450 SL 1974: Heater always on. - Mercedes-Benz Forum
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post #1 of 13 (permalink) Old 06-24-2017, 08:16 AM Thread Starter
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450 SL 1974: Heater always on.

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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post #2 of 13 (permalink) Old 06-25-2017, 06:42 AM
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Try the R107 forum
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post #3 of 13 (permalink) Old 06-25-2017, 08:06 AM
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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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post #4 of 13 (permalink) Old 06-25-2017, 12:11 PM
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Quote:
Originally Posted by S320L1999 View Post
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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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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post #5 of 13 (permalink) Old 06-25-2017, 02:54 PM
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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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post #6 of 13 (permalink) Old 06-27-2017, 03:10 PM Thread Starter
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Great, thank you. I've opted for keeping the valve shut off using a zip tie.

Thanks again


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post #7 of 13 (permalink) Old 06-27-2017, 04:18 PM
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If you go to Startekinfo, there is a section under 107 workshop manual that describes the climate control system.
https://www.startekinfo.com/StarTek/.../74450sl.html#
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post #8 of 13 (permalink) Old 06-28-2017, 05:53 PM
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Originally Posted by Ianwright40 View Post
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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post #9 of 13 (permalink) Old 07-13-2019, 09:19 AM
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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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post #10 of 13 (permalink) Old 07-13-2019, 11:21 AM
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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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