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VeggiePup · #10 05-12-2003, 01:45 AM

I have an '85 300TD, the first year (as far as I know) MB put the Klima relay in the A/C system. Although yours is a different chassis number, I think our A/C systems are very similar. As suggested earlier, check the low-pressure switch at the receiver-drier. Three blue wires total; two connected to one side, one to the other terminal on the switch. The other switch is for the aux fan, and should have short pigtail connections instead of terminals. Using a test light (cheaper and quicker that a VOM) check to see if you have power at each terminal with the A/C on. Your test light should light up when touched to either terminal. If it does not light up, check fuses. Use the same test light. If fuses are OK, your climate control switch unit may be bad. You need a wiring diagram to check it.

If you show power at one of the low-pressure switch terminals, but not the other, the switch itself is bad. IF you in fact have plenty of refrigerant in the system.

If you show power at both terminals of the switch, but the compressor clutch doesn't engage, your problem could be:
a. bad speed sensor at the compressor
b. bad tach sensor (does the tach work?)
c. shorted engine temp sensor (cylinder head near air cleaner) or shorted sensor wiring. Check to see if terminal at temp sensor is grounded. It shouldn't be.
d. Bad Klima relay. As I understand, behind the battery, behind a plastic cover on your car. This was the problem with mine; I took the cover off the Klima relay and found a transistor flopping back & forth because of loose solder joints. Resoldering fixed it. Sometimes, if the compressor clutch has shorted windings, the Klima will burn out because of the excessive current. Taking the cover off will uncover the damage. Probably best to buy a new one, in that case.
e. bad compressor clutch. Find the wire (blue with green stripe, I think) going to the clutch. The one that isn't brown (MB always uses brown for ground wires). Using a jumper to connect 12V to the blue/green wire, the clutch should engage. If it doesn' t, the the clutch or its connector has failed.

No list can be exhaustive, so this one isn't. But it should give you a start.

To summarize, power comes through the fuse box, to the switch unit on the dash, to the low pressure switch at the receiver/drier, to the Klima relay, to the clutch. The Klima relay senses engine speed (cuts out compressor at wide open throttle, for example), compares it to compressor speed (to save the serpentine belt), and looks at engine temp. If all are OK, it lets the compressor clutch engage.

Good luck.
VeggiePup