heat/vacuum valve trouble '89 190E
 

My '89 190E has a peculiar problem. The vacuum activated valve (can't think of the name of it - located just below the windshield [and a plastic screen] top, center on the engine side of the firewall) that shuts off engine coolant flow to the cabin heater, works only in cold weather.

I can turn off the heat in the cabin when it is really cold outside, but when the weather warms up, there is no vacuum to the valve (the valve itself is new and tested fine). The vacuum line to the valve has no vacuum at all (I've only tested it in warm weather, when the AC was losing ground to the constant flow of heat in the cabin).

What controls this vacuum, and what would the likely cause of temperature-dependent failure be?

Thanks.
-Brett

behind the dash,on the right side of glovebox is a set of vacuum valves that operate the flaps and the heater valve.it in turn is operated by the push buttons.the valves can only be replaced as a set.why dont you tee off the vac line to ignition unit and run vacuum to the heater valve.this will buy you time to fix when you can get to it.