SELF-DIAGNOSTIC SYSTEM
Gasoline vehicles are equipped with a Powertrain Control Module (PCM). The PCM controls engine/emission functions, and electronic-controlled transmission functions. For additional information on self-diagnostic system for gasoline vehicles, see THEORY/OPERATION and G - GAS - TESTS W/ CODES article.
Diesel vehicles are equipped with a Transmission Control Module (TCM). TCM controls only the electronic transmission. Unless specifically stated otherwise, references to PCM also apply to TCM-equipped vehicles.
Control module is equipped with a self-diagnostic system, which detects system failures or abnormalities. When a malfunction occurs, control module will illuminate the SERVICE ENGINE SOON light located on instrument panel. When malfunction is detected and light is turned on, a corresponding trouble code will be stored in control module memory. To retrieve stored codes, see ENTERING & EXITING DIAGNOSTIC MODE in this article. Malfunctions are recorded as either hard failures or intermittent failures.
Preliminary Inspection
- Check all vacuum hoses for correct routing, restrictions, cuts or other damage. Inspect difficult-to-see vacuum hoses beneath the air cleaner assembly and other engine components.
- Inspect all engine compartment wiring for proper connections. Also check wires for pinched or chaffed spots, as well as contact with sharp edges or exhaust manifolds.
- The preliminary inspection is very important and should be performed carefully and thoroughly, as it can often lead to fixing a problem without further diagnosis.
Diagnostic Procedure
- Ensure all non-controlled systems not related to engine or transmission control system are operating properly. DO NOT proceed with testing unless all other non-controlled system problems are repaired.
- ALWAYS begin system diagnosis with checking diagnostic circuit. On gasoline vehicles, see DIAGNOSTIC CIRCUIT CHECK in BASIC TESTING article. On diesel vehicles, DIAGNOSTIC CIRCUIT CHECK is covered in G - DIESEL - TEST W/ CODES article. The diagnostic circuit check determines if the computer-controlled system is working properly. If trouble codes are displayed (other than Code 12), determine if codes are hard or intermittent trouble codes.
- A hard code is one which is present while working on vehicle, and problem still exists. Hard codes will cause the SERVICE ENGINE SOON light to come on.
- An intermittent code is one which does not reset itself and is NOT present while working on vehicle. Intermittent codes are often caused by loose connections. The SERVICE ENGINE SOON light will go out 10 seconds after fault goes away. For intermittent diagnosis procedures, see TESTS W/O CODES article.
ENTERING & EXITING DIAGNOSTIC MODE
- With key on and engine off, connect a jumper wire between ALDL terminal "B" (diagnostic terminal) and terminal "A" (ground). (Scheme 173) This places the control unit into diagnostic mode.
- In this mode, the control unit will display Code 12 by flashing the SERVICE ENGINE SOON (SES) light. Code 12 is identified by the SES light flashing once, followed by a short pause, then 2 flashes in quick succession.
- Code 12 will be displayed 3 times. If no other codes are stored, Code 12 will continue to flash until the diagnostic terminal is ungrounded. To exit diagnostic mode, turn ignition off and remove jumper wire from ALDL connector.
Scheme 173
CLEARING TROUBLE CODES
Trouble codes should be cleared after repairs have been completed. Also, some diagnostic charts require that codes be cleared before using diagnostic chart. To clear codes, remove PCM/TCM memory battery voltage for 30 seconds.
If no hard fault codes are present (only intermittent codes exist), proceed to TESTS W/O CODES article for intermittent diagnostic procedures.
SPECIAL TOOLS (DIAGNOSTIC)
Special scan testers are plugged into the ALDL may be used to read trouble codes, and check voltages in system on serial data line. These testers can save a great deal of time. For additional information, see manual included with tester.
TEST EQUIPMENT
A tachometer, test light, Digital Volt-Ohmmeter (DVOM) with a minimum 10-megohm input impedance, vacuum gauge, and jumper wires are required to test and diagnose electronic transmission control system. A scan tester may also be used to access data parameters. Tester will supply a visual reading of most inputs, and some outputs, to the PCM/TCM. Some diagnostic flow charts require the use of a bidirectional (Tech 1) scan tester.
TROUBLE CODE CHARTS
The following diagnostic flow charts and circuit schematics are provided courtesy of General Motors Corp.
Note. Some charts require the use of a bidirectional (Tech 1) scan tester. If a bidirectional scan tester is not available, the PCM/TCM-controlled relays and solenoids may be energized by grounding ALDL test terminal "B" with ignition on and engine off.
