The monovalve diaphragm is to control the 'openness' when it is being modulated with voltage. The monovalve is a plunger type and is FULLY retracted when there is no external voltage. The plunger is controlled by a solenoid and comes down to close the valve. I think it is spring-loaded but I have not opened one up for a long time. The diaphragm may be torn but it should open and close OK and should not stop it from no heat.

I suspect

1) some of the vacuum pods is not redirecting/mis-direct the vent "if unplugging the monovalve with the car running and you do not have heat".

2) Is it possible the A/C is running even when you try to call for heat?

There is a lot of simple tests to do.

1) Start car and have heat on max. Measure the voltage across the monovalve connector, You should get 0V.
2) Feel hose behind the monovalve. It should be hot. The host return from car cabin should also be hot. I think the return is behind/near the oil filter.
3) Check compressor is running or not if the vent is blowing cold car. Feel the A/C expansion return hose to see it is cold or sweating.
4) Vacuum pod is a lot difficult to test as they are all behind the console.
5) Check for any vacuum leaks.

The heating system is very simple for the W126. I just want to give you some pointers. I do not want to waste my time to respond to incorrect info from other.

__________________
Not MBZ nor A/C trained professional but a die-hard DIY and green engineer. Use the info at your own peril. Picked up 2 Infractions because of disagreements. NOW reversed.

W124 Keyless remote, PM for details. http://www.peachparts.com/shopforum/mercedes-used-parts-sale-wanted/334620-fs-w124-chasis-keyless-remote-%2450-shipped.html

1 X 2006 CDI
1 x 87 300SDL
1 x 87 300D
1 x 87 300TDT wagon
1 x 83 300D
1 x 84 190D ( 5 sp ) - All R134 converted + keyless entry.

There are dozens (if not hundreds) of threads on this site that suggest just the opposite.


The vacuum pods have no affect whatsoever on whether the heat works or not.


That is a perfectly normal operating mode for the ACC. When working correctly, the heat will more than overcome the A/C.

From all indications, you also don't want to waste any time learning how the system actually works. But you are more than happy to waste other's time with your misinformation. Or is it disinformation?

First off I have owned the car about one month. The A/C didn't work until yesterday as it was low on refrigerant. I now need a switch to make my fan come on when A/C is on but that is a different issue, sigh. I have had heat issues since day one. When I first got the car I could unplug the monovalve and get full heat. That no longer works. I thought the aux pump might be an issue but I can feel the hoses and feel it running. I will test voltage. I did resolder the control unit and it seems to work properly at least in A/C mode. I can change vents and temperature. Sensor above visor even holds a tissue paper. Just no heat. Maybe it got an air bubble. I did change antifreeze. again weird thing, turn off hot engine, turn just the key back on and I have heat. start engine and it goes away.

Tango007 - With your experience, a 'Dummy guide to no heat' book is in order

__________________
Not MBZ nor A/C trained professional but a die-hard DIY and green engineer. Use the info at your own peril. Picked up 2 Infractions because of disagreements. NOW reversed.

W124 Keyless remote, PM for details. http://www.peachparts.com/shopforum/mercedes-used-parts-sale-wanted/334620-fs-w124-chasis-keyless-remote-%2450-shipped.html

1 X 2006 CDI
1 x 87 300SDL
1 x 87 300D
1 x 87 300TDT wagon
1 x 83 300D
1 x 84 190D ( 5 sp ) - All R134 converted + keyless entry.

Thank you both for info

I suspect your climate control may be reversed, when it calls for heat, A/C turns on. I am not sure whether it is possible. It could is electrical rather than monovalve. You mentioned removing the monovalve had heat before, so it is not a 'torn' diaphragm, and no clog in heat exchanger.

If I were you I would be incline to check for electrical issues. Good luck.

__________________
Not MBZ nor A/C trained professional but a die-hard DIY and green engineer. Use the info at your own peril. Picked up 2 Infractions because of disagreements. NOW reversed.

W124 Keyless remote, PM for details. http://www.peachparts.com/shopforum/mercedes-used-parts-sale-wanted/334620-fs-w124-chasis-keyless-remote-%2450-shipped.html

1 X 2006 CDI
1 x 87 300SDL
1 x 87 300D
1 x 87 300TDT wagon
1 x 83 300D
1 x 84 190D ( 5 sp ) - All R134 converted + keyless entry.

Kindly explain why removing the electrical connector at the monovalve had no affect. If the monovalve fails to open when the solenoid is completely deenergized, somethings wrong. And it has nothing to do with electrons.

The ACC never "calls" for A/C. The A/C compressor is engaged (or not engaged) as a function of ACC mode selection. Temp is then controlled by adding (or not adding) heat.

Good luck, indeed.

Think outside the box.

If you read #18. The climate control unit had/has issue. My suspicion is electrical otherwise how can heat goes away once the car is started.

__________________
Not MBZ nor A/C trained professional but a die-hard DIY and green engineer. Use the info at your own peril. Picked up 2 Infractions because of disagreements. NOW reversed.

W124 Keyless remote, PM for details. http://www.peachparts.com/shopforum/mercedes-used-parts-sale-wanted/334620-fs-w124-chasis-keyless-remote-%2450-shipped.html

1 X 2006 CDI
1 x 87 300SDL
1 x 87 300D
1 x 87 300TDT wagon
1 x 83 300D
1 x 84 190D ( 5 sp ) - All R134 converted + keyless entry.

With a torn diaphragm, coolant under pressure moves to the "dry" side of the diaphragm, where it overcomes the pressure of the hold-open spring and prevents the valve from opening. The classic symptom of a torn diaphragm is heat that works at low rpm and fails at higher rpm. There are countless threads in this forum that describe that condition.

One more time: if the problem is purely electrical, explain why there is no heat when the monovalve is not powered.

I certainly can't that is why I was baffled. First it worked a little then got worse. Luckily summer is here and the A/C is mostly working once I get a fan switch should be good to go.