MCI SERVICE INFORMATION

Passenger Blower Failure - D/DL Coaches

The D/DL model coach has a two-speed fan motor to circulate the air in the passenger area of the coach. Inside this motor is a brake circuit that regulates the motor speed.

The brake circuit receives full power through a closed relay at the F1 field terminal when operating at low speed. The relay opens when the motor is switched to high speed and reduced power is fed to the F1 terminal through a resistor (7L-8-589) that is in parallel with the relay. If the resistor should break or if the electrical connection is lost another way, the brake circuit will not restrict the motor when operating at high speed, and it will overspeed or "run away". The circuit breaker for the motor normally interrupts the circuit before the motor is ruined. If the circuit breaker is reset without repairing the resistor, there is an increased potential for damage to the motor.

If you run into the situation where the motor will run on low speed, but immediately trips the circuit breaker on high speed, the resistor is the culprit. Please refer to schematic 7L-13-3393. The relay is open on high speed, closed on low speed. The greater the voltage to field, the slower the motor runs.