W126 heater, aircon and recirculation controls

[br1anstorm]

I have a 1990 W126 560 SEC, and need help in diagnosing a problem with the heating and ventilation system.

The car has the system with two rotary thumbwheels for temp setting; the usual two rotary controls for fan speed and directing the airflow; a rocker switch for aircon; and a rocker switch for 'recirculation' - ie it does not have the row of pushbutton switches for the fully automatic/electronic setup.

On a trip yesterday, with aircon off and temp set to roughly normal, I noticed that the dash vents were pushing out pretty hot air - and then noticed that the little red light in the 'recirc' switch was flashing on/off regularly. The owners manual says nothing about what this might mean, and the light continued to flash when i tried to switch the recirc on (or off!).

Switching the aircon on, or adjusting the temp controls, made no difference: hot air continued to come through dash vents (and footwell or screen if I changed the direction settings).

Even with the roof and windows open it was a sweaty journey home...

Can anyone suggest what might be the problem and more especially, the solution? I have a 600 mile trip planned next week which will not be fun if the problem persists. The symptoms suggest that the temp controls (for both sides of the cabin) are somehow stuck on max heat. But what exactly does the flashing light on the recirc swich mean?

Any advice would be appreciated...

br1anstorm

 

[Legoman]

aircon needs a regas afaik. either that or the recirc is stuck closed so your not letting in fresh air,.

 

[br1anstorm]

Thanks Legoman, but I suspect the problem lies elsewhere. If aircon was low on gas, then the system simply wouldn't blow seriously cold when the a/c was switched on and temp to "min". But lack of gas in the a/c would not result in the vents blowing hot irrespective of temp setting on the rotary dials; nor would it (afaik) cause the recirc switch light to flash...

And I don't think the symptoms suggest the recirc was stuck closed. Although the vents from the heater/ventilation system at the end of the dash and in the footwells were blowing very hot air, the "fresh air" vents along the dash were blowing ambient air in from the outside - which wasn't that cool, but clearly hadn't been heated up any further by the car's own system.

And the sequel to this story is that on my next trip a couple of days later, the entire system (aircon and heating/ventilation) worked perfectly normally, and the recirc switch light wasn't flashing! So I'm still mystified. I have to assume that the weird behaviour (hot air and flashing light) was some sort of default setting. But why, and what did it signify?

Any further thoughts would be welcome - just in case it happens again...

br1anstorm

 

[br1anstorm]

It's a long time since i started this thread... But belatedly, I ought to record the explanation and answer for the odd behaviour of my 560 SEC's heating/cooling/aircon system.

This happens when the monovalve/duovalve doesn't function properly. Blowing hot air is set as the default mode (don't ask me why) and the blinking Recirc switch light is the signal. There's no reference to this in the handbook (why not?). I found the explanation on a US Mercedes owners' forum, and there's a useful illustration here http://mbdieseldiy.tripod.com/monovalve.htm
of what to look for and how to fix it.

br1anstorm