Case Study: 1993 Ford F150
Blower Resistor and Burn Pattern

 

Resistor pack for blower motor control, likely to have ignited combustible debris such as leaves

Examination of the vehicle and review of documents and witness statements indicated that the fire was located around the heater blower assembly located in the right rear of the engine compartment. Potential scenarios included fire origination in the a) blower assembly, b) wiring adjacent to the fan motor, or c) wiring between the heater blower and the battery/relay. Only the first scenario was consistent with all evidence. It was determined that the most likely cause of fire in the blower assembly was combustible debris (such as leaves) in the blower assembly ignited by the blower resistor pack (motor speed controller). It was also possible that faults in the wiring inside the blower fan or leading into the fan assembly was the cause of fire.

For this vehicle, variable blower speed control is accomplished by switching resistors in the circuit to drop voltage to the motor. When the fan is operating at full speed (“high”), no resistors are in the circuit; at lower speeds, a series of external resistors, capable of dropping voltage to the motor, are used to obtain the desired fan motor speed. Resistance heating of combustible materials in contact with the resistor pack, such as leaves or other debris, was a likely cause of fire.

Wiring normally connected to the blower housing in the engine compartment

An exemplar F150 was inspected to confirm the location of wiring in pre-fire conditions. A wire that had been routed inside the blower assembly and through a boot in the blower housing showed more burn damage to the section that had been inside the housing than outside the housing (and inside the engine compartment). This investigation utilized large-scale features of burn pattern in identifying paint damage, rust, and damage to the instrument panel that helped identify the region of origin. Small burn pattern features, such as the wire indicated by the arrow in the photograph above, were equally helpful in identifying the origin in a more precise way.