Diagnostic Circuit Check
NOTE:
Perform diagnostic circuit check to diagnose CHECK ENGINE light function, DLC, and no-start condition.
- Compare data from vehicle being tested with SCAN TESTER DATA table to verify self-diagnostic system is working properly. With ignition on and engine off, observe CHECK ENGINE light. If Code 12 is flashing, check for grounded diagnostic test circuit (DLC terminal "B"). See wiring diagram in CHART A-2. If CHECK ENGINE light is off, go to CHART A-1. If a steady CHECK ENGINE light is present, jump DLC terminal "B" to terminal "A".
- CHECK ENGINE light should flash a Code 12. If Code 12 does not exist, go to CHART A-2. If Code 12 exists, check whether 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 exist, see applicable chart under CODE CHARTS. Start with lowest code. If no fault codes exist, 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° |
| Intake Air Temp |
°C |
10-90° |
| MAP |
Volts |
1.0-2.0 |
| BARO |
Volts |
.3-1.0 |
| Loop Status |
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 |
| ST Fuel Trim |
Counts |
110-145 |
| LT Fuel Trim |
Counts |
115-138 |
| 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 |
| PROM ID |
0-999 |
Varies |
| Shift Light (M/T) |
On/Off |
Off |