DIAGNOSTIC CIRCUIT CHECK
The ECM provides the diagnostic logic to detect faults in the Diesel Electronic Control system that the ECM monitors or controls. The ECM, when it recognizes a fault, has the ability of turning the "SERVICE ENGINE SOON" light on and storing a code. If the condition corrects itself, the "SERVICE ENGINE SOON" light will be immediately turned off.
The ECM recognizes errors in engine speed, vacuum errors in the EGR vacuum loop, and electrical faults involving the 5-volt reference circuit. The ECM controls the following
- Exhaust Gas Recirculation (EGR)
- Exhaust Pressure Regulation (EPR)
- Torque Converter Clutch (TCC)
- System Diagnosis
- Cold Advance & Glow Plugs
To allow proper engine control, the ECM monitors the following inputs
- Engine RPM
- Manifold Absolute Pressure (MAP)
- Throttle Position Sensor (TPS)
- Vehicle Speed Sensor (VSS)
- Coolant Temperature Sensor (CTS)
All diagnosis should start with the "DIESEL DIAGNOSTIC CIRCUIT CHECK" chart. After any repair to the Diesel Electronic Control system, the diesel diagnostic circuit check procedure must be repeated.
Note. Test numbers refer to test numbers on diagnostic chart.
- This test checks for proper operation of "SERVICE ENGINE SOON" light. With ignition on and engine not running, the light should be on.
- Grounding the diagnostic terminal will allow ECM to flash Code 12 and any stored codes. The light must go on and off for a proper code. If light goes from bright to dim, this is not considered a code.
Scheme 193
Scheme 194
Diagnostic Aids
Check circuit No. 410 routing for a potential short to circuit No. 452 or ground.
"SCAN" tester displays engine temperature in degrees Centigrade. After engine is started, the temperature should rise steadily to about 90°C then stabilize when thermostat opens.
The "COOLANT SENSOR TEMPERATURE TO RESISTANCE VALUES" chart may be used to test coolant sensor at various temperature levels to evaluate the possibility of a mis-scaled sensor. A mis-scaled sensor could result in poor driveability complaints.
"SCAN" tester displays engine temperature in degrees Centigrade. After engine is started, the temperature should rise steadily to about 90°C then stabilize when thermostat opens.
If Code 12 or 21 is also set, check circuit No. 452 for faulty wiring or connections. Check terminals at sensor for good contact.
The "COOLANT SENSOR TEMPERATURE TO RESISTANCE VALUES" chart may be used to test coolant sensor at various temperature levels to evaluate the possibility of a mis-scaled sensor. A mis-scaled sensor could result in poor driveability complaints.
Disregard Code 23 if "SERVICE ENGINE SOON" light goes out as soon as throttle is returned to idle.
With drive wheels turning, "SCAN" tester reading should closely match with speedometer reading.
Solenoid coil resistance must measure more than 20 ohms. A lower resistance value will cause early failure of the ECM driver. Check the solenoid coil resistance of all ECM controlled solenoids and relays, before installing replacement ECM. Replace any solenoid or relay that measures less than 20 ohms resistance.
If there is no electrical problem found, check the operation of the cold advance solenoid in injection pump. Check the solenoid coil resistance of all ECM controlled solenoids and relays, before installing replacement ECM. Replace any solenoid or relay that measures less than 20 ohms resistance.
