GM/Detroit 6.5L ENGINE CONTROLS – DTC P1641 MIL Control Circuit

Circuit Description
A dash light is illuminated by the PCM if diagnostics have detected certain errors related to the engine performance or engine sensor status. When the PCM is commanding the MIL ON, the voltage potential of the circuit will be low (near 0 volts), When the PCM is commanding the MIL OFF, the voltage potential of the circuit will be high (near battery voltage). The primary function of the PCM is to supply the ground for the MIL circuit.

Conditions for Running the DTC
The ignition switch in the ON position.

Conditions for Setting the DTC
The voltage on MIL control circuit high (near battery voltage) when the MIL is requested ON.
Voltage on M IL control circuit low (near 0 volts) when the MIL is requested OFF.

Action Taken When the DTC Sets
• The PCM will not illuminate the malfunction indicator lamp (MIL).
• The PCM records the operating conditions at the time the diagnostic fails. This information is stored in the Failure Records.

Conditions for Clearing the MIL/DTC
• A History DTC clears after forty consecutive
warm-up cycles, if this or any other emission
related diagnostic does not report any failures
• The use of a scan tool.

Diagnostic Aids
A faulty bulb, the control circuit shorted to ground or battery voltage, will cause a P1641 to set.
An intermittent can be caused by the following:
• Poor connections
• Rubbed through wire insulation
• Broken wire inside the insulation

Test Description
Number(s) below refer to the step number(s) on the Diagnostic Table.
2. Be sure that both the ON and the OFF states are commanded. Repeat the commands as many times as necessary.
9. If no trouble is found in the control circuit or the connection at the PCM, the PCM maybe malfunctioning, however, this is an extremely unlikely failure.


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