You need specialized test equipment to verify all functions on the CCU when it's not in the vehicle. The rebuild houses have such equipment. It's not possible fo r a DIY person.

If the compressor won't engage when you jumped #5 and #7, it would appear that the clutch has failed. You can verify that you have continuity between #7 and the clutch.......just to confirm it.

Took the car to check freon level. Empty...So I had them refill it but the CCU was not hooked up so they could not test if the ac came on. Hooked it back up and no ac. Try to jump 5 and 7 and still no ac as before. But I noticed that the CCU was cutting on and off by itself. Can you replace the harness for the CCU? Brian....what clutch? Fan clutch? The car has been getting hot while idle also. Should the fan run all the time? Where do the leads go to on the clutch?
A

__________________
1987 300SDL
187,000 miles

See if you have voltage at socket #5 of the Klima. If you do, then jump sockets 5 and 7. The compressor clutch should engage. If it does not engage, then either the clutch is NG or your plug at the clutch is NG. I'm inclined to believe that it's the latter because sometimes you have the clutch engaging and sometimes you don't.

Did you ever unplug the clutch and check out the plug?. It's in a bad area and probably has not been checked in 20 years. You need continuity between the clutch and socket #7 of the Klima.

I don't understand the phrase "The CCU was cutting on and off by itself"???

I had to rebuild my plug ('82 300SD -- may be different) the leads were shot after 25 years!

__________________
1998 E300 +310,000K
1996 E300 +460,000K

"The CCU was cutting on and off by itself" I was testing to see if the AC would come on. During the timer the CCU was running, it stopped , then started up again. Sometimes it stopped running and would come back on right away.
A

__________________
1987 300SDL
187,000 miles

I hope I get this fixed soon. I am tired off having hot air blowing in.......
A

__________________
1987 300SDL
187,000 miles

The CCU doesn't run. It's a control unit that runs other devices. It cannot "start and stop"?????????????????

That is why I am wondering if there is a short in the wiring for the CCU.
A

__________________
1987 300SDL
187,000 miles

I'm not understanding any of your statements regarding the function of the CCU.

This has gotten very confusing, lets start again, now that you have a good grip on what is the Klima relay and what is the CCU (I think you mean the push-button unit inside the car).

