Caveat: I only know enough about AC systems to be dangerous.

Would be happy to get the AC on this 95 E300D up and running....but not willing to invest any significant money.

The system is pressurized (presumably with the proper refrigerant), but compressor won't engage. However, when I briefly jumper the low-pressure cutoff switch, the compressor engages. Perhaps I have a bad switch?

What are some other simple things to check and/or common failure items?

Thanks.

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14 E250 Bluetec 4Matic "Sinclair", Palladium Silver on Black, 157k miles
06 E320 CDI "Rutherford", Black on Tan, 175k mi, Stage 1 tune, tuned TCU
91 300D "Otis", Smoke Silver on Tan, 144k mi, wastegate conversion, ALDA delete

19 Honda CR-V EX 70k mi
Fourteen other MB's owned and sold
1961 Very Tolerant Wife

I remember that we had this problem on our Corolla earlier this year -- good pressure, but no compressor engagement. A $13.99 fan clutch relay ended up solving it.

__________________
"Senior Luna, your sense of humor is still loco... but we love it, anyway." -rickymay ____ "Your sense of humor is still loco... " -MBeige ____ "Señor Luna, your sense of humor is quite järjetön" -Delibes

1982 300SD -- 211k, Texas car, tranny issues ____ 1979 240D 4-speed 234k -- turbo and tuned IP, third world taxi hot rod

2 Samuel 12:13: "David said to Nathan, “I have sinned against the Lord.” And Nathan said to David, “The Lord also has put away your sin; you shall not die."

When you say 'pressurized' - have you attached gauges to system and checked?

Before assuming the low pressure switch is bad, consider that you might just have... low pressure.

A system with a leak can remain relatively pressurized, yet not quite pressurized enough to kick the compressor on. I've just been stepping through a system rebuild myself, for precisely this reason.

A leak test is always a good place to start.

__________________
'83 300 SD
'05 E320 4Matic
'06 BMW 530 xi
'68 Triumph TR 250 - The only car I ever loved more than the Mercedes; who needs electricity, anyway? - Damn, why did I sell it?!
'59 Jaguar 3.4 'Le Chat Noir' - Damn, why did I sell it?!

It's difficult to make predictions, especially about the future.
- Niels Bohr

I checked low side pressure and it's plenty high to cause the compressor to engage.

I am wondering if that relay is a likely culprit....I had one go bad on my 92.

__________________
14 E250 Bluetec 4Matic "Sinclair", Palladium Silver on Black, 157k miles
06 E320 CDI "Rutherford", Black on Tan, 175k mi, Stage 1 tune, tuned TCU
91 300D "Otis", Smoke Silver on Tan, 144k mi, wastegate conversion, ALDA delete

19 Honda CR-V EX 70k mi
Fourteen other MB's owned and sold
1961 Very Tolerant Wife

if pressure is above 75psi, but compressor will not engage, and jumpering the low pressure switch lets it run, you need a new switch. you can leave it jumpered and check the pressures and temps, and if it's ok, you can safely use the a/c this way. but YOU will need to monitor the system for pressure. with out the LPS safety, you can destroy your compressor, and the entire system by drawing in moisture if there is a leak...

__________________
John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

My drivers:
1987 190D 2.5Turbo
1987 560SL convertible
1987 190D 2.5-5SPEED!!!

1987 300TD
2005 Dodge Sprinter 2500 158"WB
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!

Go figure....I jumpered switch again and this time the compressor does NOT engage. Sigh....

__________________
14 E250 Bluetec 4Matic "Sinclair", Palladium Silver on Black, 157k miles
06 E320 CDI "Rutherford", Black on Tan, 175k mi, Stage 1 tune, tuned TCU
91 300D "Otis", Smoke Silver on Tan, 144k mi, wastegate conversion, ALDA delete

19 Honda CR-V EX 70k mi
Fourteen other MB's owned and sold
1961 Very Tolerant Wife

Is there another relay in the car that you could temporarily swap over to the AC to see if it is a relay problem or not? For example, our Corolla uses the same relay for the AC clutch as it does for the horn, so I just made sure that the horn worked and then swapped the relays. Lo and behold, the AC came on!

__________________
"Senior Luna, your sense of humor is still loco... but we love it, anyway." -rickymay ____ "Your sense of humor is still loco... " -MBeige ____ "Señor Luna, your sense of humor is quite järjetön" -Delibes

1982 300SD -- 211k, Texas car, tranny issues ____ 1979 240D 4-speed 234k -- turbo and tuned IP, third world taxi hot rod

2 Samuel 12:13: "David said to Nathan, “I have sinned against the Lord.” And Nathan said to David, “The Lord also has put away your sin; you shall not die."

To the best of my knowledge, there's only one relay like that in the car.

__________________
14 E250 Bluetec 4Matic "Sinclair", Palladium Silver on Black, 157k miles
06 E320 CDI "Rutherford", Black on Tan, 175k mi, Stage 1 tune, tuned TCU
91 300D "Otis", Smoke Silver on Tan, 144k mi, wastegate conversion, ALDA delete

19 Honda CR-V EX 70k mi
Fourteen other MB's owned and sold
1961 Very Tolerant Wife

When I replaced the wiring harness with one I had rebuilt, I noticed one of the connections controls the AC system. Have you had any issues with your harness and the switch(es) it is connected to? Don't want to send you down the wrong hallway but you might want to consider.

__________________
-- Chris

'95 E300, 216k miles, Silver Surfer
'05 E320 CDI, 138k miles
'07 S550 4matic, 69k miles

Gone but not forgotten:

'76 300D, 350k miles?, SOLD in 1995
'75 240D, 300k miles, SOLD in 1991

The PO removed some stuff at one point, so that's certainly a possibility.

The main thing that's confusing me now is that I'm certain I was able to get the compressor to engage by jumpering the switch (this was a few weeks ago), but now I can't.

__________________
14 E250 Bluetec 4Matic "Sinclair", Palladium Silver on Black, 157k miles
06 E320 CDI "Rutherford", Black on Tan, 175k mi, Stage 1 tune, tuned TCU
91 300D "Otis", Smoke Silver on Tan, 144k mi, wastegate conversion, ALDA delete

19 Honda CR-V EX 70k mi
Fourteen other MB's owned and sold
1961 Very Tolerant Wife

sounds like a dodgy klima/kickdown relay if you get no A/C compressor engagement at all with low pressure switch wiring shorted and the A/C set to cool from the pushbutton unit.

It may also be a bad magnet clutch on the compressor.

__________________
2012 BMW X5 (Beef + Granite suspension model)

1995 E300D - The original humming machine (consumed by Flood 2017)
2000 E320 - The evolution (consumed by flood 2017)