After not having A/C since I bought the car, I tried to diagnose why the A/C was not working on my '87 300TD. I went through the test procedure section A of "83-605 Checking compressor cutoff and remedial measures". I pulled the Klima relay of and this is what I found:
Step 1: The measured voltage was nominal between terminal 5 and 1, 13V, OK
Step 2: The measured voltage was nominal between terminal 5 and 10, 13V, OK
Step 3: I bridged terminals 5 and 7 and briefly started the engine. A/C compressor clutch engaged and I had cold air blowing inside. OK
Step 4: Terminals 5 and 7 remain bridged and I measure the Volts (AC) at terminals 9 and 11. The nominal value is at least 0.3 VAC but I only measured 0.15 at idle and this went up to .25 when I revved the engine to 1500RPM. Since it did not pass the test, It says with engine off, to measure the Ohms between terminals 9 and 11. The nominal values should be between 530 and 650 Ohms. I was at 620, right in the range. Is this sensor good or bad???
Step 5: Since I have a diesel, I measured the Volts (AC) from the engine speed sensor between terminals 1 and 2. The nominal value is > 4 VAC and I got 5.68 VAC at idle. It also mentions to test the Ohms of this sensor and the nominal range should be between 1.7 and 2.1 kOhms. I measured 8.6 kOhms. Is this sensor good or bad??
Step 6: The measured voltage was .02 DC well in the < 1 V DC nominal range. OK

How do I interpret these results? By failing Step 4, I would think the Ohms would also fail, but they were OK. Does it mean the compressor is slipping? The belt is tight. By failing the second part of Step 6, I would think this sensor is bad. However, this is the same sensor that drives the tachometer and it seems to be working fine. If all these tests were completed OK, that would mean the Klima Relay is bad.

Do I buy:
a) Klima Relay $120
b) Engine Speed Sensor $50
c) A/C Compressor Speed Sensor $120
d) Rebuilt A/C Compressor $200 +

Any input of experience in this diagnosis would be appreciated!

1987 300D 186Km
1987 300TD 198Km
2004 CLK 500 Convertible 2Km