Scenario: 200 mile round-trip (each way).

Weather: Winter 28-35 degrees F.

Climate control system: works great each way for first 30-40 miles, then blows super hot air from all vents for remainder of trip.

Result: Driving with front and rear windows down in dead of winter to keep interior below 90 degrees F.

Switchology: Doesn't matter, any setting produces same results. Have pulled and inspected solenoid valve inside mono valve multiple times, no tears in rubber, looks OK (new one on order). A/C switch assembly pulled, cleaned, checked. Works perfectly at dealer, he swears all is well.

Requirement: Any and all advise on how to fix climate control. Has been recurring problem over last year, no one can fix. car.

JoeB

Try disconnecting the electric (aux) water pump that supplies water to heater core durning idle & low RPM conditions. Any short circuit in that circuit will send the A/C into fixed operational mode (FOM). Also the windings in the mono heat valve will do the same thing.
If the incar temp sensor goes open or short you will have the same results.

------------------
MERCEDES BENZ MASTER GUILD TECHNICIAN
ASE MASTER TECHNICIAN
27 YEARS DEALER M.B. TECH
190E 2.3 ITS RACECAR

In car temp. sensor
 

Could you tell me the location of the in-car temp sensor on a 1989 420 sel?

if it's the same as the 86 it's on the left side of the panel on the roof between the sunvisors.........
William Rogers........

Only the interior front cabin light is between the sun visors. Any other choices?

hmm, mines a euro model and has the sunroof control dome light and climate control sensor all on one panel there. got to be be someone with an 89........
William Rogers.........