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Old 04-03-2004, 05:26 PM
pmizell's Avatar
Benz Zealot
Join Date: Jun 2001
Location: Houston, Texas
Posts: 838
Angry Low pressure compressor cut-off switch

Talk about frustrating...

After 2 weeks of totally breaking down, fully flushing my a/c system, including replacing exp valve and receiver/drier as well as resealing my compressor, I go to charge system and compressor fails to engage. (compressor worked fine before this work)

No problem I think, I'll just jump the pressure switch to engage compressor clutch and get R-12 flowing thru system.


I remove compressor again to double check the solenoid coil ground wire and that the speed sensor was put back on correctly... all looks fine.

The only thing I can imagine is that the pressure switch went bad when I R&R'ed it to the new drier. But that begs the question, when a failed compressor low pressure switch is jumpered, will the compressor clutch still engage?

If someone knows the answer please inform me before I go thru the trouble of testing the low pressure switch.


'01 E430, Sport 72,000 mi
'98 C280, 126,500 mi
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Old 04-04-2004, 05:36 AM
Registered User
Join Date: Sep 2000
Posts: 97
different car, but on my 300SD I just charged the system up this morning. You can bypass the pressure switch and the compressor should come on....

But I had the same probelm this morning as you...the compressor wouldn't engage and realized that I didn't have the climate control inside the car turned on(simple thing, but easily overlooked...i.e. just jumping the switch is no good if the A/C isn't switched on).

But check your fusebox for the compressor as well and the connector to the compressor.
1) 84 300SD 285K+ Miles (Ver Calif)
2) 84 300SD 175K+ Miles (Ver Federal)
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Old 04-04-2004, 10:53 AM
Posts: n/a
Start your troubleshooting by jumping DIRECTLY to the clutch. If it kicks in, then trace backwards from the clutch with a test light of voltmeter.

Good luck,
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Old 04-20-2004, 01:19 PM
Registered User
Join Date: Jan 2004
Location: houston
Posts: 51
Jumpering Clutch

90, 300E

How do you jumper clutch? Where & which wires?
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Old 04-20-2004, 01:54 PM
Posts: n/a
You do just what the word suggests, you use a jumper wire from the positive post of the battery to the clutch wire MOMENTARILY and see if the clutch clicks. DON'T do this for more than the time it takes to check the clutch.

I would not suggest doing this unless you UNDERSTAND the circuit and electrical troubleshooting procedures. It is not the END in your troubleshooting, it is only part of the MEANS to the end. You are checking the clutch and then you can go back through the circuit to methodically find the source of the problem if the clutch is good.

Good luck,

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