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1980 300SD Climate Control Monovalve Manipulations

Started by SmokeyBen, 15 February 2017, 01:04 PM

SmokeyBen

Feeling posty today.

Have scrutinized the service manual diagrams for the climate control monovalve operation and can't quite get a clear understanding of operation.  It has a gear (motor) driven operator and I don't have the magic test panel with all of the test inputs.

Has anyone delved into how it works without destroying it in an autopsy?  I would like to identify which of the input pins I could put voltage to to make the motor move, thinking that it might just be stuck in park and just needs a little action to loosen up.  Everything else in the system seems to work individually, but the monovalve is the key to switching between modes.

UTn_boy

The first thing I'd do is check your separate fuse in the fuse box. It'll be an inline fuse in a housing that twists apart. 

For testing, refer to the following:

1.    Remove the rubber manifold that all of the colored lines go into.  Unplug the left electrical plug (the one closest to the front of the car, pins 1 to 5)
Connect a volt ohm meter to pins 1 and 2 and measure the resistance of the feedback potentiometer.  Set your volt meter  on a scale to read 2,000 ohms.  If the servo is in the park position, then the meter will read 1,400 ohms, plus or minus 50 ohms. 

2.    Next, connect a 12V positive supply to pin4, and negative to pin 5.  (the pins are numbered in such a way that the pine furthest to the left is number 1)  The servo should run smoothly to the HOT position.  At this point, the resistance of the feedback potentiometer should decrease slowly and stop between 200 and 350 ohms when the servo stops.  To confirm that the heater valve is open, make sure all coolant passages on the bottom are free flowing and open. 

3.    Next, reverse your power connections: 12V positive to pin 5, and negative to pin 4.  The servo should run smoothly to the COLD position.  The resistance of the feedback potentiometer should increase slowly and stop between 1,800 and 2,100 ohms when the servo comes to a stop.  At this point, two of the coolant line connections on the bottom should be obstructed, not allowing any passage at all. 


                        Side note: At each end of it's travel, the servo should come to a stop with a noticeable "clunk"-it should not gradually slow down to a stop.  If it does, then then the water valve in the bottom or the gear train in the top of the servo may be damaged.  causing the servo to intermittently hang up in the full HOT or full COLD position, or anywhere in between.  At this juncture, buy another servo. 

4.    If all is ok up to this point, then connect 12V positive to pin 3, negative to pin 5.  The servo should move part-way toward HOT and stop at the PARK position.  At this point, the feedback potentiometer should be giving a reading of 1,400 ohms, plus or minus 50 ohms. 

5.    If the servo appears to operate properly and the feedback potentiometer has no "open" spots, the the servo is probably ok.  If the feedback potentiometer does not read  1,400 ohms +/- 50 ohms at the "park from cold position", then remove the top cover of the servo.  Loosen the two screws that hold the potentieometer tight and rotate the potentiometer until you get the +/- 1,400 ohm reading. 

If the servo has faltered in the past having gotten stuck, then the amplifier will likely be burned up.  Source a new one. 

These servos can be taken apart and cleaned/serviced.  However, I do not recommend doing this if you've not ever done it before.  At the very least, have two or three extras lying around for parts and reference.  Go through the testing first, and then proceed from there. 

There are likely other threads on this very issue.  Just do a search and you'll find them.  You'll see that many people do not like this system, as it's all vacuum operated.  Just one good sized vacuum leak will render the system useless.  There is also a scan of the factory repair/diagnostic manual on here somewhere, as well.  It lays out the wiring schematic, as well as the vacuum schematic.  However, the factory manual does not make any mention of servicing the servo, but only to replace it.  Be prepared to spend several hours diagnosing.  You'll likely have to replace most of the vacuum actuators underneath of the dash, too, as these are a very common source of vacuum leaks.  All are still available new, but the time involved is off putting to most.  Best of luck. 
1966 250se coupe`,black/dark green leather
1970 600 midnight blue/parchment leather
1971 300sel 6.3,papyrus white/dark red leather
1975 450se, pine green metallic/green leather
1973 300sel 4.5,silver blue metallic/blue leather
1979 450sel 516 red/bamboo

SmokeyBen

Thank you for the very concise troubleshooting guide.  It is exactly the information needed.  The MB service manual instructions concentrate on using the MB test panel device.  Will report back experience. 

UTn_boy

If you'll refer to Job number 83 section 601/1 through 601/9 in the factory repair manual, it gives a quick test program with remedies in the event a tester isn't available. 
1966 250se coupe`,black/dark green leather
1970 600 midnight blue/parchment leather
1971 300sel 6.3,papyrus white/dark red leather
1975 450se, pine green metallic/green leather
1973 300sel 4.5,silver blue metallic/blue leather
1979 450sel 516 red/bamboo