I think I figured out how the AC is controlled on this car and was hoping that someone could confirm whether I got it right. As I understand it, the AC compressor is an on/off affair. If the system calls for full cooling the monovalve stays shut and the compressor stays on. If the system calls for cooling at less than full cooling capacity, it turns the compressor on and then modulates the monovalve open to let some hot water go through the heating coil. If the system calls for no cooling or for heat, the compressor turns off.

Am I close?

I am not concerned with how it controls the compressor for defrosting, I just want to know what happens with respect to cooling and heating.

On the '85 the only things that will drop the compressor out is low freon pressure and WOT cut out. The low pressure switch is located at the dryer and the WOT cut out is the microswitch located on top of the valve cover in front of the throttle linkage.

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Jim

Now that ain't true and we discussed it.

The original statement is sort-of correct. The exception in both cases is that whenever any button but "EC" is pushed the refrigeration is engaged. This doesn't not mean the compressor stays on always. It means that the compressor is solely in the hands of the ETR switch. As such the evap is kept to a temp probably between 35-40 degrees F. if measured at the center duct the air temp should get below 40 before cycling takes place (and then the temp should raise 3-5 degrees before the compressor is switched back on)..

The reason why the original statement is sort of right is that there is no compressor control related to cabin temp control. The evap is held to a minimum temp (controlled extraneously by the ETR switch) and cabin temp control is accomplished by a combination of blending and heater core temp control (due to pulsing the mono-valve).

if the compressor were to just stay on the evaporator would turn into a block of ice (if the system were worth a hoot).

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician

That's what I needed to know. So when I convert to manual HVAC control can I control the compressor with an on-off switch on the dash and let the ETR switch cycle the compressor to maintain the evaporator at 35-40 degrees?

Then your only concern will be regulating cabin temps by some means of a blend door.. The AC is self regulating.. When powered up, it runs wide open, regulated only by the evap. temp. switch, to cycle the clutch...To keep it from freezing...

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89 300SEL Money Pit

92 Blown Buick Ultra Pimpmobile 220K and adding 1K per week

88 Wagoneer Slightly modified (Not for soccer moms)

04 Kia Sedona with every option... NICE

Great. Thank you.

if I were going to all the trouble of going manual, I would put a dash and evap case from a 240D in the car (for 123 models anyway).

The problem otherwise will be to make a lot of functions manual, such as door control and heater control.

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician

I still might try to find a manual system from a 240D, but I have never liked Mercedes's approach to HVAC. The Japanese companies have always had a better approach, a point that I first noticed this back in high school when I bought an early 70's Dodge Colt (made by Mitsubishi). The ventilation system was everything you need to control the environment in a car. It was simple, flexible, and effective.

As poorly designed (in my humble opinion) as the Mercedes manual systems are, I think that their automatic systems are even worse. For that matter, I don't see why anyone would want automatic climate control in a car. There are too many variables that the automatic system can't control. All the system knows is cabin temperature and whether the occupant wants defrost, or bi-level, or whatever air distribution. Human comfort is affected by numerous other variables that the system can't control such as the level of sunlight coming in, humidity levels, whether the windows are open or closed, and probably other factors that don't come to mind right now. To me, a manual system gives the highest comfort level. Part of why I think that is the psychological benefit of being able to tell the system what to do rather than depend on an automatic controller to figure it out.

My plan is use the HVAC control unit from a junkyard Honda Accord. If it works, I will post pictures. If it is a debacle, I will disavow any knowledge of this thread.

Sorry about that. As always, you are right Steve.

I forgot about the pressure switch wiring through the evap temp sensor. My brain is fried right now. We had a 4160 v conductor blow up yesterday evening about the time we were getting the paper machine going and things have been real busy around here since then. I replied prior to putting enough thought into it.

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Jim

You need four controls... A manual slide for temp control ( blend door)... a swith to control where the air blows out (with different combinations possible).. ..A fan speed control.... And a switch to turn on/off the AC compressor...

THat's it.. Nothing complicated.. I'm not even 40 yet, but I guess I'm just old..

Troubleshooting would go something like this... Hmmm.. My temp control isn't working?? How many things could it be????

__________________
89 300SEL Money Pit

92 Blown Buick Ultra Pimpmobile 220K and adding 1K per week

88 Wagoneer Slightly modified (Not for soccer moms)

04 Kia Sedona with every option... NICE