Help from the Tecnical dept

[Keith D]

I have been through the forum re the Heater probs, but am still a little confused. I seem to have the same problem as others IE Heater stuck on Hot, regardless of possition of the temp thumb control, not a problem at the mo, but would like to re gain some control.

I am not sure if its an electrical or a mechanical problem, and how to repair.
1991 230E Auto non aircon w124.

Help and advice greatfully accepted.

Cheers, and a Great New year to all.

 

[Myros]

sounds like the duovalve has had it

mine did the same last year, red-hot regardless of setting. Cost me about 115 from MB (ouch).

 

[Keith D]

Ouch indeed, having just put new dist cap, plug leads, roto arm. and a trip to RAF Lossimouth from Morecambe, 800miles or so. Thank goodness for LPG.

 

[jberks]

Do a search for "Duovalve" and "Monovalve" (depending on whether you have left/right side control).

 

[Keith D]

Thanks, jberks, but same heat both sides (hot) the only cool air I can get is from the 2 center vents.
Qustion is, are the 2 thumb temp controls electrical (quick squirt with WD) or mechanica (strip down dash to re attach something)

 

[tom7035]

Not sure, but the thumbwheel heat controls could be vacuum operated.

 

[c13tay]

Check the re circ switch too

My 202 was like that when I bought it - could only get hot air - recirc light wouldnt illuminate- just flashed when pressed - also the heater lighting flashed with lights off and went out when lights on

Initially cured it with a quick thump on the panel - recently took the controls out cleaned all the connections etc and all seems fine now although the airflow direction dial now lights up slightly brighter than the fan switch !!

 

[jberks]

Thanks, jberks, but same heat both sides (hot) the only cool air I can get is from the 2 center vents.
Qustion is, are the 2 thumb temp controls electrical (quick squirt with WD) or mechanica (strip down dash to re attach something)

On the 123 the centre vents only supply cold air anyway - not sure but I suspect the 124 is the same. The temp is controlled by a 2 stage process. The voltage is supplied by the thumbwheel, then goes through a temp sensor which modulates the current dependant on the ambient temp and then to the duovalve. I believe if the valve gets no current then it's full hot, so it could be the thumbwheels, but the valves fail far more often than the wheels so either you have 2 duff thumbwheels, a duff duovalve or poss a duff temp sensor. As I say, by far the most common failure point is the duovalve, other items are less common though admittedly possible. I'd check the current at the valve first, if you have none, it's electrical, if you do, its the valve.

 

[joe bloggs]

The duo valve springs open (hot) if no current is applied, pull the 3 pin plug off the valve, ignition on the centre pin should be live, the two outer pins are either earthed (cold) or not (hot) the ecu cycles the valves on/off until the correct temp is reached. Check the fuse, it may look ok, but the ends have a habit of corroding away, take it out for a look.

 

[Keith D]

Cheers, and thanks will have a look tomos. Will let you know.

 

[Keith D]

Thanks Joe, is it easy to find the 3 pin plug on the valve, I guess its in the engine compartment. Good thinking re the fuse, as I have had probs with the wiper fuse.

 

[Keith D]

Heater Tech Help

Thanks to all that gave valuable advice re the heater stuck on Hot.
After checking the 3 pin plug I found no voltage, so checked the fuse and hey presto a twiddle with B7 fuse all now well.

Cheers
Keith D