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What does these pressure and fan changes tell me about AC system? 900 1994

posted by  Philip on  Fri Aug 28 08:28 CST 2009
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last visit: Thu Oct 23 10:21 CST 2014
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I put in a rebuilt compressor, dryer and oriface. The AC works OK but not at cold as the original compressor. At idle or stop lights, it is especially lukewarm.

At idle and 90 degrees outside, the radiator cooling fan starts when the low side pressure rises to 45. It stays on for 4 seconds and then the fan stops and the pressure starts to drop. When the pressure drops to 38, the pressure then starts to rise again. In about 10 seconds, the pressure is back up to 45 and the fan kicks on. This cycle then continues on and on.

In comparison, when my other 940T is at idle, the fan comes on and stays on. The pressure stays at 33 without any change. And the air is nice and cold.

Is this a compressor problem, freon problem, or fan problem, or sensor problem?
(when I vacuumed and put r134 in the system, I put in 3 (12 oz.) cans, and the system calls for 30 oz so there should be plenty of r134)

Thanks for any ideas.








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    The Answer -- What does these pressure and fan changes tell me about AC system? 900 1994

    posted by  Philip on  Sun Aug 30 05:53 CST 2009
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    last visit: Thu Oct 23 10:21 CST 2014
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    So it turns out that I had bad info on the brown vs grey pressure switches. It is actually the grey that controls low speed on the fan. I jumpered the connections to the grey and all I got was a click from the fan relay. I took out the relay and found one of the contacts rattling around in the box -- broken off. So I am ordering a used one off of eBay.








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    What does these pressure and fan changes tell me about AC system? 900 1994

    posted by  hardknocks! on  Fri Aug 28 12:33 CST 2009
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    hello
    sounds like your pressure switch is incorrect, some are reported adjustable but mine is not.
    you also put too much freon in there.
    i believe the pressure cut in and cut out numbers are in the FAQ.
    good luck
    Mike








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      What does these pressure and fan changes tell me about AC system? 900 1994

      posted by  howdy_doody on  Sat Aug 29 16:06 CST 2009
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      last visit: Sat Feb 27 14:47 CST 2010
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      You may have too much 134A in there, these systems are very sensitive to too much refridgerant.

      I would jump the fan switch on the radiator so it runs all the time and see what your pressures are with the compressor cycling on and off.








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        What does these pressure and fan changes tell me about AC system? 900 1994

        uid 18083's pic
        posted by  lucid on  Sun Aug 30 06:59 CST 2009
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        last visit: Fri Oct 24 12:31 CST 2014
        [RELATED]

        Here's how I read the 700/900 Climate Control "Green Book" (TP8701210 Section 8 (87):

        Switches at bottom front of condenser:
        • Purple Safety switch stops compressor if pressure exceeds 435 psi (R134a)
        • Gray High pressure switch turns fan on High speed if pressure exceeds 334 psi
        • Brown High pressure switch turns fan on Low if pressure exceeds 261 psi.

        In my experience (moderate climate, '93 940NA), neither the Gray or Brown switches ever made the fan run. The only fan operation I ever got was High speed, when triggered by thermo-switch in the radiator, with prolonged idling on a very hot day. Then it would cycle on and off—on just long enough to drop the coolant temp, then off till heat rose again. (Much like the subject line in your other thread.)

        The point is that those two switches only come into play at what I read as abnormally high pressures. But at the others have noted, maybe you need to be concerned about high pressures with an overfill of R134a. The spec is 950 grams x .035 = 30.25 oz, so you're about 2.75 oz over. I don't know what the symptoms of an overfill are.

        You also say, "In comparison, when my other 940T is at idle, the fan comes on and stays on." My '93 diagrams show the the Turbo ECU can control both the High and Low speed relay coils. Which raises the question as to why the 945T ECU do the same thing as the 940T? Are they the same years?


        --
        Bruce Young, '93 940-NA (current), 240s (one V8), 140s, 122s, since '63.







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