Diesel Diagnostic Circuit Check - GMC G Service Manual

6E2-12 DIESEL EMISSIONS
DIESEL DIAGNOSTIC CIRCUIT CH ECK
Circuit Description
The ECM provides the diagnostic logic to detect faults in
the systems that the ECM monitors or controls. The ECM,
when it recognizes a fault, has the capability of turning a
"Service Engine Soon" (SES) lamp "ON" and storing a
code. Furthermore, if the condition corrects itself, the "Ser­
vice Engine Soon" light signal will be turned ''OFF" immedi­
ately following the correction. The ECM recognizes errors
in engine speed, vehicle speed vacuum errors in the EGR
vacuum loop, via the MAP sensor, and electrical faults
involving the 5 volt reference circuit.
The ECM is a multifunction engine controller that con­
trols the following:
1. Exhaust Gas Recirculation (EGR)
2. Exhaust Pressure Regulation (EPR)
3. Transmission Converter Clutch Control (TCC)
4. System Diagnosis
5. Cold Advance and Glow Plug
The ECM monitors the following inputs, to allow proper
engine control of the above.
1. Engine rpm
2. Absolute Pressure (MAP) used to monitor EGR
vacuum
6.2L (LH6) DIESEL
3. Throttle Position (TPS)
4. Vehicle Speed (VSS)
5. Coolant Temperature
All diagnosis should start with the Diesel Diagnostic Cir­
cuit Check on the facing page.
After any repair to the Diesel Electronic Control System,
the Diesel Diagnostic Circuit Check must be repeated.
A brief description of operation is included with each
System Check or Code Chart.
Test Description
Numbers below refer to the circled numbers on the diag­
nostic chart.
1. Checks for proper operation of the "Service Engine
Soon" lamp. With the key "ON", and the engine not
running, the lamp should be on steady.
2. Grounding the diagnostic terminal will flash a Code
12 and any stored codes. The lamp must go "ON" and
"OFF", for a proper code. If the lamp goes from
"bright" to "dim", this is not considered a code.