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Old 05-23-2008, 09:31 PM
Professional Novice
Join Date: Oct 2007
Location: Chattanooga
Posts: 190
86' 300SDL AC question

Ah Spring, that magical time of year when we find out how well our AC will work for June-October in Tennessee! Mine is acting pretty funny!

At idle and low RPM's the AC is nice and at 70mph it wasnt blowing cool air! I turned it off, then on again when RPM's were at 2000 rpm and nice and cool again! Whats goin on!? The system is obviously working...what gives!?

Any ideas...ask me more specific questions...i'll supply!
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Old 05-24-2008, 04:59 AM
Registered User
Join Date: Feb 2006
Location: San Antonio, TX.
Posts: 199
The wonders of old technology. There are guys on this forum who are extremely well versed in the AC systems on these cars much more so than than I am. Hopefully they will post a reply.
However I'll put my 2 cents worth in for you.
From what you write , it sounds as though your compressor is engaging and functioning and then disengaging at high speed 70Mph , like 2700 RPM.
Does your AC function correctly and continuously for extended periods at low speeds like 40 Mph. ?

You might start by looking at the items below.

Compressor speed sensor failure at higher RPMs. Compressor speed sensor signal must sync with tach signal at correct ratio. Belt slippage from water or whatever can cause this.

Low (or low/high)pressure cutout switch (very unlikely, but happened on my Swedish car) where the pressure is getting low enough to switch off the compressor. Pressure may as well be too high cruising - not normal though.
Pressure would usually drop when you have good airflow across the condenser.

Tach signal failure from EDS. Required to start the compressor.

Klima relay failure (they are called this but really are logic based ).

Over temp. cutout switch (cuts out at 120C). Was your temp getting high?

Control module ( the one in the dash with push buttons and temp wheel).

Evaporator temp sensor old and out of range since last year - probably not, but may be an issue since it is I believe an NTC thermistor.

Clutch on compressor going bad. You know, it's magnetic , gets hot , fails, cools down , works... etc.

There are just a number of things that could cause your system to stop cooling. There is probably someone on this forum who has experienced this same issue. If you have more info , might be helpful.
1986 300SDL 201K Daily Driver
1984 300D In Progress
1989 240GL 196K Swedish Brick
1984 300SD -- Sold 289K
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Old 05-24-2008, 08:04 AM
JimFreeh's Avatar
Benz addict
Join Date: Mar 2007
Location: Hampton Roads, Virginia
Posts: 3,308
Get a set of gages on it and check the pressure.
The previous post gave a pretty inclusive list of possible failures.

If it comes back on without intervention when the rpms drop, I'd suspect low freon pressure on the suction side.

If not, it's likely a control issue.

The factory A/C manual contains a comprehensive diagnosis procedure, and that's really where you need to be. Don't just start throwing parts at this problem. Verify pressure before doing any replacement.

Now, if you had access to another W126, you could swap out suspect stuff.....

14 E250 BlueTEC black
94 E320 Cabriolet Emerald green
85 300TD 4 spd manual dark blue
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Old 05-24-2008, 09:24 AM
Registered User
Join Date: Jul 2007
Location: USA
Posts: 2,156
I would check your evaporator first then your accumulator.
1986 300SDL, 211K,Dealership serviced its whole life
1991 190E 2.6(120k)
1983 300D(300k)
1977 300D(211k)
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