Hi Guys, I just bought a 300SD and my first job was to put in a new the AC compressor/line flushed and recharged. The compressor engages only if I jump wire it (connect it to a 12V terminal). The A/C blows cold! I suspect the climate control unit and possibly a relay gone kaput.. Has anybody got a wiring diagram of the system so I can debug it effectively. Don't worry I am quite familiar with electronics and circuit and I even have a friend who is an electrical engineer. I would by the manual but it will not get here in time I need the information.. Thanks, any help is greatly appreciated. ..

Try jumping the switches on the dryer, the switch by the expansion valve (behind the dash) and swapping the two relays under the black cover to see if one of them makes it work

On the 300SD the evaporator is above the gas pedal. Where could the relay be? Under the glove box is the blower and it has some sort of control boxes. Relatively big for a relay? Thanks..I try the low pressure relay and the temperature switch now .. I let you know what is going on. Cheers

I have a similarly defunct AC on my 300SD, however, got the R12 pressure tested and its pretty low. If the R12 is low, will the compressor/clutch still engage? I'll try the above mentioned steps. The blower blows fine, and all the ACC switches work as performed. Just nothing is cold, and the compressor never engages.

I followed roughly the instructions on www.dieselgiant.com but on the 300SD the expansion valve is right above the gas pedal. The system is pretty simple. I just freeze 12 a replacement for R12 changed all the o-rings and the schrader valves. The material cost was about $400. However, now I have to debug the elctronics to figure out why the clutch does not get 12 V. In the worst case I just install a switch once I figure our why my blower stopped working today. HOpe this helps.. Let me know if you have further questions..
BTW. You can wire 12Vdirectly from the battery to the blue wire and manually control the clutch..

It was the ETR switch behind the cover on the right side of the gas pedal. It senses the temperature at the evaporator. The switch was open and it should be closed.

Its a Fuel pedal


And I've been looking for this information for a while. Bless you.

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77' 300D, "Cartman" SOLD @ 150K (didn't know what I had)
83' 300SD, "The Superdon" 325k+ @ 28mpg
95 E320 wagon, "Millennium Falcon" 231k+ @ 24 Mpg
95 E300D, "Sherley" 308k @ 33.69 Mpg, currently anticipating a head
99 Suzuki Intruder "Trudy" @ 45 mpg