-88 560 cabin heater problems

Hi, all.
The heater in my car live its own life. Warm, then cold, independent of the settings.
I have repaired the mono valve, with no change. I suspect it is the vacuum system which is failing. I have studied the vacuum diagrams, but I dont know where to start. Is it anyone out there with tips on how to proceed, or where I should look first?

Rgds Erik

Hello Erik,

as first you have to check if the temperature wheel is locked on Maximun or Minimun, it must always heating, or coolling. In these case the regulatio is overruled.

In all positions between the temperature regulation is working. If is not correct working, i think the inside temperature sensor, will not work correkt. The most problem here is a short foam-tube form the inside temperature sensor to a plastic tube behind the glove box. The plastic tube sucks air thru the temp-sensor to measure the correkt temp-value.
If the foam tube is missing, the sensor measures the temperature under the dashboard. And these temperature increases with the car is running, during all the components generates heat.

The temperature sensor is mounted in the dashboard over the outside temperature indicator. And the plastik tube, you see in in the picture here:
sternzeit-107.de/de/forum/forum-107/2475...-schlauch-hin#261740

Hi Hans,

thanks for fast reply.
I have tried with the wheel in all positions, also on max and min, without any difference.
I have also replaced the dash sensor you mention, as it was broken. The "new" sensor was second hand, so i dont know if it actually works.
I am told that the air pods under the glove box compartment, or in the dash generally, maybe are leaking, so I wonder how to test these without taking out the complete dash...

Rgds Erik

Hello Erik,

sorry, in this case i can´t help you. Right behind the glove box, the ECU for temperature and venting regulation is mounted, but I don´t know how it works and what it do in case of malfunction.

Hi, ok thanks for the tip. Maybe I should check this first.

Rgds Erik

Hi Erik,

regards the air-pods - the various air-pods do only have the function to steer airflow through the various outlets or if fresh air from comes from the outside vs. circulation inside the cabin.

Even if non of them in function - temperature should be constand and not fluctuating.

Regards,
Ralf

Hi Ralf, you have a good point there. Maybe it is the ECU for temperature and venting regulation as Hans mentioned, or the electronics in the adjustment wheel which is bad.

Rgds Erik

Hi Erik,

being the author of Reparing mono and duo valve I am suspicious whether you have used previous MTC 3065 to replace valve actuator in mono valve and not just diaphragm. Your symptoms sound like that.

Remove connector from mono valve and apply 12V. It must close then. When you drive it must stay cold. If it does not, you can clean mono valve inside and if it does not help, you have to find an original actuator and install only new diaphragm on it.

Hi Dr-DJet,

I purchased a monovalve repair kit, the MTC 3065. I have tested the movement with and without 12V, and it is moving up and down accordingly. The hoses are warm, both before and after the monovalve. I have driven with the monovalve disconnected from 12V, but still problems with the heat. Does the ECU play a role here? or the electronics in the dash, with the regulating wheel?
I will have a look at your article.

Rgds Erik

Dear Erik,

the Monovalve is a complex hydraulic part. For correct function the pressure drop across the valve is very important. Also the pressure above and below the diaphragm is key. I had a closing monovalve without any electrical actuation in my car just caused by the pressure ratio across the valve at high engine revs. The only cure was using the factory valve (instead of aftermarket replacement) with a new (aftermarket) diaphragm.


Regards Andreas

Dear Andreas,

based on your comment, and also from the others, it sounds like the problem is the monovalve. The aftermarket MTC 365 was not original, so it is very possible that this is the problem. When I bought the car, was the monovalve unplugged. The small aux pump was not there, so I installed a new one. But it did not make any difference.
Can it not be a vacuum problem as well?

Rgds Erik

when you unplug the monovslve electrically, you must get hot air at all engine speeds. Then you have no influence from electrics or vacuum. If this is not the case, you need to work on the monovalve.

I do not see a reason to question the vacuum system now. First you need to be sure on the monovalve

Regards Andreas

Hi,

forget the MTC 3065, it never worked hydraulically. You have to follow instructions in my article to install a proper daiphragm on old original actuator. Your problems are know to be caused by MTC 3065. That is most probably the reason why it is not on the market any more.

Hello Erik,

When I bought my 560SL in 2018 the monovalve was leaking pretty badly due to a fitted MTC whose diaphragm was squeezed and rimpled (not torn though). In Volker's heating workshop I was able to buy another MTC with the objective of stopping the leakage. However, that was more difficult because the wavy washer included in the set is flatter than the original one and hence, another regular washer was required. After the leaking stopped, the valve worked to some extend, but stopped usually after some time which was a nuissance on longer trips to southern vacation spots where the aircon was used during highway driving.

Since a new trip is foreseen in summer, I had decided to fix the issue as in Volker's description, and purchased a few used duovalves from the W126 with functioning "Stössel" but torn diaphragms and used üro kits to fix them. After mounting the fixed stössel it worked instantaneously and without leaking since the original washers were also included.

I wouldn't mind providing one of my refurbished valves (please write me a private message if interested) but in order to prevent a disappointment you need to be sure that the electric unit is still signaling the monovalve to close. There's a nice description of how on this site, but personally I simply checked the voltages while going through various settings of the temp. wheel. At the extremes of the wheel there should be continuous full voltage (cold) or little voltage (hot), but intermittant pulses should come from intermediate settings like 22 C for example. If this isn't working properly, changing the monovalve will also not lead to a fully functioning system.

Hope this helps, cheers,

Maurice