Completely at loose for how this circuit works

Thread Starter

MB107

Joined Jul 24, 2016
122
I just posted another thread about this circuit on relay switching and now I realize that relay switching is not my issue but rather a lack of understanding on how this circuit works. So I started another thread to discuss how this circuit works.

I went and tested my all stock all working 560SL which is wired exactly according to the diagram below and realized that I do not understand how that works at all.

Looking at the diagram below, I would have assumed that if I pulled the 0.75 BK/RD wire off the B10/6 Evaporator temps sensor, and measured voltage at the end of the wire, I would get ~13 volts with the AC in either Norm, TV or Defrost positions. I did not, I got 0 no mater what I did Key off, key on AC of or AC on. This originally led me to believe that my push button control unit was bad. But its also like that on the stock 560SL I just tested which works perfectly.

Also looking at the diagram below, I would have assumed that if I pulled the 0.75 BK/RD/GN wire off the opposite side of the evaporator temp switch, I would get 0 volts, I do not, I get ~ 13V with the key on and the AC in either on or off. This led me to believe my Base module was bad. Except this is the same performance I get on my stock 560SL which performs perfectly. The actual performance on the stock 560SL was ~13.3 volts with the AC off and ~12.4 Volts with the AC on.

So the question is 1) Why is there no voltage at the 0.75 BK/RD wire at the B10/9 temperature sensor at any time. 2) Where does power come from on the other side of the B10/9 temperature sensor 0.75 BK/RD/GN wire.

I also did check that the wire BK/RD wire was going to terminal 6 of the push button control. Keep in mind the push button control is not a manual switch as shown but more of a relay controlled switch.

1603308602336.png
 

Externet

Joined Nov 29, 2005
1,546
That schematic is very clear. Top right; replace the word "hot" with "+12V"
If fuse 7 is good, +12V 'flows' to pin 12. --> Probe!
If pushbutton is in position Def, T/V or norm, +12V flows to pin 6 --> Probe!
If temperature sensor is above 40F, +12V is now continuing to low pressure switch. --> Probe!
If there is refrigerant pressure above 10 psi, +12V continues to pin 10 'enable input' --> Probe!
If any of the above is not right or their wiring/connectors have failed, the AC is disabled to work.
Plus other conditions that should be met at the rest of the wiring shown.

Instead of a voltmeter, much better to probe with a light :1603311424159.png
 
Last edited:

Thread Starter

MB107

Joined Jul 24, 2016
122
That schematic is very clear. Top right; replace the word "hot" with "+12V"
If fuse 7 is good, +12V 'flows' to pin 12. --> Probe!
If pushbutton is in position Def, T/V or norm, +12V flows to pin 6 --> Probe!
If temperature sensor is above 40F, +12V is now continuing to low pressure switch. --> Probe!
If there is refrigerant pressure above 10 psi, +12V continues to pin 10 'enable input' --> Probe!
If any of the above is not right or their wiring/connectors have failed, the AC is disabled to work.
Plus other conditions that should be met at the rest of the wiring shown.

Instead of a voltmeter, much better to probe with a light :View attachment 220238
I certainly understand it being faster with a light but all I have at the moment is a volt meter. All good up to step 3. Zero voltage at pin 6. Including on another working car.
 

Thread Starter

MB107

Joined Jul 24, 2016
122
By pin6 , do you mean no output from the switches ?
Yes pin 6 is the output of the switches.

I'm starting to believe MB messed up here. I think fuse 7 feeds power for the relays only within the push button switch control and that the relays within the pushbutton switch actually switch pin 6 to ground.
 

Thread Starter

MB107

Joined Jul 24, 2016
122
It is not about being faster, it is about misleading readings a high resistance meter may yield.
I've never heard of that with a volt meter but I certainly have with an Ohm meter. I certainly could have used a light this time around. Makes things much easier when your upside down under the dash and trying to read a digital auto scale volt meter, especially when you get a reading like 14.2 and then you take a closer look and its really 14.2 mV.
 

Thread Starter

MB107

Joined Jul 24, 2016
122
I found a picture of the real innards of the N22/2 switch, it was the size of a postage stamp but I took a picture of it and now I can read it. As you can see, its a far cry from the over simplification presented earlier. I will be looking through this today to see if with any of those buttons pressed does it route pin 6 to 12V or ground.
 

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Thread Starter

MB107

Joined Jul 24, 2016
122
I got it. The green line traces out the path for pin 6 to ground. Depressing the EC or O switch will break the ground an shut off the compressor. Turning the compressor on by pressing any of the three left switches in the diagram will reset the EC and O stitches to the state shown grounding the compressor. None of the relays are involved for this operation. I cant believe MB published the first diagram in there ETM's. Well, I guess I can believe it. I have seen many errors but this one appears to be the worst so far.

1603390388340.png
 
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