1996 E300 Diesel. Only 50,000 miles and otherwise is great repair. When car is first operated, heat/AC system works perfectly. Then, when desired temperature is apparently achieved, fresh outside air comes into driver's side. In summer, hot air on left. In winter, cold air on left. The other side is the opposite as it tries to counter-act the error on the left side. I replaced mono valve in the engine compartment. Did not fix. Dealer replaced a couple of vacuum connections, thought that fixed it, but they did not drive long enough. They seem to be without a clue. Can anyone help? I'd really appreciate. This is my 3rd MB diesel but this is very frustrating.

 

You need to replace the heater control valve. It is located under the hood close to the firewall on the opposite side of the break master cylinder. The valve looks like two aluminum cylinders side by side. They have valves in them that sometimes get stuck in the open or closed position. Sometimes while you have the engine running you can tap them with the back side of a screwdriver and they will unstick. I usually do most of my own work but I did not attempt this. There is not much room to get your hands under the valve to disconnect the coolant hoses. I think in order for this to be done you must remove the black box next to it containing the computer. I didn't want to take a chance screwing it up. I did this on my old 89 300e and it was a piece of cake. .

 

thanks for the response, cb. I did replace (a real mechanic did it) the heater valve that you describe. still have the problem. any other suggestions? appreciate your help

 

I would guess that you have some vacuum pods that are bad. Not sure on your car, but on the W124's there's a manifold visible after you take the glovebox out, that has about seven vacuum lines to it. You check each line with a Mityvac to see if they hold vacuum...if not, you have a bad pod associated with that line. From what I understand, this is pandemic in older MB's.

Kevin

 

Any closure? I have a 99 E300 Turbo Diesel that has started to show this same problem (hot air when cooling and hot on one side while blowing cold on the other). How did this get resolved for you? My car has 167k on it and the problem started maybe 40k miles ago.