I was installing a new Pioneer AVIC- Z2 unit in the C5 and was hooking up a latching relay for the AVIC for delayed power to the head unit when a ground wire touched the blower motor case and I got a little spark. Hooking up the volt meter between the motor case and ground it showed 13 volts. Looking at the print on the circuit don't think the voltage should be on the motor case. Any ideas???

 

Removed the blower motor today, should have read the yellow sticker on the fan motor first. The case of the motor has 12 volts on it at all times. The motor has a floating ground on the case and 12 volts applied to the positive side of the motor windings. When the motor is grounded by the circuit the fan runs when the ground is removed the motor case has a 12 volt potential to any ground. Well, got a chance to clean the fan blades. So, be careful running wires in that area of the car.

 

or disconnect battery....

 

The BCM controls a lot of things by grounding them. Also the use of digital "signals" to control things make use of a volt ohm meter to trace "problems" a real challenge.

Here is the description of the operation of the manual blower motor, not exactly simple or straightforward:

The blower motor relay is supplied ignition positive voltage to the coil side through circuit 41 and battery positive voltage to the switch side through circuit 1342. When the ignition switch is turned on the relay coil is energized and the switch is drawn closed. Voltage is supplied through the relay switch to the blower speed control switch through the supply circuit 42. When the blower speed control switch is in positions 1, 2, 3, or 4, voltage from the switch supply circuit 42 is supplied to the blower motor resistor through one of the resistor supply circuits 60 (position 1), 63 (position 2), 72 (position 3), or 73 (position 4). The blower motor resistor is made up of 4 resistors in series. When the blower speed control switch is in position 1 voltage is supplied to the beginning of the blower motor resistor and current flows through all 4 resistors before reaching the blower motor. When the blower speed control switch is in position 2 voltage is supplied to the blower motor resistor between the first and second resistors and current flows through the last 3 resistors before reaching the blower motor. When the blower speed control switch is in position 3 voltage is supplied to the blower motor resistor between the second and third resistors and current flows through the last 2 resistors before reaching the blower motor. When the blower speed control switch is in position 4 voltage is supplied to the blower motor resistor between the third and fourth resistors and current flows through the last resistor before reaching the blower motor. Voltage from the blower motor resistor is supplied to a high speed relay within the blower motor control module. When the relay is de-energized voltage from the blower motor resistor is supplied through the relay to the blower motor. Battery positive voltage is supplied to the high speed relay through circuit 1342. When the blower speed control switch is in position 5 voltage is supplied to the high speed relay coil through the high speed control circuit 2. Turning the blower speed control switch to position 5 energizes the high speed relay and the relay switch is drawn away from the pole supplied by the blower motor resistor to the pole supplied by the battery positive circuit 1342 which is supplied to the blower motor.