1983 Mercedes Benz 300sd Heating system will not work

1983 MB 300SD, when you turn the heater controls on, by depressing any of the buttons on the control module, you hear a "hum" low sounding hum but nothing happens with heat. The blower motor works, I jumped it from the battery and it spins just fine. Anyone with any ideas about what the problem my be, I would be every so grateful. Many thanks.