I have a problem with my 85 300D I have never experienced in my 30+years of driving MB diesels! When I push the button to turn on the AC compressor, the vents switch from center to side vents and I have heat. When I push the EC button, I get fresh (non-heat) out of all the vents. This occurs even though the thermostat setting is on cool. Again, cool fresh air comes out nicely until I turn on the AC & I have heat? If I go from AC button back to EC, fresh air returns to all vents (with no heat). The only way I can get the AC on is to move the thermostat wheel to minimum. Is this a problem with push-button main unit? I don't think it is with the mono valve as I have fresh non-heated air with the EC button on? Any ideas why it would switch to heat with the AC buttons?

Richard

It's all but impossible that setting the temp wheel to MIN turns the a/c on. What is most likely happening is that that setting is the only thing (in the selected mode) that turns the heat off. The a/c compressor should engage in all modes except EC and OFF, regardless of temp wheel position.

All the vents, meaning windshield, dash and foot vents?

I was not clear about "all vents". When I meant all vents, I meant the center vents and side vents. Air is NOT coming out of the windshield defrost vents and not coming out of the floor vents. The system is working perfectly in that when heat comes on, the center vents close;when fresh air or AC is on, all the vents blow air (center & side vents...not defrost or floor vents). I have checked the compressor and indeed it is on when the the AC buttons are pushed and is on when the system switches to heat and to the side vents. When I move to the min setting, cold air returns as the compressor continues to work. Just not sure why the system switches to heat when the AC buttons are pushed and the compressor is running?

This is a monovalve system????

__________________
Did you just pass my 740 at 200 kmh in a 300SD?????

1978 300SD 'Phil' - 1,315,853 Miles And Counting - 1, 317,885 as of 12/27/2012 - 1,333,000 as of 05/10/2013, 1,337,850 as of July 15, 2013, 1,339,000 as of August 13, 2013



100,000 miles since June 2005 Overhaul - Sold January 25th, 2014 After 1,344,246 Miles & 20 Years of Ownership

Found the problem. It was the temperature control box (silver about the size of the cruise control amp). I replaced it with a another unit and now the system is back to normal. This was AFTER 3 hours of testing/replacing the push button unit, temp. sensor in the roof and testing the side control pods which directs which vent to open/close! Hope this will help someone else with same problem! This is the first time in my many years if driving MB the temp. control box has gone bad!

Where is the temperature control box located? Was it easy to get to/inspect?

__________________
Spencer

87 2.5L 190D Turbo 5-speed - 102k (Daily Driver)
82 300D Turbo - 145k - Light Ivory (near mint)

Gone: 85 300D Federal - mileage unknown