Heater randomly on and off
 

tonight was the first night ive driven where it was pretty cold, and noticed that my side vents in my 1993 190E 2.6 was sometimes blowing hot air through and sometimes cold air.

i have the temperature switch turned all the way to cold, and im using the button that looks like a rectangle with a line down the middle.

my car wasnt doing this before (as in last week), and since then, i have switched my MAS control unit out, and have been flooring it quite a bit because of the new found power in my car.

anyone have ideas why its doing this?

If the center vent was staying COLD then you must have a vacuum supply issue allowing the heater valve to open at times.

center vents were staying cold, so you think i have a vacuum leak somewhere? maybe i was flooring it too hard and one of the vacuum hoses popped off?

Most likely a vacuum actuator, the problem is finding which one.

Mine's doing that, again, but at least there's two I don't suspect;)

i checked it out today, and the vacuum from the red vacuum line to the heater valve is pretty weak, is that how its supposed to be?

I am guessing I should look for a similar problem with my 1985 190E. When I switch to heat, it takes about 10 minutes for heat to come on and it only comes out of the side vents. Center vents stay the same temperature no matter what. The switch from heat back to normal is much quicker. Should I be looking for vacuum issues as well?

it shouldnt take 10 mins for heat to come on unless your coolant is cold.

and by design, it should only come out of the side vents

Nope, car was at operating temperature. I also repeated the expirement several times while driving with the same results.

does the MAS unit control the heater in any way?