test procedure, refer to the appropriate shop manual. The computer system has a self-diagnostic capability. Always begin computer system checks by accessing the computer's diagnostic circuit. All of the sensors which send information to the computer are assigned a two-digit trouble code. If one of these sensors malfunctions, the computer will store the sensor's trouble code in its memory and usually activate the "Check Engine" or "Service Soon" light. When the computer is in the diagnostic state, it will read out the trouble codes stored in its memory. You then have a place to start looking for the malfunction.
Diagnostic Circuit Check
^ Turn the ignition "ON" and have the engine "OFF".
^ The check engine light should be "ON" steady. (If the check engine light is "OFF", check the bulb).
^ If the bulb is good, or the light flashes intermittently, refer to the car's service manual for further checks).
^ Connect a jumper between pins A and B of the 12 pin ALDL.
^ The check engine light should flash a code 12. (If it does not flash a code 12, refer to the car's service manual for further tests).
^ If you get a code 12, note and record any additional codes.
^ If a 50 series code is stored, refer to the car's service manual for further tests.
^ Clear the computer's long term memory, and go for another road test.
^ Retest and record codes.
^ If no codes were present in EITHER test, the computer doesn't see any malfunctions. (This does not mean that there is no malfunction).
^ If codes were only present in the first test, they are intermittent.
Make a visual check of the circuit involved.
^ If coded were present in BOTH tests, the computer is seeing a current malfunction. The following codes are most likely to affect transmission performance.
^ Code 14 = Shorted Coolant Temperature Circuit
^ Code 15 = Open Coolant Temperature Circuit
^ Code 21 = Throttle Position Sensor Circuit
^ Code 24 = Vehicle Speed Sensor Circuit
^ Code 32 = Barometric Pressure Sensor Circuit
^ Code 34 = MAP or Vacuum Sensor Circuit
HOW TO READ TROUBLE CODES
Trouble code 12 will show as one flash of the check engine light followed by a pause and then two more quick flashes. This will repeat two more times. Code 34 will show as three flashes followed by a pause and then 4 quick flashes. All codes in the computer will flash three times, starting with the lowest code, until all codes have been displayed. The computer will then start the entire sequence again beginning with code 12. If more than one trouble code is present, always start your checks with the lowest number code. Exception: A50 series code is always checked first. An example: if a code 21 and a code 32 were present, you would diagnose code 21 first.
Clear Computer.
^ Turn the key "off".
^ Remove the jumper between A and B at the ALDL.
^ Disconnect the pigtail lead on the positive battery cable or remove the ECM fuse for 10 seconds.
^ Reconnect the pigtail or replace the fuse and the codes are erased.
^ Drive the car at operating temperature for at least 5 minutes before re-checking for trouble codes. Go back to test # 13.