pc820

Strange thing happened to me a couple times this weekend. It was in the upper 90s and I was driving along with the A/C on when all of a sudden, hot air started coming out of the driver's side and foot vent. I turned the climate control on and off a couple times, but that didn't help. Then stopped for about 30 mins, got back into the car and everything worked fine again.

Later in the weekend, hot air started coming out of the Drivers side (warm on passenger side )vent. Turned off the climate control for a bit, turned it back on, and all was ok again. Seems to be working fine again...for now.

Someone mentioned the Evap in an earlier thread. Any info on how to check that? Do I need to remove the whole dash..yikes!

CJWalker

There is a good possibility that your heater duo valve (assembly of 2 separate valves) is malfunctioning. The valves are located on the right hand side nearest the ECU and ABS units against the firewall. You will see a heater line running out of each valve and one inlet from the engine. The valves send coolant to each side of the heater core so if one of the valves is stuck open, hot coolant from the engine can end up on one side of the heater core causing one side (or in some cases both side vents) of the vehicle to blow hot air even when the AC is engaged. This is due to the routing of the heater box and this particular type of valve is notorious for sticking on the Mercedes; I replace them frequently. Before you replace the Duo valve assembly, be sure to check that the valves are getting a ground (closed) signal and voltage from the ACC controller. Start the vehicle, engage the AC, and check for a ground on the left hand and right hand sides of the 3 pin connector on top of the valve assembly. The positive connection is the center pin and should be reading ~12v.

-Chris
European Auto Repair