Two thoughts on heating issues
 

I had some heat trouble with my 300SDL recently, but all has been fixed. After searching tirelessly for some answers, I may have some for you if you ever suffer this same situation:

I was having the trouble of "no heat at high rpms". Basically I had heat at idle / below 45mph / under 2000 rpm. I replaced the monovalve with an MTC unit and that did not help.

Well, turns out that I had a very loose (but still connected) hose coming from the tee above the BFS that goes to the vac reservoir. Check that line! It cured my problem right away. It is also worth mentioning that my CCU is no longer clicking on and off relentlessly, although I don't know why vacuum would affect that, I just thought it was bad solder joints. I'm guessing that there was not enough vacuum available at higher rpms to keep the mixing flap open for heat.

Secondly, while trying to solve this issue, I found that many have had issues with the MTC valves being too weak or fitting too tightly. Did you all know you can disassemble the monovale quite easily? I ended up just taking the diaphragm from the MTC unit and putting it on my original. You just need to remove the small c clip and everything will come off. With those only being $15 or so, it's still worth it IMO for the new diaphragm.

Anyway, hope this helps someone. Winter is coming!

Another way to fix heat issue is to move to a temperate climate like ours. :P

Good idea about checking the vacuum hoses! :thumbsup:

Also, the inner seal has a clip just like the diaphragm. So, all three rubber bits then can be new. I however didn't for not the right reason.

Please explain all of those initials such as CCU and MTC. I am interested in what you are saying and I am an old engine builder and racer, but I don't know the jargon. Thanks

Richard

Richard, these are regularly used and never seen spelled out. So, guess we never stop, as no one specifically asked for definitions. Bet if you searched, you find. :P

CCU is climate control unit, which is the device with the selector buttons.

MTC is the manufacture of the rebuild kit for the heater valve, which is controlled by the CCU from the temperature wheel setting. All modes except ECC (economy, or compressor off) use the compressor to dry the air. In all modes except ECC and defrost, the amount of hot water to the heater coil is adjusted for temperature based on temperature called for. Meaning, dial set to full cold, nothing going to heater coil and heater valve fully closed and set to fully warm, heater valve fully open. In all modes, the cycle of on/off is regulated by the temperature wheel, so as not only getting the correct supply, but maintaining. In short, the CCU is very complicated, but obtains comfortable and safe cabin environment (versus other systems with no logic, just off and on).

Hope this helps. :D

BFS is the "blue flying saucer", which is a transmission vacuum modulator type of thing on the driver's side fender wall. I believe w123s have this in addition to w124s and w126s