Quenching Diodes

In order to break down the voltage peaks that occur when the solenoid valves are switched, a quenching diode is provided on the program switch printed-circuit board for each solenoid valve.


If a quenching diode develops a fault, the following malfunctions may occur:

- Quenching diode blocks in both directions:
Peak voltages are no longer suppressed. The result is increased wear at the switch contacts (program switches, airflow selector wheel, flap program in control unit).

Remedy: renew program switch.

- Quenching diode passes current in 'block' direction:
This will result in a short-circuit in the program switch. The fuse will blow or the paths on the printed-circuit board in the program switch will burn out.

Remedy: renew program switch or fuse.