FFFFreeeezing!!
 

Hi,
I have an 83 500sl euro with no heat.
I had the car for 14 years but didn't need it, mostly it sat. Drove it maybe 10 times a year a short distance to take my kids to Dairy Queen with the top down, when they were young. Unfortunately, as circumstances would have it. It has now become my daily driver. It runs great and I repair things as they come along.
However, the climate control has me baffled. Seems regardless of what I do I cannot get consistant heat, and its freezing. I have read tons of threads about this problem but have not found a definitive fix. I will have heat for a few days. I will then have no heat for a few days. I will have heat if I keep the rpms below 1500. I will have heat when driving along normally and then the air turns cold cannot get it warm again. I will not have heat, blowing cold air, turn of climate control, turn it back on and immediately get warm air. But who knows for how long.
I replaced the push button console probably 12-13 years ago. At the end of last winter I installed a new monovavle rebuild kit. Seemed to help, or maybe the weather just got warmer. Anyway since the problem was back. I did check and the diaphragm did have a small hole again. So I have just installed another monovalve repair kit within the last 2 weeks. I get tons of heat at idle and low rpms when I have the electrical connection disconnected from the valve. However normal driving or highway speeds just blows cold air. When the valve is connected, who knows? Sometimes it seems to work but, mostly I have no heat. I have also noticed at the times that I do have heat I hear a clicking sound from behind the console, as if a relay is kicking on and off. I should also note I have replaced the thermostat. The car does heat up and run at normal operating temperature. You can watch the temp gauge and see the temperature go up and tell when the thermostat opens.
I am an IT guy so I am analytical and understand how to test. Don't have a ton of tools though. The fact that I can get heat when the rpms are low and the valve is disconnected leads me to believe that the valve is funtioning. Also since there is no heat when connected. As I understand it the failure is an open valve so something is making it close. At this point I think that it is the push button console. However, I have removed it, opened the bottom and resoldered the connections. Although this did repair my cruise control, no such luck here. But, what is forcing the valve closed at higher rpms when the electronics are disconnected from the valve (is has to be vacuum, right?). This would point away from the push button console. What other items affect this climate control system? Temperature sensor? What is the Vacuum changeover valve? Does it just control the ducts?
At this point I just need to be able to get some heat. Is there a workaround? A way to force the valve to stay open?
Would greatly appreciate any insight.

Happy ThanksGiving to all!

If you wanted to migajig it you could just bypass it....at least for the winter.

Also...are all your vacuum lines and connectors in good shape?

and the temp sensor wire that goes to the top center of the dash has a foam insulation on it that disintegrates with time. At that point it is sensing the temp behind the dash and not cabin temp. Just make sure the connection is good and put some foam insulation around it.

Good point. Hose repair tutorial. 560 SL Cabin Temperature Sensor Hose Repair - Mercedes-Benz Forum

The foam hose had all but disintegrated. I performed this repair several years back but I will go back and check it. I doubted that this sensor could cause such a wide margin of error. I also assumed that if I turned the temperator wheel to max warm. That is just ignored the sensor and heat should be on all of the time. Any ideas on what could be making the valve close at higher rpm's and how I can prevent this.

Thanks again for all of the responses.

Again...are your vacuum hoses and connectors good?

You get the most vacuum at idle. Less vacuum at higher rpms.

I'm not exactly sure how all the flaps and valves open and close....but I do know it requires some vacuum.

If you have a breech in the system the higher vacuum at idle might be able to overcome it and conversely...not so much at higher rpms.

I'm not sure how to answer that. Everything that I know of, that is vacuum related on the car works. Except, for the passenger side door lock. Now, I have never investigated this to see if its a vacuum or mechanical problem. The little plastic door lock lever got broken several years back and as I recall that's when it stopped working.
So, vacuum is used to control the ducts, I get that. But when I'm driving along and warm air is blowing out of the vents and it just turns cold. What is happening? The monovalve has to be closing, correct. But the electronics are not even connected to it. I don't know where else to look. Is there a vacuum line that runs to the monovalve? I don't recall that.
Again, I appreciate the assistance and enjoy the holiday.

I dunno. I'm not HVAC expert because I've never really cared. I try to drive in fair weather. But the times I do drive in the rain or cold....I experience similar issues. If I choose heat while cruising....I get cold air. If I come to a stop....the warm air starts and stays on no matter what speed.

I was just giving you options to think about....one day I'll look at mine.

I found some posts on another site where people were losing heat on the highway. They don't specify a resolution but they are talking about a leak in the vacuum reservoir. These were on earlier years then my 83'. I don't even know if my car has one. Anyone have any thoughts or experience in this area? Has anyone lost heat at higer rpms and found to have a leaking vacuum reservoir?

I don't know about your specific car, but some cars have vacuum check valves in the vacuum lines which actuate the vacuum motors connected to the mechanical doors and flaps in the vents. If the check valve fails, when the vacuum drops during hard acceleration or idle, the vacuum motors revert to the no vacuum applied position. As stated above, a leaky vacuum reservoir, cracked hose, or cracked connector can all cause a problem. You really need a vacuum diagram to trouble shoot the issue.

Do you have heat in the Defrost setting? This is a "failsafe" that may work when other settings don't.

No, no heat in defrost mode either. When it goes cold everything is cold.

I still think it is under dash vacuum related.

There is a slew of vacuum lines and connectors under the dash.

They come through the firewall in places and there are check valves, as well.

Plus the vacuum pods.

Something in this thread is bound to help you: http://www.benzworld.org/forums/r-c107-sl-slc-class/1453268-107-vacuum-diagrams.html

Thanks for the info.
I'm kind of leaning towards a vacuum problem myself at the moment. I did find a diagram and some info on the CD. Need to get out and get a vacuum pump/tester. Haven't had a chance to do that yet. Appreciate your input.

Well, as I've tracked down vacuum leaks and corrected them my idle is smoother than ever and the car is running great. However, it's colder then ever as well. :) All of the the vacuume leaks that I have found have been under the hood. But, where with the monovalve disconnected and I used to drive around at low rpms and have heat. Now I have none. Even at low rpm, and the car is at operating temperature. If I turn on the heat I will get very warm air. Within I would say 20 seconds the air turns cool. Turn of the heater for a few seconds and back on again. I'll have heat again for 20 seconds. Do these symptoms ring a bell for anyone??
Thanks again, everyone.
Happy New Year to all!