140 Climate Control Logic

[wbrian63]

I read a thread last week here on the forum where someone detailed the logic behind how the climate control worked (But I think it was for a 126-chassis).

By this I mean, when you select X on the dial and the cabin temp is Y, the climate control will do Z.

It would be very helpful to know what my car's system is supposed to do at various combinations of settings/values. I've got the MB DVD for the car, is that info out there?

Thanks!

[softconsult]

Care to share the year of your W140.

It's detailed in the Owner's manual, but maybe you don't have one.

If not, then PM me with your e-mail address and I will pdf the appicable pages.

Mine is '97S320.

Steve

[softconsult]

Saw your second post with Behaviour in the subject line.

Obviously, you want more technical info, than the owner's manual provides.

The outside temperature reading, in my experience, with W124's and current W140, is that it sometimes reads higher than outside air temperature. Hot parking lot, engine heat, sunlight beating down on the front bumper, contribute to the difference.

Steve

[wbrian63]

You are correct - I want the "inside skinny" on why it does what it does.

BTW - the outside temp sensor for the indicator in the instrument cluster is different than the one for the A/C, at least on my '92.

The instrument cluster sensor is attached to the tow-hook door on the driver's side.

The A/C sensor "plugs" into the A/C fan "gallery" under the hood right next to the ABS pump. That's the most accurate location for gauging the temp of the air being pulled into the system. That's why it's so important to have good seals for the hood, especially the one that runs across the car from fender-to-fender. Without that one, hot underhood air can be pulled into the system.

I think the only purpose that sensor plays in the "climate control game" is to determine whether or not the compressor should run - I believe system programming turns it off at temps below 32? Anything above that and it runs as needed by the CC unit.

Most of the programming logic I've seen on automatic climate controls (across all makes, models and years) doesn't require any input from outside temps to make the system work correctly. They "do what they do" based on requested temp settings and internal temperature readings. The farther apart the 2 numbers are, the more aggressively the system runs. I'm just trying to figure out what the system should be doing when the various sensors and settings are in a particular relationship to one another.

[68882]

Ok well the w140 system has a temp sensor on the evaporator to judge how cold it is, plus two temp sensors on each heater box to judge how warm the air actually is, plus the temp sensor in the overhead lamp unit to judge cabin temp and an outside temp sensor in the air intake box, not to be confused with the sensor on the bumper for the dash display. This all makes the system work so it does rely on that outside temp reading.

Now from experience in my w140, if someone removes the air intake box,say to service the wiper system, and drops the sensor under the box versus putting it back in the box, the sensor will more likely measure the engine air temp, versus the air temp of incoming air. This makes the climate control unit think outside air is hot and then it ensures there is a slight amount of cold air coming out of the upper dash area to cool it down. Which freezes hands on steering wheel, yet air from other vents is fine. Very subtle yet real. You can use the w140 climate control diagnostics to see the values from the different sensors and confirm the readings are rational. In my case although outside temp was 10C, the sensor would say 42C, so we thought it was faulty until we discovered it was not where it should be.

In my w124 if some idiot cell phone installer breaks off the tab holding the passenger side temp sensor in the heater box and then stuffs it under the transmission tunnel carpet, the system will try to make passenger side hot by asking the duo value to open more on the passenger side, but doesn't see it getting hot in the heater box, and since overall temp goes up based on the sensor in the dome lamp it will cool down the driver side to balance. The dual temp zones then fight. So hot hot on passenger side, freeze feet on driver side. Find sensor, replacing sensor, and put back in heater box, fixes issue.