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1974 (W107/R107) 350 SL
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6 Posts
Discussion Starter #1
1974 350SL while using the air conditioning and as the revs. drop as when entering lower speed limit the demister turns on. Even if slowing in traffic the same happens. I have replaced the vacuum hoses,only under the bonnet, and all the fittings but still the problems persist. At the moment the heater tap is tied in the off position but this is inconvenient in the rain when the window mists up. Any ideas as to my “hot” problem.
 

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1972 220 D, 1974 450SL, 1995 E300D 2002 S430, 1976 300D, 1986 SDL, 1982 SD
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2,708 Posts
Please complete your profile so somebody is able to assist you. No idea where you are and which model car, so any opinions given aren't to worthless.

best guess is that the vac pod has failed and default mode is for heat
 

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1974 (W107/R107) 350 SL
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6 Posts
Discussion Starter #3
Please complete your profile so somebody is able to assist you. No idea where you are and which model car, so any opinions given aren't to worthless.

best guess is that the vac pod has failed and default mode is for heat
I thought 1974 350SL might have giving it away! In oz where it is starting to heat up.
Having trouble logging in so I will update my profile once again.
Thanks for your reply.
Col
 

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R/C107 Moderator
1986 560SL: '84 500SL: '84 280SL 5 speed: other 107s
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32,488 Posts
350SLs are everywhere.
To control the heat you need a new valve. I think the '74 uses the black plastic one with one vacuum line. Even if it is the old two part valve the new one can be used.
 

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1974 (W107/R107) 350 SL
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6 Posts
Discussion Starter #5
Right start again! 350sl. 1974. Heater tap replaced with single vacuum inlet. This has two solenoid on the firewall. Vacuum from manifold to tee one side to vacuum tank under drivers side guard. The other to top of left hand solenoid. y piece to levers in dashboard.
Vaccum hose bottom of solenoid to heater tap actuating lever, normally closed?
Levers left or right operate to heater tap. Works as it should while at speed but as soon as I back off the vacuum increases and the heater comes on overcoming air conditioning.
There are one way valves in the lines as well. Today I replumbed the hoses to remove the second solenoid to try to uncomplicated the system but same result. Has anyone got a simple line diagram of the heater vacuum circuit? The door locks, boot and fuel flap works as they are supposed to.
photos are nearly impossible hence the long winded description.
Help please!
 
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