Diagnostic Circuit Check
NOTE:
Perform diagnostic circuit check to diagnose SERVICE ENGINE SOON light function, ALDL connector and no-start condition.
- Compare data from vehicle being tested with SCAN TESTER DATA table. This ensures self-diagnostic system is working properly. With ignition on and engine off, observe SERVICE ENGINE SOON light. If Code 12 is flashing, check for grounded diagnostic test circuit (ALDL terminal "B"). See wiring schematic in CHART A-2. If SERVICE ENGINE SOON light is off, go to CHART A-1. If a steady SERVICE ENGINE SOON light is present, jump ALDL terminal "B" to terminal "A".
- The SERVICE ENGINE SOON light should flash a Code 12. If Code 12 is not present, go to CHART A-2. If Code 12 is present, check if scan tester displays ECM data. If no data is displayed, go to CHART A-2.
- Start engine. If engine does not start, go to CHART A-3 in BASIC TESTING
article in this section. If engine starts, check for fault codes. If fault codes are present, see applicable chart under CODE CHARTS. Start with lowest code. If no fault codes are present, compare scan tester data with typical data values. See SCAN TESTER DATA table. Values should be within typical ranges.
- If values are okay, see TESTS W/O CODES
article in this section. If values are not within typical ranges, see I - SYS/COMP TESTS article in this section.
SCAN TESTER DATA
| Scan Position |
Units Displayed |
Typical Value |
| Engine Speed |
RPM |
0-200 RPM |
| Desired Idle |
RPM |
ECM Idle |
| Coolant Temp |
°C |
85-105° |
| MAP |
Volts |
1.0-2.0 |
| Open/Closed Loop |
Open/Closed Loop |
Closed Loop |
| TPS |
Volts |
.45-1.25 |
| Throttle Angle |
0-100% |
0 |
| Oxygen Sensor |
mV |
100-1000 |
| Inj. Pulse Width |
Milliseconds |
.8-3.0 |
| Spark Advance |
Degrees |
Varies |
| Fuel Integrator |
Counts |
110-145 |
| Block Learn |
Counts |
118-138 |
| Idle Air Control |
Counts/Steps |
1-50 |
| Park/Neutral |
PNRDL |
P or N |
| MPH & km/h |
0-255 |
0 |
| Torque Conv. Clutch |
On/Off |
Off |
| Battery Voltage |
Volts |
13.5-14.5 |
| Cooling Fan Relay |
On/Off |
Off |
| A/C Request |
Yes/No |
No |
| A/C Clutch |
On/Off |
Off |
| Shift Light (M/T) |
On/Off |
Off |