Diagnostic Aids
Code 28 will set if PCM/TCM detects one of 2 "illegal" PSM combinations. See VALID PSM COMBINATION for various combinations. Check pass-thru connector for good contact.
| Application | A | B | C |
|---|---|---|---|
| Park | 12 | 0 | 12 |
| Reverse | 0 | 0 | 12 |
| Neutral | 12 | 0 | 12 |
| 4th | 12 | 0 | 0 |
| 3rd | 12 | 12 | 0 |
| 2nd | 12 | 12 | 12 |
| 1st | 0 | 12 | 12 |
| Illegal | 0 | 12 | 0 |
| Illegal | 0 | 0 | 0 |
VALID PSM COMBINATION CHART
Scheme 174
Scheme 175
Clear codes and re-check for Code 39. If code resets, problem could be internal to the transmission. Code 39 will only set in 3rd gear. TCC slip in 4th gear will set Code 68.
Scheme 176
Scheme 177
Check harness routing for a potential short to ground in circuit No. 1227. Scan tester displays transmission temperature in degrees Centigrade. After transmission is running, the temperature display should rise steadily to about 100°C then stabilize. Test the transmission sensor at various temperature levels to determine if sensor is out of calibration. See TRANSMISSION SENSOR - TEMPERATURE TO RESISTANCE chart. An out-of-calibration sensor could result in delayed shifts, or TCC enabled complaint.
Scheme 178
Scheme 179
Scan tester displays transmission temperature in degrees Centigrade. After transmission is running, the displayed temperature should rise steadily to about 212°F (100°C) then stabilize. A faulty connection or an open in circuit No. 455/452 or circuit No. 1227 will result in a Code 59. Test the coolant sensor at various temperature levels to determine if sensor is out of calibration. See TRANSMISSION SENSOR - TEMPERATURE TO RESISTANCE chart. An out-of-calibration sensor could result in firm shifts, or TCC enabled complaint.
Scheme 180
Check for spread connectors at pass-thru connector. Code 68 will set when going to default (2nd gear). Refer to TESTS W/O CODES article.
Scheme 181
Scheme 182
Check for poor connections at PCM/TCM and transmission pass-thru connector.
Scheme 183
Scheme 184
Check all connections, especially those at the transmission pass-thru connector. If code is intermittent, see INTERMITTENTS in TESTS W/O CODES article.
Scheme 185
Scheme 186
Check all connections, especially at the transmission pass-thru connector. If code is intermittent, see INTERMITTENTS in TESTS W/O CODES article.
Scheme 187
Scheme 188
Check all connections, especially those at the transmission pass-thru connector. If code is intermittent, see INTERMITTENTS in TESTS W/O CODES article.
Scheme 189
The PCM/TCM relies on the Power Switch Module (PSM) to indicate what gear range has been selected. PSM must be functioning properly or Code 85 may be set. Check all connections, especially those at the transmission pass-thru connector. If code is intermittent, see INTERMITTENTS in TESTS W/O CODES article. Compare scan tester gear ratio reading to specifications in GEAR RATIO table.
| Gear | Less Than | More Than |
|---|---|---|
| 1st | 2.38 | 2.63 |
| 2nd | 1.43 | 1.58 |
| 3rd | .95 | 1.05 |
| Reverse | 1.97 | 2.17 |
GEAR RATIO
Scheme 190
Scheme 191
If code is intermittent, see INTERMITTENTS in TESTS W/O CODES article.
Scheme 192
Check all connections, especially those at the transmission pass-thru connector. Fault may be an internal transmission problem.
Scheme 193
Check all connections, especially at the transmission pass-thru connector. If code is intermittent, see INTERMITTENTS in TESTS W/O CODES article.
Scheme 194
Scheme 195
Scheme 196
Check all connections, especially at the transmission pass-thru connector. If code is intermittent, see INTERMITTENTS in TESTS W/O CODES article.
Scheme 197
Scheme 198
Check all connectors. If code is intermittent, see INTERMITTENTS in TESTS W/O CODES article.
Scheme 199
Scheme 200
If code is intermittent, see INTERMITTENTS in TESTS W/O CODES article. Check customer driving habits and/or unusual traffic conditions (i.e. stop and go expressway traffic).
Scheme 201
Scheme 202
Check all connections, especially those at the transmission pass-thru connector. If code is intermittent, see INTERMITTENTS in TESTS W/O CODES article. The TCC solenoid is pulse width modulated, and is designed to keep the TCC right at the point of engagement. Therefore, some slight slippage is normal.
Scheme 203
Check all connections, especially those at the transmission pass-thru connector. If code is intermittent, see INTERMITTENTS in TESTS W/O CODES article.
Scheme 204
Check all connections, especially those at the transmission pass-thru connector. If code is intermittent, see INTERMITTENTS in TESTS W/O CODES article.
Scheme 205
Check for poor connections at PCM/TCM, especially at transmission pass-thru connector.
