I have been having on/off cycling of my ACC system in my 87 300D when it is in certain modes and at certain ambient temps. I found a troubleshooting tree in my cd manual and tried to perform some of the tests. The manual of course calls for a special mb device/tool called a "socket box". This seems to be just a handy way to hook a multimeter into various inputs and outputs of the PBU. I found that the troubleshooting tree gives jack numbers to attach your multimeter to in order to test different parts of the system. Since I don't have the "socket box", I have tried to figure out which wires in my plugs which plug into the PBU correspond to the jack numbers on the socket box. Fortunately I found that the troubleshooting tree also gives wire colors along with jack numbers. So I ran the tests on the temperature sensors up to step 5 on the troubleshooting tree, measuring ohms at the appropriate wires and using an infrared thermometer to check actual temps at the sensor locations. I got the appropriate values at the PBU connector to all sensors except the coolant temperature sensor (CTS). At the CTS I got infinite resistance. I then checked the CTS itself by putting the meter on the CTS and got the appropriate value. I also tried testing the CTS by hooking my tester up to the CTS wire at the PBU plug and then hooking the other wire of the tester to negative terminal of the battery. I got the appropriate value. I also tested the wire from the plug which hooks to the PBU to the sensor for continuity. The wire tested fine. I am confused.
The troubleshooting tree says to hook the ohmeter to the following jacks of the socket box
Step 1: jack 1 and 17, interior air sensor (cable 10-brown/yellow and cable 2 grey/yellow)
Step 2: jack 1 and 24, outside air temp sensor (cable 9 grey/black)
Step 3: jack 1 and 19, evaporator temp sensor (cable 4 grey/red)
Step 4: jack 1 and 22, heat exchanger temp sensor (cable 7 grey/green)
Step 5: jack 1 and 23, coolant temp sensor (cable 8 green or blue/grey)
I assume from this that jack 1 of the socket box corresponds to cable 10 brown/yellow. Under this assumption I have tested all of the above. I got appropriate values for 1-4, but infinite resistance at step 5. If I put the ohmeter on cable 8 and on the neg terminal of the battery, I get the appropriate value. If I put the ohmeter on cable 8 and the other lead of the ohmeter on cable 10 like I did for all the other sensors, I get infinite resistance.
How can my sensor, sensor wire, and the whole assembly test fine against ground but not against the specified test cable number 10 (jack 1 of socket box)? The fault can't be in cable 10 because all the other sensors tested fine using cable 10. I don't get it.

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87 300D 162,000 miles: It's not just a car, it's an adventure!