W140 ACC - No Heat
 

'93 500 SEL, 125K miles... climate control producing NO heat at all, regardless of setting. Heater core temperature essentially same as vehicle's interior temperature, per pushbutton control unit.

Fault code check revealed two codes (E074, E075), both indicating a faulty auxiliary coolant pump. Approx. 11 volts present at aux. pump harness. Connected new pump to harness and visually confirmed pump operation, prior to installation... pump noted to cycle on/off, relative to temperature thumbwheel position.

Cleared fault codes and installed new aux. pump... but still no interior heat. Fault code recheck reveals no ACC faults. Aux. pump warm and duovalves hot to the touch, with engine at operating temperature.

Any suggestions? Duovalve assembly at fault? Is a diaphragm repair kit available for the duovalve? (None listed at Fastlane). TIA.

USE A TEMP GUN TO CONFIRM TEMP GOING INTO DUO VALVE AND COMING OUT. ALSO CHECK TEMP AT THE HEATER LINES GOING INTO THE FIRE WALL.

With the engine at operating temperature and the ACC set for max temp in economy mode: the coolant supply line to the aux. pump is hot; the lines running from the duovalves to the heat exchanger are warm (likely due to the elevated engine compartment temperature).

Checking the heat exchanger's temperature, via the pusbutton control unit, reveals the temperature to be essentially the same as the vehicle's interior temperature ... less than 90 deg F. Obviously, no engine coolant is entering the heater core.

Fault code check using the M-B HHT and the pushbutton control unit diagnostic mode reveals only the heater supply unit circulation pump (aux. coolant pump) was defective. No other codes are found.

Removal of the existing pump revealed the pump was defective. Installation of a new, functional pump, however, made no difference. So, in addition to the pump, either the duovalve assembly is defective or the pushbutton control unit is not functioning properly when heat is requested. A overhaul kit is not available for the duovalves.

Does the heater work in defrost mode? If it blows out the same temp as the engine then your duovalve is probaly fine. I would get a used push button from Potomac German for testing purposes. The push buttons are very expensive.

Nope. No heat in defrost mode, even with the new aux pump installed.

And no fault codes for either the duovalves or the pushbutton control unit.

My A/C went out on me in New Orleans last week. Fortunately the weather cooperated... somewhat. Does the S350 have an OVP device? It quit at about 90 MPH on I-10

NHODGES

Do you have any blower motor operation? And if so will the fan recognize your input for high and low? Also will the fan run at full speed when you hit the defrost button?

JGL1
Sounds like you are heading in the direction of replacing the duovalves.

A/C
 

Yes, everything works except the cooling. The fan works as it is suppose to, defroster works, Just no chilled air from any where. At all settings , the air comes from the correct vents. I gotta stop going to New Orleans... my a/c quits on me every time I am down there. Naaaah.

Installation of a new duovalve assembly rectified the problem.