W124 Heater Stuck on!

Mercky

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96 W124 320 Coupe
Hi All,

The heater on my 320 124 coupe is acting up, more and more regularly it is throwing out heat with all dials to off, the car has AC but this has never worked properly due to suspected gas leak. The heater controls used to work fine and then intermitently they started acting up and now it's nearly a permanent fault.
Heat comes from both driver and passenger sides with dials set to min or clicked to off position. Getting a bit tired of driving around with the windows all down to stay cool so any help would be great!
Thanks in advance

Mercky
 

television

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2002 SL500, 216 CL500, all fully loaded
As Roadhog above this should be the duo valve,,we do a an article in the DIY
 
OP
M

Mercky

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96 W124 320 Coupe
  • Thread Starter
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Thanks all, yes I suspected the duo valves but thought unlikely that both would be faulty at the same time and wondered is the control signal to the valves the culprit? If you disconnect the connector to the valves they default to open it seems. Does the temp sensor over the mirror have any relevance to this?
Thanks again
Mercky
 

television

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You can measure the output from the control unit,,but the duo valve must be plugged in,,it is a pulsed voltage and will read something like 8v on a Meter, that is fox max cold. there temp sensor does monitor what the dials are set to
 

wireman

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nice 201 2.5D 1993 & very nice 129 SL500 1994
There are 3 sensors, two are in the heater and measure the temperature of the heated air. The one over the mirror has an aspirator fan that sucks air from the cabin down a tube to behind the glove box where it lives in a foam rubber block tied to the framework behind the dash, it measures the cabin temp and if the fan is duff the heat will work at first and then goes cold for ages untill the sensor finds the cooler air - then it all goes hot again etc etc etc. You should be able to stick a small piece of paper to the sensors vent if the aspirator is working.

Measure the volts at each duovalve coil and adjust the temp dials either side of ambient temperature say from 18 to 24 deg or less, the valves cycling (2-3 per sec) should change as it passes through the actual temperature of the roof sensor, being pulsed at near temp and either off for too cold or on for too hot. On my car there seems to be a band of about 1 or 2 degrees between fully open and fully closed.

The 3 wires to the valve are centre is common and each side to each coil.
Take care not to short out anything, the elecronics won't like it.

Check that any fuse supplying the controls is intact and that its holder has not gone corroded. I do not have a diagram to hand so can't tell you which fuses it could be that may cause these troubles.
 

Alex Crow

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hi mercky. have you got a multimeter? test the resistance of both solenoids in the duovalve assembly by testing central pin to both outside pins in turn.

let us know what you find.
 

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