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2001 SL320
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51 Posts
Discussion Starter #1
I can't turn the heat off on my 1985 UK 500SL (with no a/c).

Just had a new vacuum tank fitted (the old one fell off) so now my central locking works but the temp sliders have no effect. I checked the diaphram on the heater valve on the bulkhead & a "suck test" closed the valve. I zip tied the valve closed but there's still heat coming in to the cabin via the footwell vents. Is there another valve I'm not aware of?

You may think excess heat wouldn't be a problem in North East England but global warming & all is making anything but short journeys a pain.
 

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Premium Member
1983 380SL, 2000 S430, 1991 420SEL (retired) - RHD
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5,533 Posts
By "footwell vents" do you mean the lower heater box or the fresh air vents?

You can't get hot air via the heater box without hot water flowing through the valve forward of the scuttle. Try clamping the hose between the valve and the scuttle. If you are still getting warm air then it's coming from the motor. Maybe you have holes in the scuttle or grommets missing somewhere.
If no hot water gets through it shouldn't matter where the levers are.
 

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1983 380SL, 2000 S430, 1991 420SEL (retired) - RHD
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5,533 Posts
The red vac line from the valve should connect to lever 1, then another one goes from lever 1 to lever 4. Both the levers s/be down for no heat.There is a thread on lever adjustment in the sticky section.
 

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1981 380SL
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256 Posts
Also, there are two vacuum reservoirs. One for central locking, and another for the HVAC. The HVAC reservoir is found behind a panel by the driver's left foot.

Each system operates from engine supplied vacuum through an individual check valve. Perhaps something got pulled loose and you have no vacuum to the HVAC controls? Check the vacuum connections under the bonnet by the brake booster.
 

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Premium Member
1983 500SL, 1984 500SL, 1978 450SLC 5.0, 1979 450 SLC 5.0, 1980 450SLC,1999 ML430
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37 Posts
Your monovalve servo may have died in the open position. When this happens hot water is constantly circulating through the heater element. Unfortunately, you can't just by the servo or relay, you have to buy the entire unit. You could try a wrecking yard for the servo.
 
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