This is a A/C step by step DIY troubleshooting guide I found online. Works for the w201/w124, may work for rest.
1. Turn Car Off
2. Pull Klima A/C Relay Out. Its next to the OVP relay & fuel pump relay behind the battery.
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. 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
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.
19. 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.
20. 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.
21. Change the setting of the meter from volts to ohms.
22. You should get a low resistance about a few ohms.
23. If you don’t get a low resistance, need one last test.
24. 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).
25. Put one of meter’s terminals on battery ground and the other terminal on the compressor’s clutch coil pin (polarity does not matter).
26. 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.
27. 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.
28. If you don’t get a low resistance in step 23, then the compressor coil is open, normally this would require a new compressor.
29. 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.
30. 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.
31. Turn on the car and your compressor should come on.
32. Turn car off
33. 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.
34. No matter whether it is 29a (shorted coil) or 29b (seized), compressor is bad, replace it.
35. 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.
36. 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
37. Put the meter back on voltmeter setting. Do this test with engine cooler than 105 deg-C, better when just cold.
38. 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.
39. If you get about 12V, then the temperature sensor is bad.
40. 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. If disconnecting the temp sensor cable does not work, there is still another problem. Remove the Klima relay and move on.
41. 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. 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.
42. Change the meter setting to measure resistance.
43. 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.
44. 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.
45. If the resistance is o.k., then, jumper pins 5 and 7 on the socket.
46. Change the meter setting to ac volts.
47. Turn on the car.
48. 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.
49. Turn off the car.
50. To check engine speed signal, put the meter setting to ac volts
51. Connect one terminal of the meter to pin 1 of the socket and the other to pin 2 of the socket.
52. Turn the car on.
53. 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.
54. Stop the car
55. 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.
56. 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.
57. If in step 15, you determine that the push-button unit is not sending the control signal, replace the push-button unit.
1. Turn Car Off
2. Pull Klima A/C Relay Out. Its next to the OVP relay & fuel pump relay behind the battery.
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. 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
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.
19. 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.
20. 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.
21. Change the setting of the meter from volts to ohms.
22. You should get a low resistance about a few ohms.
23. If you don’t get a low resistance, need one last test.
24. 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).
25. Put one of meter’s terminals on battery ground and the other terminal on the compressor’s clutch coil pin (polarity does not matter).
26. 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.
27. 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.
28. If you don’t get a low resistance in step 23, then the compressor coil is open, normally this would require a new compressor.
29. 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.
30. 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.
31. Turn on the car and your compressor should come on.
32. Turn car off
33. 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.
34. No matter whether it is 29a (shorted coil) or 29b (seized), compressor is bad, replace it.
35. 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.
36. 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
37. Put the meter back on voltmeter setting. Do this test with engine cooler than 105 deg-C, better when just cold.
38. 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.
39. If you get about 12V, then the temperature sensor is bad.
40. 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. If disconnecting the temp sensor cable does not work, there is still another problem. Remove the Klima relay and move on.
41. 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. 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.
42. Change the meter setting to measure resistance.
43. 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.
44. 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.
45. If the resistance is o.k., then, jumper pins 5 and 7 on the socket.
46. Change the meter setting to ac volts.
47. Turn on the car.
48. 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.
49. Turn off the car.
50. To check engine speed signal, put the meter setting to ac volts
51. Connect one terminal of the meter to pin 1 of the socket and the other to pin 2 of the socket.
52. Turn the car on.
53. 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.
54. Stop the car
55. 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.
56. 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.
57. If in step 15, you determine that the push-button unit is not sending the control signal, replace the push-button unit.
