I just bought a '79 450 SL w/ 68,000 miles. Driving home, it seemed as though the heat was on and blowing on my feet even though the controls indicated that it was off. Is this common or does a vent need turned off?

I just bought a '79 450 SL w/ 68,000 miles. Driving home, it seemed as though the heat was on and blowing on my feet even though the controls indicated that it was off. Is this common or does a vent need turned off?

My 76 450sl was doing this as well. What turned out to be the issue was the vacuum controlled valve that lets hot water circulate the heater was not closing completely. I cant remember the name of the part. Its located against the firewall just above the right valve cover. The part was east to change and not very expensive. I hope this helps solve your issue.

I just bought a '79 450 SL w/ 68,000 miles. Driving home, it seemed as though the heat was on and blowing on my feet even though the controls indicated that it was off. Is this common or does a vent need turned off?

Most likely a bad ACC Servo, but it could also be a vacuum issue.

My 76 450sl was doing this as well. What turned out to be the issue was the vacuum controlled valve that lets hot water circulate the heater was not closing completely. I cant remember the name of the part. Its located against the firewall just above the right valve cover. The part was east to change and not very expensive. I hope this helps solve your issue.

Here is an image of the valve I am talking about:
The lever to the heater connects via vacuum hose and controls the opening and closing of the valve letting more or less hot water through. In my 450SL the valve wouldn't close completely and it caused the heater to be on all the time.

http://www.benzworld.org/forums/attachments/r-c107-sl-slc-class/310852d1275224205t-ac-1972-450sl-heater-connt.jpg

^ On your car, it's a $25 part; on his, it's a $700 part called an ACC Servo.

Thanks for the info from you all. I think I might wait a while to replace a $700 part. Luckily I usually only drive with the top down and can't feel the heat.