Heat remains permanently on

Last summer my air conditioner (cooling) stopped working. However the heat has remained on and is very high, even though it is not turned on. How can I turn the heat off?

Too many variables to give you an answer. Did you get the car looked at when the a/c stopped working and what was the diagnosis.

I think that the water valve on the heater will be normally open. The heater control has to send it power and supply the ground so that the solenoid can shut the valve… If you not getting both power and ground at the plug at the heater valve, I suppose you could add the necessary signal to force the valves shut. Not sure if that would damage anything.

You could also pinch one of the heater hoses shut, or remove the hoses from the heater valve and tie them to each other with a short nipple.

I can’t tell what we’re talking about here or what it means to say that your heat is permanently on. In your mind, separate the fan that turns on when you run the A/C or heat from the temperature of the air coming out and try again.

The fan turns on an off normally but the air that comes out is always hot? Is that what you meant when you said that the A/C stopped working? I.e. if you try to run the A/C is the air just about the temp of the outside air? Or does it come out as heated air? Did anyone look at the A/C system, and is there sufficient refrigerant charge in it?

The A/C and heat run through the same path in the car. There is a little door in the duct work that controls whether the air will get heater or cooled or nothing. It sounds like yours is stuck to always direct the circulated air over the heater core. Unless you’re also talking about a fan that always blows on high. I can’t tell.

First of all, you AC may still be working, you just are unable to turn off the flow of coolant to the heater core. even if the AC is working perfectly, it will not overcome the heat from the heater core. Manolito referred to a solenoid to shut off the coolant, I am not aware of anyone who uses a solenoid for this, but I have never worked on a Mercedes either.

Most cars use a cable from the control panel to a ball valve in the heater hose. It can get out of adjustment so that it doesn’t quite close off the heater valve. When that happens, it is not uncommon for the vent temperature to reach 100°F even with the AC on. Fortunately this is adjustable, but on some cars, it is only adjustable at the control panel, which means a lot of dashboard disassembly.

As usual Cigroller is right on the money again…Ho Hum… LOL… Sharp he is…

Like Cigroller said he is trying to get a clearer picture of what is going on.

For us mechanics the heat works like this.

When you turn you TEMP KNOB from cool to hot you are literally opening a water valve…which routes HOT engine coolant into a miniature radiator inside your car under the dash…this is the HEATER CORE…

So when on cool…the valve slows or stops alltogether this hot coolant from entering the heater core…When on hot…obviously you open the valve and let hot coolant INTO the heater core…

Then you have the Blower motor speeds…this pushes air thru the heater core and the air is warmed by the core and then it comes out your vents as heated air…

THEN you have the A/C…which is also like a small radiator in your car except this one pulls heat out of the air…so your AC might be getting cold…but the heater core can overpower the cooling affects and make it seem like your AC doesnt work…and in this instance…it means that the Cool…Hot control knob may not be stopping/slowing the hot coolant from passing thru the heater core…so that valve may be faulty

In your instance since I know your vehicle…this is exactly what is happening…your Hot cold knob for your heater is probably electrically adjusted… and may not be actuating the valve that stops or allows the hot coolant from passing thru the heater core. DO THIS…KEY ON - ENGINE OFF - WINDOWS UP -BLOWER MOTOR OFF…

THEN…move your heat control knob from full cool…to full hot rather quickly and LISTEN…do it again and again…DO you hear the motor adjusting the valve under your dash? If you pay attention…You WILL HEAR IT…if you DO NOT HEAR IT…then you know the electrical heater valve is not being actuated…due either to a faulty electronic COOL/HOT knob/wheel/switch in your dash…OR THE MOTOR that controls this valve adjustment…(more common is the switch) IF YOU DO HEAR IT…the valve internals may be melted or messed up in some way…or the arm that controls the valve may have broken.

THE LAST ditch method to stop this or TEST the theory is to locate the heater core hoses going into your vehicle…once you find them…(they are the only rubber hoses appearing to go thru your firewall and into your car) YOu can use a pair of long nose vice grips to gently pinch off the hose so as to cut off the flow of hot coolant into the heater core…thus acting like a control valve shutting off the coolant flow…if you then notice that the “heat” temp has been turned down…and suddenly the AC seems much cooler…you found the culprit…you just need to find out if its the switch, the motor, or the valve that is faulty… Hope this helps