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Old 05-22-2012, 01:01 PM
engatwork's Avatar
Join Date: May 2000
Location: Soperton, Ga. USA
Posts: 12,685
91 300D climate control issues

I've been trying to help a guy out with his climate control and have done a bunch of troubleshooting and still having issues. The system is charged and holding pressure and the pressure switch is made up showing charge. I do not have 12v at pin 7 on the pushbutton unit to supply power to the pressure switch. I've tried another pushbutton unit and it did not make any difference. In addition when I put 12 volts on the line from pin 7 the klima is not pulling in. I've tried another Klima relay and it did not make any difference. Any thoughts/comments are appreciated.

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Old 05-23-2012, 08:19 AM
Join Date: Mar 1999
Location: Tucker, Ga USA
Posts: 12,153
The A//C control panel (pin 7) is a GROUND signal to low pressure switch & then to pin 10 on the Klima relay.
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Last edited by mbdoc; 05-23-2012 at 11:27 AM. Reason: WRONG Pin number
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Old 05-23-2012, 10:08 AM
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Join Date: May 2012
Posts: 33
I hope you don't mind me tagging on here. I am a newbie to MB, having bought my first 300d (1985 - 85k) just 3 weeks ago. I just returned from a 2700 mile trip and noticed that my climate control system is not quite right. As it's been cool, it was not apparent at first. I had the system checked while on the trip. It was indeed upgraded to the R-134a, but it had an odd looking threaded low side shrader valve. This was replaced with a standard valve, and the system was charged with R-134, oil, and dye.

1. The AC does not cool at highway speeds (2900-3300 rpm). It works OK (meaning I wish it were cooler, but it's still better than highway speeds) just driving around town.

2. If I select ECON with a cold temp selected, it blows warm (not totally hot) out of the top vents. Selecting back to AC will cool it a little, but it's still warm....even with MIN selected on temp wheel.

3. Last night night I had it in cool outside temps. I selected ECON, and set the temp wheel to MIN. Air came out of the proper vents (upper), but it was warmer than OAT. If I set the temp wheel to MAX, the air would flow out of the foot vents, but at the same temp as the upper vents, i.e. no hot air, just mildly warm.

4. AUTO fan appears to work OK.

This seems to be an issue with the heater core valve. Am I on the right track, and is this something that I can take care of myself? Do they tend to stick? I am assuming it's vacuum controlled like almost everything else.

Thanks in advance,

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Old 05-23-2012, 10:42 AM
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Location: Soperton, Ga. USA
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Old 05-25-2012, 10:49 AM
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Join Date: May 2012
Posts: 33
I'm betting it's the mono valve......
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Old 05-25-2012, 11:39 AM
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Join Date: Nov 2010
Location: TX
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tap the heater valve with a small hammer.
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Old 05-25-2012, 01:40 PM
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Location: Matthews, NC
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Did you get the a/c working? Here are some things to keep in mind and/or look at.
Compressor won't come on for 10 seconds after the prm is at or above 600rpm.
WOT switch open. (I don't know exactly where it is but I think it is on the valve cover.
2 function pressure switch closed.
Over boost switch open. could be stuck closed.
If compressor comes on, the RPM and compressor speed sensor have to match with in a few seconds.
Could just be a bad clutch.

Does the car run good?
Does the tach work?

The 90-93 300D 2.5's are very much different from the older 300D's.


84 500 SEL (307,xxx miles)
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