Good thing I got the A/C working on my back-up Honda last night 'cuz the '91 300-SELs A/C decided to go on vacation running an errand awhile ago. Worked on the way to where I was going(blowing cold air), but was dead after restarting about a min. later. No clutch engagement.

I know I have many things to test, but wanted to confirm something. I'm looking at a layout of the "Compressor Clutch Control" - 1990 300SE/300 SEL - page 304/3 in the ACC climate section - 126 car.

It looks like I can jumper 30 and 87 in the MAS to test the A/C compressor clutch control.

Am I reading this correctly?

Thanks for your time.

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Mike Murrell
1991 300-SEL - Model 126
M103 - SOHC
"Fräulein"

I'm not intimately familiar with your chassis, but there are two things that I would suspect in your case.

First is the ignition switch. After starting the engine only to have the a/c system not come on, wiggle the ignition switch. If you can make it come on this way, the problem is the switch.

Secondly, inspect the belt and its tension. If the belt slips a rotation sensor sends a signal that drops out the system. It may come on after starting again if it has had enough time to bleed off some high side pressure, decreasing the load thus allowing it to start without making the belt slip.

The other more obscure problem could be a bad clutch or locked up compressor. With engine not running reach down and turn the forward most part of the compressor to ensure that it turns smoothly. If it does, jump 12 volts directly to the clutch with engine off and listen for the clutch to click. If it does not, then the clutch coil is open or the air gap is too wide.

Good luck,

Larry:

Thanks for the advise.

Steve Brotherton confirmed what I believed I was seeing in the MB schematic in an older post below.

190e 2.6 A/C

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Mike Murrell
1991 300-SEL - Model 126
M103 - SOHC
"Fräulein"

Larry nailed it with his first suspect -> ignition switch.

I first checked fuses and belt tension; all looked ok.

Car was then started and an attempt to activate the a/c from the push button controller(PBC) was made. The compressor clutch did not engage.

I then jiggled the key in the ignition with the car running and the a/c activated.

Time for a new ignition switch. Hope there's an elect. piece that can be replaced w/o requiring a new key as well.

Thank you Larry.

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Mike Murrell
1991 300-SEL - Model 126
M103 - SOHC
"Fräulein"

Just screws to back of mechanism

http://catalog.eautopartscatalog.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=1D00YU6RY1K70NTVG4&year=1991&make=MB&model=300-SEL-006&category=M&part=Ignition+Switch

Arthur:

That's good news. Thanks for your response.

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Mike Murrell
1991 300-SEL - Model 126
M103 - SOHC
"Fräulein"

Wow! Great call.

I never would have suspected the ignition switch.

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2012 E350
2006 Callaway SC560