heat always on 1973/450sl - PeachParts Mercedes-Benz Forum

zeggate · #1 07-07-2003, 06:58 PM

Even with the air temp and levers in the off and closed positions I get heat in the passenger compartment. Is there a valve in the engine compartment that will shut off the heat . Any pictures,thanks

jerryb · #2 07-07-2003, 07:54 PM

Under the hood, center of the firewall. Follow the hose. There is a vac. controled heater valve, just ty it off in the closed position.

Darrell Woods · #3 07-08-2003, 10:09 PM

JerryB, I've been having the same problem. If I found the right parts, it is actually two pieces. There's a plastic, vacuum operated unit (which I think is called a monovalve), and that is connected by a plastic arm to an in-line metal unit that I assume is the actual shutoff gate to stop the flow of coolant. I thought about replacing the monovalve, but decided to tie it closed as you suggested. When I did, it started leaking coolant at the in-line shutoff valve. When I let it return to the open position, the leak stopped. Any thoughts as to whether I am just forcing the arm too far, or do you think the in-line shutoff is defective? OR, am I totally confused and don't even have the right parts identified?

zeggate · #4 07-09-2003, 03:39 PM

To Darrell Woods ,what year car do you have I don't believe I have a monovalve. My car is a 1973 model

Darrell Woods · #5 07-09-2003, 07:07 PM

Zeggate, it's a '75, and I may be calling it the wrong thing. As described above, it's a two-part setup. I did find it on my repair CD. MB calls the plastic piece simply a "Vacuum Element", and the stainless part that's actually in the coolant line a "Heater Valve". When you put both outside levers (on your dash) in the down position, they actuate a microswitch (interrupts power) which in turn tells two vacuum control valves what to do. Working correctly, the vacuum will move an arm on the vacuum element, closing the heater valve. Not sure if yours is the same or not. I'm trying to attach a picture of my controls, and of the parts on the firewall. If it comes through, the Vacuum Element is on the left, arm goes to heater valve on right. This picture shows "heat on" position. For heat off, arm would move to left toward vacuum element. You can only see a small piece of the arm in the photo. If yours is similar, you might be able to push the arm to closed and tie it off. Mine leaked when I tried it, so I'm ordering both replacement parts and hoping the problem is not in the microswitch. Hope this helps.

Darrell Woods · #6 07-09-2003, 07:09 PM

Sorry, missed the controls inside.

And if anyone is curious about the "thing" with a green and red light and missing center, I think it was at one time a security alarm.

jerryb · #7 07-11-2003, 08:14 PM

Darrel, you have the right part but it's not a monovalve, that came in later cars. If it leaks you will have to get a new heater valve, the vaccum valve and the water shut off come as 1 piece and are not very expensive.
You could also join the 2 hoses with a piece of pipe to bypass the valve.

Darrell Woods · #8 07-11-2003, 09:51 PM

Thanks JerryB, you're suggestion to Zeggate to tie off the valve got me to the right parts. I plan on ordering the new parts this weekend.

slsmag · #9 07-12-2003, 12:47 AM

Quote:

Originally posted by Darrell Woods
Thanks JerryB, you're suggestion to Zeggate to tie off the valve got me to the right parts. I plan on ordering the new parts this weekend.

The heater control valve is inexpensive and easy to replace, but it will only work properly if you have good vaccuum - so while you're in there you might want to use a mighty vac to measure the vaccuum on the line. (I don't know what the spec is but maybe someone else here can look it up for you).

- michael

zeggate · #10 07-21-2003, 11:27 AM

Has anyone found the part number for the heat valve for the 1973/ 450sl. thanks

tommy76Benz · #11 07-22-2003, 02:01 PM

Hey Darrell,

My 74 450SL is missing the lower A/C knob from the control panel like the one in your pictures. My cab also fills with hot air, but without this knob, I don't know if my problem is with the vacuum units that you guys are talking about or if it's just a problem with not having the A/C knob? Do you know where I can get this knob or what the correct name for it is? I've checked Fastlane, but the pictures and descriptions for the A/C selector, don't look like the right one. Any help would be great because I would like to drive my car sometime this summer without melting! p.s. I don't have an owner's manual for this car, other than Ebay, do you know where I can find one?

Darrell Woods · #12 07-22-2003, 07:56 PM

TBenz, you can try: http://www.autobooksbishko.com/mercedesmenu/mercedes.htm
Haven't ordered from them myself, so can't vouch for 'em. I believe the Classics section of MBUSA also has manuals. Take a look there. There are several reputable parts suppliers on the net. If I can find the knob, I'll let you know. In the meantime, you can see from the photo that max cooling is clockwise...just turn yours with some small pliers and see if it helps.