Can't Shut The Heat in 190E
 

Setting the temperature level to cool does not shut the heat off. My 1987 190E A/C still works but with the heat still on it works against the cooling. Even with the temperature on low it's still warm/hot air coming out of the vents in the winter.

What should be checked to fix?

You have a vacuum leak, somewhere, or if you're lucky a vacuum hose came off. When the car looses vacuum the default (all vacuum actuators in their no-vacuum state) is heater on, defroster on.

Except the leak could be anywhere:eek:

There is something called the monovalve (a vertical can in front of the battery, you can trace a water hose to it, one end goes into the car), which opens and coolant water passes thru the heat exchanger inside the car. This brings heat thru your vents. The monovalve is opened by the pushbutton unit. If you measure the voltage across the terminals of the monovalve, you should get very low value (close to 0) when the heater is not needed. If you get close to 12V or -12V (depending on the polarity of your meter terminals), when you dont expect the heater to be running, then the pushbutton unit is sending the wrong signal. If you dont get +-12V and you still get hot air, then the valve is bad.

Another component that comes into play when heater is needed is the coolant pump. This is a pump that can again be traced thru the water hose connected to the monovalve. On my 87 260E it is located near the coolant tank. When heater is needed, this pump also comes on and pushes the coolant thru the monovalve into the heat exchanger. You can also measure the voltage across the terminals of this pump. If you get + or - 12V when heater is not needed, the pushbutton unit is sending the wrong signal.

Why the PB unit is sending a wrong signal ?

1. trivial - it is bad
2. it is receiving a wrong signal from the interior tempertaure sensor, so the PB unit thinks it is too cold and turns on the heater.

I am thinking you can test the interior temperature sensor using a hair dryer. The resistance across its terminal should change with increase in air temperature, most likely decrease with increase in temperature, I am not sure about this sensor, never touched it.

A quick fix I did to get the heat to turn off was to unplug the auxilliary coolant pump and run a ground wire from the heater valve to the negative battery terminal to keep it closed. This is assuming that your heater valve is good. It needs ground to stay closed. The default position is open. Got another pushbutton control unit and problem solved.

Sorry, cmcdonnell is right, when there is approx +-12V across the terminals of the monovalve, the valve is actually closed and this is what you should get when you dont expect the heater to be on. This is opposite to what I posted earlier. If you get close to 0V across the terminals, then the pushbutton unit is sending the wrong signal. If you get approx +-12V and you still get heat in the vents, the monovalve is bad, replace it. The rest of the text is still o.k. in the previous post.

The idea cmcdonnell has is a good idea, but when you ground the terminal on the valve, make sure it is the terminal that does not already have 12V, otherwise there will be a big short and you will know it for sure. The pin that receives the signal from the pushbutton unit should be grounded.

Removing power from the coolant pump is much simpler, that should prevent circulation, but if the valve is open, you could still end up getting some heat.

the 87 190e does'nt have monovalve it has vacuum operated heater valve that is fed vacuum from the electric distribution block hidden behind dash on the right of glove box.the defult for the valve[no vacuum] is open.now if you follow the line and make sure that there are no breaks it could be the distribution block or the push buttons that send electric commands.it could also be diaphram on the valve.if that is good you can run vacuum from a good source and turn valve to off.

Oops !! I was assuming my 87 260E and the 87 190E are largely similar except for the engine and the body. Now it makes sense why others also were pointing towards vaccum leak and I was getting puzzled.

hello everyone,
well hope i do not sound silly but was wondering does the 93' 190E as well have the vaccum valve ???
just courious as my 190E as well the heat is on "ALL the Time"
driving in the summer is kinda out... :)
just wondering as i had seen were someone asked for a 92' i think?
but mine is a 93' 190E 2.6L
any help would be great !!!
by the way good place for help on various automotive help !! : )

Now it is nearing mid-summer leading to lots of AC problems. Thank God, my 18 year old 300CE has only minor issue with the adjusting wheel. My AC works best when the inside temperature is set to 72F.

the 93 190e should also have vac operated heater valve.if all else fails go buy needle nose locking pliers and carefully adjust so you can close off one of the two heater hoses.