W201 HVAC vacuum control module problems

[w123fanman]

I am having an issue with the HVAC in my 190E. The defrost flap is always open and the heat would come on with the AC.

I first checked the heater valve and it was fine, so I suspected either the vacuum module or the HVAC control panel. I swapped the control panel with another one which I am not sure is good but it worked the same way as the original so I decided to check the vacuum control module. I had swapped it out before with another one when I first started having issues earlier this year but it didn't change anything. I just figure it is probably this module because it gets incredibly hot and it works for a few of the vacuum flaps but not others. I tested each of the vacuum flaps and found the defrost flap circuit is not holding any vacuum so either there is a line broken or disconnected or the pod is bad. Everything else held vacuum. The module can open and close the floor vents and recirculate flaps but heater box flap and heater valve are always open.

My guess is the solenoids for those valves have gone bad or are failing, which is why the unit gets incredibly hot. I might try to upload a picture tomorrow but the circuit board for the module has definitely been hot, right at the solenoids for the heater box flap and the heater valve.

I'll try testing the function and resistance of the solenoids and swap some around between the two units at the Invention Studio at GA Tech but I am wondering if anyone else has had this issue. The module in the W201 is behind the glove box and there are two versions, the early version is two separate pieces with a vacuum connector between the two (IIRC) and the later style is one piece.

[Mxfrank]

First off, the "normal" failure mode is for the heat to be on full blast. The system is vacuum controlled, so any leak in the vacuum circuit can cause this problem. Begin by checking the various rubber connectors under the hood. There aren't that many in a gas car, so it's easy enough just to replace them all as a first step. If the defroster pod isn't holding vacuum, that's enough to screw up everything else. You will either have to replace the pod or put a stopper in the line to test.

There are no solenoids involved. There is multiport vacuum switchover valve located behind the dash, on the right side. It can be accessed by removing the glove box liner. Sometimes these fail, and produce strange problems.

The heater valve has nothing to do with regulating temperature. There is an air blend flap at the top of the heater box which mixes cold and hot air to produce the desired temperature. The bad news here is that the only way to replace a pod is to remove all or part of the dashboard. For the defroster pod, you may be able to get away with just pulling the instrument cluster. For the heater flap and recirc pods, you should be able to pull just the radio console. But for the air blend flap, the entire dash has to be removed. Here is some reference material:

Troubleshooting the Mercedes-Benz 190 Tempmatic ACC Climate Control System

190 Restoration (click on any photo to enlarge)

[w123fanman]

I got to the defrost pod and sure enough one of the diaphragms was popped like a balloon. Hopefully replacing it will resolve my problems.

[w123fanman]

Fortunately I had a good spare, that fixed the problem. I also fixed the interior lights issue as well as the parking brake and a new speedometer cable while I was at it.