1. Turn Car Off
2. Pull Klima Relay Out
3. Put your multimeter on DC Volts setting
4. Connect the ground terminal of the meter (black color usually) to the battery's negative terminal
5. Connect the positive terminal of the meter to pin number 5 in the socket (Hopefully, you know which pin is 5)
6. You should get close to 12V, if not, the fuse is blown (#7 I think). Replace fuse. Put the Klima relay back in and check a/c again.
7. If the fuse is o.k. and the compressor clutch still does not run, then lets move on
8. Remove the Klima relay. Now connect the negative terminal of the meter to pin number 10 in the socket. (Hopefully you know which is pin 10). Put the positive terminal of the meter on the positive terminal of the battery.
9. Turn on the car and press one of the a/c switches on the push button unit.
10. You should get close to 12V when the a/c switch is on.
11. If you dont get close to 12V then there are one of two possibilities:
a. the push button unit is not sending the correct control signal
b. the pressure switch is not closed
For now, I am ignoring the trivial possibilities that wires or connectors may be broken. We can dive into these once we have some answers to the basic tests.
12. Turn the car off.
13. If you dont get the 12V, disconnect the two cables from the pressure sensor (should be at the drier), and jumper them.
14. Repeat steps 9 and 10.
15. If you still dont get the 12V, the push-button unit is not sending the signal.
16. Turn car off, dont keep the car running for a long time with the pressure switch cables jumpered.
17. If at step 14, you did get 12V, then the pressure switch is not closed
18. If the pressure switch is not closed there could be three possibilities:
a. refrigerant pressure is too low (lower than 2 bar)
b. refrigerant pressure is too high (higher than 30 bar)
c. the pressure switch is bad
you will need pressure gauges to seperate these possibilities.
16. If at step 11, if you did get 12V, then the push-button unit is sending the control signal and the pressure sensor is o.k. and we will move on.
17. If push-button unit is sending a signal and the pressure sensor is o.k. but the compressor clutch will still not run, then need to check the compressor clutch coil resistance.
18. Connect the positive terminal of the meter to pin number 7 in the socket and negative terminal of the meter to negative terminal of the battery.
19. Change the setting of the meter from volts to ohms.
20. You should get a low resistance about a few ohms.
21. If you don’t get a low resistance, need one last test.
22. Get to the compressor and disconnect the connector for the compressor clutch coil (there is a speed sensor cable that connects to the compressor connector also, you have to trace the clutch wire to the compressor connector to pick the right pin on the connector).
23. Put one of meter’s terminals on battery ground and the other terminal on the compressor’s clutch coil pin (polarity does not matter).
24. You should get a low resistance, about a few ohms. If you do get the low resistance at the compressor then compressor clutch coil is o.k.
25. If you get low resistance in step 23 but do not get a low resistance in steps 18-19, then the cable from the Klima relay to the compressor is bad, replace it.
25. If you don’t get a low resistance in step 23, then the compressor coil is open, normally this would require a new compressor.
26. If the compressor clutch coil is o.k. and the cable from the Klima relay to the compressor is o.k. then let us move on.
27. At this point, if the fuse is o.k., and the compressor coil’s resistance and cable are o.k., then jumper pin number 5 and 7 in the socket.
28. Turn on the car and your compressor should come on.
29. Turn car off
29. If the compressor does not come on, there are two possibilities:
a. the fuse is blown
b. the compressor has an internal mechanical problem, is likely seized.
30. No matter whether it is 29a (shorted coil) or 29b (seized), compressor is bad, replace it.
31. If the compressor comes on at step 28, but does not come on when the Klima relay is put back in and the push button switches are on a/c, then the Klima relay is not producing the 12V signal at its pin 7 to drive the compressor.
32. If Klima relay is not driving the compressor at step 31, then there are five possibilities:
a. the temperature sensor is bad
b. faulty throttle cutout microswitch
c. bad compressor speed sensor
d. bad engine speed sensor
e. bad Klima relay
33. Put the meter back on voltmeter setting. Do this test with engine cooler than 105 deg-C, better when just cold.
34. Put the positive terminal of the meter on the positive terminal of the battery. Put the negative terminal of the meter in pin 12 of the socket.
35. If you get about 12V, then the temperature sensor is bad. I am not sure about the physical location of this sensor but on my 87 260E, it is a 3 pin sensor on the engine I think third from last from the windshield side. You can ask your parts vendor about it.
36. If the temperature sensor is bad, then disconnect the sensor cables, put the Klima relay back, and try you’re a/c. If this was the only bad component, your compressor should start working. But again, don’t keep running like this, replace the sensor as soon as you can. If disconnecting the temp sensor cable does not work, there is still another problem. Remove the Klima relay and move on.
37. Put the positive terminal of the meter on the positive terminal of the battery. Put the negative terminal of the meter in pin 4 of the socket.
38. If you get about 12V, then the throttle cut-off switch is bad. This switch is supposed to be open and only close when the throttle is wide open. I am not sure about the physical location of this switch but you can ask your parts vendor about it. Once again, if you can find this part, you can disconnect the wire, put the Klima relay back and check a/c. If it still does not come on, then there is some other problem and move on.
39. Change the meter setting to measure resistance.
40. Put the positive terminal of the meter on pin 9 of the socket. Put the negative terminal of the meter in pin 11 of the socket.
41. You should get about 350 – 450 ohms. If you don’t get the low resistance but get very high resistance, either the compressor speed sensor is open circuit or the wire is broken somewhere. Do the same test as close as possible to the speed sensor (i.e. on the compressor connector) to rule out a broken wire. Replace sensor if it is bad.
41. If the resistance is o.k., then, jumper pins 5 and 7 on the socket.
42. Change the meter setting to ac volts.
43. Turn on the car.
42. You should get about 0.3V ac at idle of about 750 rpm. If you don’t get the ac voltage, and the resistance was kind of o.k., replace the sensor.
43. Turn off the car.
43. To check engine speed signal, put the meter setting to ac volts
44. Connect one terminal of the meter to pin 1 of the socket and the other to pin 2 of the socket.
45. Turn the car on.
46. At idle of about 750 rpm, you should get a voltage greater than about 4V ac. And the voltage should increase with increase in RPM.
47. Stop the car
48. If you don’t get the voltage, switch the meter to resistance setting and readout the resistance between pins 1 and 2 of the socket.
49. You should get about 2 Kohms. If you get a very high resistance, check the resistance closer to the sensor to rule out a cut in the cable. Otherwise replace the sensor. Ask the parts person about its location.
50. If in step 15, you determine that the push-button unit is not sending the control signal, replace the push-button unit. This has happened to me once, the electronics driver inside the push-button unit is blown and it will not send the control signal to the Klima relay. What I did is bypassed the electronics and just used the mechanical switches to send the control signals. The downside is that there is no more auto modes, so when I feel like it is too cold, I would press the economy switch and I designated that switch to send a signal to cut-off the compressor. So my brain does what the box would otherwise do, have still not fixed the electronics, just a big headache to test this unit once it is out of the car. If you just bought a new one, you can ask for a replacement. Most likely the problem you are having is not in that unit.

This is awfully long and I may have missed some possibility, but you will have enough info to test this problem in a systematic manner and when you come to a point where you think something is missing, let us know, some one will help.

__________________
Saumil S. Patel

It is..........but you didn't.

Thanks for the post. We'll see if folks can follow it from beginning to end.

Wow, great details.
That should become a "sticky"

__________________
1987 300SDL
187,000 miles

Great info, Mr. Patel.

Am I correct in assuming those steps apply to the earlier cars, before the fuel pump relay was incorporated into the Klima relay?

Thansk

__________________
2012 E350
2006 Callaway SC560

I think these steps should work where there is a seperate Klima socket as my 87, 260E and if the socket pin layout is similar. This one can find out from the parts vendor.

__________________
Saumil S. Patel

That's very thorough!

Isn't there an evaporator temp switch between the PBU and the Klima to cut the compressor if the evaporator ices?

Sixto
87 300D