BenzWorld Junior Member
Date registered: May 2004
Vehicle: 1990 560 SEC
Location: United Kingdom
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RE: 450 SEL Issues
I had a 450SEL for about 11 years, and had the same problem as you, but although mine had aircon this was not the automatic type, so my solution may not work, but it's worth a look.
On mine, when the heater switch for either passenger or drivers side is moved up, this moves a vaccumm switch which goes through the firewall and directly to another vacumm switch next to the heater pipe. This switch literally lets the flow of hot water from the engine into the heater matrix. This switch can corrode over time, but can be freed with a little WD40. Look for a heater pipe going from the engine to inside the vehicle. From memory, this pipe on my car was located at the top of the engine bay around about where the the wiper motor is situated. Look for a pipe with a vacumm unit connected. The vacumm unit will have an small arm on it which fits inline to the heater pipe.
I found that the heater switch inside the car was working (you can hear the vacuum hissing if this is working), but as the arm was corroded there was not enough air pressure to move the switch. Mark the position of the arm, start the car. move the temperature completely in the opposite postion and see if the arm moves.
This is at your own risk, but I did find I could move the arm "manually" until I freed the vacumm unit up. This should block the flow of heated water into your matrix.
As stated my 450 was slightly older and UK spec, but it did have aircon, so I am presuming the heater setup was similair.