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Hello,

could someone point out where the heater valve is for a 450 SL, the controls inside the car seem to be inoperative and there is hot air blowing in...

It's summer here and it's very hot now, if I could temporarily just close off the valve it would be great.

Thank you

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The failure mode for the vacuum actuated valve on the firewall is for the heat to be on. If you can't figure out how to get it to stay off, you won't be the first person the put a plumbing valve in the hose. :)

Moving to r/c107 forum


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Hello,

could someone point out where the heater valve is for a 450 SL, the controls inside the car seem to be inoperative and there is hot air blowing in...

It's summer here and it's very hot now, if I could temporarily just close off the valve it would be great.

Thank you

Sent from my EVA-L19 using Tapatalk
If the control of the 74 is the same as my 73.... The slider on the far right has a micro-switch which operates at the very end of the downward travel of that slider. If that adjustment is out that could cause your problem. If I recall, removing the glove box and the trim panel under the dash will give you access to that slider and you should be able to manually activate the switch or bypass it too test.
 

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The quick and dirty way is to simply block off the hot water inlet hose. Either plug it or squeeze it. No negative side effects. Then you can trouble shoot.

Edit
It only has low pressure but you need to make sure the plug is well secured
 

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If the control of the 74 is the same as my 73.... The slider on the far right has a micro-switch which operates at the very end of the downward travel of that slider. If that adjustment is out that could cause your problem. If I recall, removing the glove box and the trim panel under the dash will give you access to that slider and you should be able to manually activate the switch or bypass it too test.
I believe both outboard sliders have micro switches (my 72 does). If either outboard lever is anywhere other than full down position, the vacuum actuator for the valve is commanded open.

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The quick and dirty way is to simply block off the hot water inlet hose. Either plug it or squeeze it. No negative side effects. Then you can trouble shoot.

Edit
It only has low pressure but you need to make sure the plug is well secured
I am glad I saw this thread. My 1972 350SL is having the same issue. Just throws heat, mostly at the feet, especially when moving. The levers don't work and I am not interested in fixing it in any complicated way -- its a summer fling car, driven only with the top off. I don't need AC, or any airflow whatsoever to the cabin. So... quick and dirty sounds good to me... but can you help me identify the "hot water inlet hose" so I can pinch it off?
 

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If yours is original it connects to the metal valve pictured. You can manually close the valve.
 

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If yours is original it connects to the metal valve pictured. You can manually close the valve.
Great. I looked and found this (pictured). So, no downsides to closing that valve? And if it's not original or there's no way to close it, am I to understand that this is where the warm air flowing to the cabin is coming from? Thanks.
 

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Those are original parts. That is the heater control valve. If your system was operating as it should the vacuum solenoid on the left of it would open and close it. There should be two vacuum lines connected to the solenoid.

To answer your question. There is no downside to closing the valve to keep hot water from entering the heater core.
 

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That valve is what controls the water flow to the heater core.

One other thing you might check is the large grommet just outboard and a bit lower than the valve. It is a firewall feed through for wire bundles. I found mine rotted out and was allowing hot engine air through into the cabin.


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