Possible issue with accessory mode not shutting off.
Whats the best way to test this and not have your battery be drained if this is the issue.

Thanks

 

Is this for a 65 convertible as this is what I see in your profile ??...please post the model year as a minimum or update your profile as all C'6's are NOT the same...if you have a factory level scan tool you can see what "power mode" the car is actually in...if you have no electrical diagnostic experience you can just change the IMS (Ignition Mode Switch) or BCM and hope for the best....it may be a wiring issue too …below info is for an '08…electrical testing should always be done with battery voltage above 11.45 volts…you can also try a “cap discharge” by removing both battery cables and touching them together…if you have a scan tool that can read the BCM see if you have a B0978 01 or 06 set…last paragraph is a description of that DTC…I can look at the wiring diagram if you are capable of testing the circuit.

Serial Data Power Mode

On vehicles that have several control modules connected by serial data circuits, one module is the power mode master (PMM). On this vehicle the PMM is the BCM. The BCM has multiple B+ circuits that feed into it. Each of those circuits are partitioned within the controller to drive certain outputs of the vehicle’s body functions. An open or short in any one of the B+ circuits may induce multiple codes/or a section of non-functionality within the BCM with the rest of the BCM functioning normally. In this case it is useful to refer to the power distribution schematics to determine if the non-functional partition of the controller shares a common B+ circuit. The BCM will monitor the mode switch only with battery voltage above 7 volts to prevent invalid input readings during low voltages encountered during vehicle cranking. If the mode switch input or the battery voltage input changes, a ratiometric calculation of mode switch input value is performed. The BCM discerns four thresholds:

• Inactive
• Start active
• Off active
• Short detect

To determine the correct power mode the BCM uses:

• The mode switch data received from the ignition mode switch
• The status of the engine run flag (ERF)

The following power mode states are transmitted on the class 2 network:

• Off
• RAP
• Accessory
• Run
• Crank

The following power mode states are transmitted on the GMLAN network:

• Off
• Accessory
• Run
• Crank

The power mode message is a periodic with event message type and is transmitted by the PMM or BPMM at every 2 seconds on class 2 link and at every 250 ms on GMLAN link.
The Accessory power mode times out after 20 minutes and will transition to Off/Awake or RAP power mode.

Fail-safe Operation

The remote control door lock receiver (RCDLR) is the back up power mode master (BPMM). The both BCM and RCDLR receive signals from the ignition mode switch, representing the mode switch data, through 2 different circuits. Therefore if BPMM becomes enabled, it can determine and transmit all the system power modes per the lists above, except RAP. There is no degradation in system operation if the BPMM is the RCDLR. Since the operation of the vehicle systems depends on the power mode, there is a fail-safe plan in place if the power mode message is not received from the BCM or RCDLR. The fail-safe plan covers modules with discrete ignition signal inputs as well as those modules using exclusively serial data control of power mode. The engine control module (ECM) behaves differently.

Serial Data Messages

If no power mode message can be received, the modules remain in the last power mode received and check for the ERF message. If the ERF serial data is true, indicating that the engine is running, the modules fail-safe to "Run" power mode. In this state the modules and their subsystems can support all operator requirements. If the engine run flag serial data is false, indicating that the engine is not running, the modules fail-safe to "Off" power mode.

Discrete Ignition Signals

Those modules that have Run/Crank discrete ignition signal input also remain in the state dictated by the last valid power mode message from the BCM or the RCDLR. They then check the state of their Run/Crank discrete ignition signal input to determine the current valid state. If the discrete ignition input is active, the modules will fail-safe to the "Run" power mode. If the discrete ignition input is inactive, the modules will fail-safe to "Off" power mode.

PCM and TCM Power Mode Backup Strategy

The PCM and TCM remain in the last known power mode state until it can determine its backup system power mode based on their discrete Accessory/Wake up and Run/Crank signal inputs.