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Discussion Starter #1
According to wiring diagram, fuse 7 feeds the pushbutton unit at pin 12 (on the 12 pin connector on passenger side of pushbutton control). When any button except off and EC is pressed, 12v is fed from pin 6 via bk/rd wire to evaporative temp sensor. If this is correct, I have two problems. #1 - I have 12v from fuse 7, but 0v at pin 12. #2 -When I do a resistance check on the pushbutton control, I would expect to get continuity between pin 12 and pin 6 when any button is pressed except off and EC. This is not happening. I have a 1988 560sl. Schematic is 1987. I read something about the later controls switching the ground, but cannot find another schematic. Do I have correct wiring diagram? Any guidance would be appreciated. Thanks in advance.
AC wiring diagram.jpg
 

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One of the BW Old Guard/R129, W204 Moderator
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You have, I believe, the correct diagram. Be advised, rpm sensors and temp sensors prevent compressor activation under certain conditions. Also, the Klima relay is integral to the function. I can't recall specifics just now, but this info should steer you in troubleshooting. The EGv107 contains a wealth of info, including a tutorial by member GlueckAuf, I recall.

Push-button units are also prone to "cold Solder" joints, creating problems. I see the terminals you refer to, and believe you may have such a problem. You can reflow solder, or replace with rebuilt. Programma sells reasonable priced units (google and you will find them). I believe the link may be in the EGv107 as well.

Good Luck.
 

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Are you sure the system adequately is charged? If the pressure is too low the compressor will not engage.
 

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One of the BW Old Guard/R129, W204 Moderator
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Discussion Starter #5
Here is from the tutorial I found by GlueckAuf:

Test 5—Is the Klima relay receiving the ACC Pushbutton Unit’s enabling ground signal?
WARNING—Beginning with the 1986 R107 model year, the ACC Pushbutton Unit sends a ground signal
through this circuit. Prior to 1986, a +12 volt signal was used. Performing this resistance test on an
R107 model year prior to 1986, where this circuit is under live voltage, may damage the VOM.
Settings.
1. Ignition switch to Position 2 (Run).
2. ACC Pushbutton Switch set to  (Off).
3. Thermostat set to MIN detent (Coldest).
4. Set the Volt-Ohm meter (VOM) to OHMS R x 1.
5. Connect the VOM’s BLACK probe to the glove compartment lamp BROWN ground cable.
6. Connect the VOM’s RED probe to Klima pin 10 (KL).
7. Press ACC Pushbutton  (Normal), note reading on VOM.
8. Press pushbuttons  (T/V) and  (Defroster) and note each VOM reading.
9. Press pushbuttons  (Off) and EC (Economy) and note each VOM reading.
Desired results.
1. In positions  (Normal),  (T/V) and  (Defroster), VOM needle should move to 0 ohms
resistance, indicating that the Klima relay socket is receiving the ACC Pushbutton Unit’s
command to engage the compressor, and the serially-connected Evaporator Temperature
Sensor and the 2-Function Refrigerant Pressure Switch are not vetoing that command.
2. In positions  (Off) and EC (Economy), VOM needle should remain at Infinity (no continuity).
The Fix.
1. Check that fuse #7 (8 amp) is not blown. This is the fuse that powers the Pushbutton Unit.
2. Check that the ACC Pushbutton Unit is sending a ground signal to the Evaporator Temperature
Sensor’s incoming BLACK/RED wire. If no ground signal is present when the Normal, T/V, or
Defroster buttons are pushed, with ignition switch in position 2 (Run), and thermostat set to the
MIN detent (Coldest), the ACC Pushbutton Unit is probably defective. Further diagnose, repair,
or replace the Pushbutton Unit.

GlueckAuf talks about a ground signal being sent to the bk/rd wire from the pushbutton control. The schematic shows 12 volts being sent to the bk/rd wire (which he states was used on earlier cars). The schematic I have is incorrect. I should be looking to see that pin 6 on the acc switch is grounded except when in the off and ec position. If the evaporative temp sensor and refrigerant switch is operating properly, they will be grounded. Pin 10 on the klima relay should be grounded also.
I am currently in the process of charging the ac system. Only have 30psi in system. Can I ground the bu/rd wire on the refrigerant pressure switch to get compressor to operate, so more refrigerant can be added?
What bothers me is that if I went by the schematic, 12v would be at the pressure switch, rather than a ground.
 

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One of the BW Old Guard/R129, W204 Moderator
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Well damn... Bass-ackwards again. The story of my life :).
...what I don't understand is your schematic clearly designates as of MY 1987, after teh 1986 change...?

I still think you should be able to read continuity between 12 and 6 to verify contacts of the PBC (push button controller).
 

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Discussion Starter #7
Well damn... Bass-ackwards again. The story of my life :).
...what I don't understand is your schematic clearly designates as of MY 1987, after teh 1986 change...?

I still think you should be able to read continuity between 12 and 6 to verify contacts of the PBC (push button controller).
On the PBC I get continuity between pin 6 and 3, and pin 6 and 9, when any button is pressed except EC and off. I get continuity between pin 3 and 9 on all buttons except off. The wire from pin 3 in the socket goes to ground. So this means that when any button except EC and off is pressed, pin 6 on the PBC is grounded (this is what I hoped to find). If the two sensors are closed, this ground is sent to the Klima relay.
I can hear the Klima relay click while turning the temp control to minimum with the ignition on.
Tomorrow I'll put everything back together. With only 30psi in the system the pressure switch is not closed. I'll have to bypass the switch to get the compressor to engage (hopefully). Gettin' closer...
That schematic is dangerous. According to the circuit shown I would have to apply 12v to the Klima relay instead of grounding it - not a good idea. I cannot find an updated schematic.
 
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