Mercedes-Benz W124 heater blows lukewarm
Mercedes-Benz W124 heater blows only lukewarm air even though the engine warms up. What is wrong?
Check the thermostat first: a poorly opening thermostat keeps the heat low. Hot enough air comes when the temperature gauge rises above 80 degrees. Another cause is the foam seal of the heater core, which has slipped out of place and lets cold air past the core.
Mercedes-Benz W124 heat disappears when standing still but heats when driving. Is the circulation pump broken?
Possibly. The electric circulation pump helps in traffic jams, at low revs and when standing still; on a long drive its significance is small. The pump should run only when the control unit opens the heater valves. If the pump is dead, the cause may be an oxidised connector or worn brushes.
Mercedes-Benz W124 temperature control gives only hot or only cold. What causes it?
The heater solenoid valves close with electricity, and without electricity they stay open. The fault is often in cracked solder joints of the adjusting rollers on the climate control panel, which are re-soldered. Stuck valves can be tapped, and the fault can be traced to the panel by unplugging the valve connectors.