I've read quite a bit on this forum about troublshooting the A/C compressor.

The system has good pressure, no bubbles visible in the drier window, the belt is snug, and the fuse is good. The system was recharged last year and worked fine, but the A/C has not been used for many months.

Please give a forum and Merc newbie some assistance.

I read a thread that suggests to use a +12v hot wire to engage the compressor. Which terminal on the compressor should I touch this to, and should the other wire be connected or disconnected?

I also read a thread that suggests to bridge the low pressure switch wires to test whether the switch is bad, or is engaging due to low pressure.

Which set of wires should I bridge?


Is this the dreaded Klima?


What do I need to check at the thermostat to see if the temperature sensor is de-activating the A/C relay?

__________________
85 300SD (270K)

If the compressor is not running, the sight glass isn't telling you anything and the hi and low pressures tell you next to nothing, since as long as there is one drop of liquid refrigerant in the system you will get equal gas pressure readings (hi/lo) that correspond in psi to the ambient temp in Fº.

The black spade connectors below the red switch are what go to the dark stubby low pressure switch. Bridge them momentarily to see if compressor clicks. My guess is it will. If so, low charge (common) or defective switch (rare). It's not uncommon for a shaft seal leak to occur with disuse of the R4 compressor over the winter.

The low temp cut off switch (i don't remember if it's default ground open or default ground closed, and I'm not sure even if that's the one pictured). Disconnect the lead to it and see if anything changes; if not ground it to the head and see.

Good luck

__________________
The Golden Rule

1984 300SD (bought new, sold it in 1988, bought it back 13 yrs. later)

Thanks very much!

OK, assuming low pressure is the culprit, how does one charge the system?

I have a 1-lb. can of r-12, and a charge hose assembly. From what I remember about charging an old Ford I once owned, the compressor is supposed to be running when you charge the system.

Also, which R-12 connector valve is the proper one to hook the hose to? I assume it's the one closer to the windshield, not the one at the front of the radiator. Am I correct, or stupid?

__________________
85 300SD (270K)

The 1985 300SD has a completely different system than any other W126 SD!

That car has a "klima" relay that controls the compressor! IS the tachometer working?? IF not then neither will the A/C!!

__________________
MERCEDES Benz Master Guild Technician (6 TIMES)
ASE Master Technician
Mercedes Benz Star Technician (2 times)
44 years foreign automotive repair
27 Years M.B. Shop foreman (dealer)
MB technical information Specialist (15 years)
190E 2.3 16V ITS SCCA race car (sold)
1986 190E 2.3 16V 2.5 (sold)
Retired Moderator

Tach is working.

I will check the low pressure switch today.

__________________
85 300SD (270K)

How else is the 85 300SD A/C different? Also, my red sticker with the R-12 charge weights ha rubbed off. Anyone out there with an 85 300SD (federal) have a good sticker, the details of which you can share?

__________________
http://i284.photobucket.com/albums/l...aman/Fleet.jpg

Peach Parts W124.128 User Group.

80 280SL
85 300SD
87 300TD
92 300D 2.5 Turbo
92 300TE 4Matic

Tell me where the charge weights are listed and I'll be glad to check.

I did the low pressure switch bypass yesterday, no luck getting the compressor clutch to kick in.

__________________
85 300SD (270K)