W126 - heating and recirculation controls

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br1anstorm

Active Member
Joined
Mar 14, 2007
Messages
95
Location
Scotland
Car
1990 Mercedes Benz 560 SEC
I have a 1990 W126 560 SEC, and I need help in diagnosing a problem with the heating/ventilation/aircon controls.

The car has the system with two rotary temperature controls, one for each side of the car, and rocker switches for the aircon and for "recirculation" (ie it does not have the system with the row of pushbutton controls across the central console).

Soon after starting a trip yesterday, with the aircon off and the temp set to roughly normal, I noticed that the dash and footwell vents were blowing seriously hot air, and the little red light in the "recirc" switch was flashing regularly on and off. Normally, the light is off, and comes on (continuously) when recirc mode is switched on.

Adjusting the temp controls made no difference: hot air continued to come through the vents. Changing the ventilation controls to blow to screen or footwells just meant more hot air to those areas. Turning on the aircon made no difference either: though the switch illuminated, no cold air came through - only very hot air!

Even with sliding roof open, fresh air vents open, and windows down, we had a pretty sweaty journey home!

The owners manual section on the recirc switch explains its functions but says nothing about what the flashing light might signify... Can anyone explain?

The symptoms suggest that the ventilation and heating (ie presumably the rotary temperature controls) are somehow stuck on maximum heat. If so, how might this be sorted, and are there any other things I might check?

Any advice would be welcome: we have a 600 mile journey planned next week, and if I can't fix this problem it won't be a comfortable trip!

br1anstorm
 
Some of your heater control will be vacuum operated (switched electrically) your system may have developed a leak in the vacuum system ( actuator diaphragms can fail ) or possibly a failure in vacuum supply-- normally from the engine so check the pipework in the engine compartment which is relatively easy to do. If its in the innards of the heater some dismantling may be necessary to effect repair. Sometimes a temporary repair can be made which will restore part function sometimes not.
 
Having read your post again it could be a duo valve problem. Its found on the w124 series in front of the brake servo not sure of its position on the w126. The assembly consists of 2 valves and maybe a small circulating pump. The valves control the return flow of water from the 2 sides of the heater heat exchanger. If they stick open then the heater stays on all the time. They can be repaired and there are posts on this. this one refers to a later model but will give you an idea. http://mbclub.co.uk/forums/showthread.php?t=26854&highlight=duovalve

and parts here
http://www.detali.ru/cat/oem_mb2.as...&L=765.706&CT=F&cat=569&SID=83&SGR=180&SGN=05
 
hot air and possible duovalve problem

Thanks for all the replies so far: seems pretty clear where the problem might be. I read on another site that when there's a fault the heating/ventilation system goes into a 'default' setting which blows hot air (why?). Maybe this is what the flashing light on the recirc switch is supposed to signify - but it would be nice to have an authoritative explanation.

And an update... after this 24 hour pause, I started up the car and everything seems normal (no flashing light, no hot air). But I haven't yet run it for enough distance to see what happens when everything warms up. Does make me wonder, though, if whatever went wrong on yesterday's hot and sweaty trip has has re-set itself again overnight.

We'll see what happens on a 300 mile trip tomorrow!

br1anstorm
 
If your temp controller is like this one below (used on many MB models and all interchangeable AFAIK) they get crook soldered joints on the pcb and where the pots join on.
Resolder normally fixes.
Some have black dials, some white.

Bazzle
 

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So there you go--- 3 possible culprits
1. heater control pneumatics due to vacuum failure/leakage
2. heated coolant duovalve failure or jammed.
3. temperature controller electrical connections

Will be interested to hear how you fared on your latest trip. Consensus would seem to indicate 2 --duo valve as most likely cause??
 
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Thanks Graeme... and others. My centre console set-up is similar to, but a rather older version of, the one in Bazzle's picture (the controls on mine are all separately installed rather than incorporated into a single plastic moulded housing). I tend to agree that a malfunctioning duovalve looks the most likely culprit. I wonder if such a fault might be occasional/intermittent (ie the valve only jams sometimes?).

And I'd still be intrigued to know how things are wired up to make the red light in the recirc switch flash - and why the owner's manual says nothing about this feature...

Will report in due course how things behave on the road over the next few days.

br1anstorm
 
The promised update... but the mystery isn't solved.

In the past week I've done a couple of 300-mile journeys - and there's been no sign of the problem. Temp controls work fine and air blows hot or cool through vents as it should. A/c switches off and on as normal. Recirc switch back to normal (when it's off, no light, when switched on, light comes on).

So I'm still puzzled - and inclined to believe that the flashing light/hot air syndrome is some sort of default setting when something in the system isn't working as it should. It's clearly a setting that's been "designed into" the system. So I'd still be interested to know exactly what triggered it - just in case it happens again.

PS - Have now found that post in another forum about the problem of the "blinking" recirc switch. It's in benzwag's post in the thread on the (US) benzworld.org forum. Here's the link http://www.benzworld.org/forums/w12...-blinking.html?highlight=recirculation+switch

I'm not sure I fully understand the technicalities, but I post the link in case it sheds any light on the subject...

br1anstorm
 
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I've bumped this 'cos I'm still curious.

Does anyone know what the flashing light in the recirc switch is supposed to signify, and what triggers it? It's clearly meant as a warning signal of some kind, which is why I'm surprised it isn't mentioned in the driver's handbook. Is it referred to anywhere in the official service manuals (of which I don't have copies)?

br1anstorm
 
Hi Br1anstorm, did you ever get to the bottom of this one?, mine has developed same problem, see new post
 

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