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Old 07-13-2004, 07:07 PM
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Revamping A/C System and Controls

I just picked up a 1979 300D with a non-functioning A/C. Iíve read about converting from the OEM compressor to an aftermarket compressor. I was thinking about doing something similar, although I would like to go with a more economical Ford or GM standard compressor and drier (and other parts, if necessary).

Although I am not an A/C expert, it looks to me like the 300 D A/C system controls are vacuum operated. [There is a unit next to the compressor that has several vacuum lines running to it and an electrical connection running from it to the clutch.] As most of the vacuum lines are shot on this car, I'd kind of like to convert it to electrical operation/control.

Any thoughts on where to start? Anyone done this before?
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Old 07-14-2004, 10:53 AM
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Join Date: Jan 2001
Location: Alexandria, Virginia
Posts: 5,441
These cars have the notorious climate control servo - an electro-mechanical combination heater valve, blower & vacuum control unit, originally used on early '70s Chryslers.
It's mounted under the hood, on the passenger side fender well and it's connected to a vacuum harness, a wire harness and 4 heater hoses. The problem is the heater valve leaks and corrodes the internal motor servo mechanism. I'm told there's also an electronic control unit in the dash that is damaged when the servo fails.

First, does your climate control work OK on all functions except for the AC compressor or is the climate control system mostly inoperative?

If only the AC compressor is inoperative, can you spin it by hand or is it seized? If it spins, can you jumper a wire from the battery to see if the AC clutch works?

I experimented with bypassing the bad servo on a friend's 300D. His system only supplied unheated air with the defrost button and he really only wanted heat so his son could drive to school in winter. I tried rigging up a cable to manually operate the servo but that didn't work. I was able to bypass the heater valve part of the servo by installing a cable-operated heater valve at the firewall and get him hot air with the defrost button.

There are expensive replacements for the servo - either OE or an aftermarket aluminum unit - try a search. If you can't find it here then try the Tech-help or Diesel Forums.

Happy Motoring, Mark

Last edited by Mark DiSilvestro; 07-14-2004 at 11:06 AM.
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Old 07-14-2004, 01:22 PM
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Join Date: Apr 2000
Location: Falls Church, VA
Posts: 5,316
George Murphy at 865 482 9175 has a guide to troubleshooting these systems that is very helpful.
Chuck Taylor
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe
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