Mechanics

MERCEDES BENZ 450SL ENGINE PROBLEM

1980 Mercedes Benz 450sl • 75,000 miles

1980 450SL with digital heater control valve installed to replace original equipment vac/elec junk one! Any button pressed except defrost, quits when power applied to engine. Have to press def then the button for whatever function desired to get it to come on.

A/C & heater work but will stop when accelerating. If leaving from a stopped position, seems to quit around 2500 RPM. If driving at speed, seems to quit as soon as acceleration begins (i.E. Passing another vehicle, going up a grade, etc). Was the same with the original vac/electric valve. Sometimes it will return on its own but to get to come back, you have to press the defroster button then the button for the desired flow (i.E. Auto hi, auto lo, etc).
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Check the wiring and connections. Including the grounds. Does it happen when revving the engine while standing still?
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Rivermikerat
Feb 21, 2012.
Thanks for responding. No, not while revving the engine. Again, I had this identical issue with the original vac/elec valve. So in my thoughts, it's not the digital valve itself but maybe vacuum source supply, vacuum tank, etc. However it mystifies me because the flow stops and the blower quits also? Then when pushing the top (defroster) selector button, it takes right off and cools/heats. I love the car and it's fun to drive-just wish it wasn't so "over-enginered" in some vacuum/elect areas! Now, off to my current repair-fiber optics! Thanks again from a 71 year old guy who spent almost 50 years in the auto dealership parts business.

I'm a certified fiber optics splicing technician. And a mechanic.

See if you can find some specs as to minimum required vacuum for the system to work. Engine vacuum decreases under acceleration. You may need a vacuum amplifier inline.

Rivermikerat
Feb 21, 2012.

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