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  1. #1

    Default AC Cycling too much?

    Vehicle: 1985 K20 w/ 6.2 + TH400.

    Overview of what I have done so far:

    -> Replaced compressor (reman)
    -> Replaced Accumulator (new)
    -> Replaced orifice tube
    -> All new o-rings (greased and properly tightened)
    -> Pulled ~28hg for ~15 minutes
    -> Charged with R290

    Works great. Ice cold - will run you out of the cabin. Issue is: the compressor cycles a LOT - like every 2-3 seconds when driving at 55mph.

    Now, before you get to saying "it needs more refrigerant" - read on.

    At aprox 1500 RPM around 75 degrees f ambient, I am holding a solid 38psi. I have tried less and I have tried more. The issue here is this: the damn gearing on this truck is so low that at 55mph, the poor detroit diesel is running upwards of 2500-3000 rpm. At such a high RPM, the compressor is working overtime - hence the rapid cycling.

    I can rev to 1800-2200 all day long and the compressor occasionally dis-engages/engages - as expected from a cycling clutch system, but I feel like this was a design flaw on GM's behalf. At highway speed, the RPM is simply too high to maintain a sensible compressor cycle.

    Of coarse, one fix is to compensate with more coolant - then I would be running way too high at idle. I could also completely by-pass the low-pressure switch, but not sure what this could lead to. I would not be afraid of running the low side at 5-10psi (hell, I have run PC coolers like this in the past) - I am just afraid of actually pulling a vacuum at some point and starving the compressor of oil...

    Perhaps this is fine? A cycle every 3-4 seconds is roughly 15-20 cycles a minute. At 2500 RPM, thats a lot of stress on any clutch I would think. My initial guess is that this is very hard on the clutch - am I wrong here?

    I have not tested it above 78 degrees f ambient yet - perhaps it will not cycle as bad once it warms up a bit more.

    The system had the same issue before I replaced everything and when it ran r134a.

    Thoughts? Surely I am not the only one who has an old 80's silverado w/ a diesel and the th400 - this has to be common. Should I not worry about it?
    Last edited by deanrantala; 06-16-2011 at 09:25 PM.

  2. #2
    stephan's Avatar
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    Sep 2010
    Far West, Oregon USA,
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    I agree this is tough on the clutch & will shorten it's life. Have you checked to see if there is an another pulley available to reduce the compressor rpms?
    1988 Chevy C-3500 2wd (no pic)
    350 c.i. 5.7 L Stock Block, 4 Bolt Mains
    L-31 Vortec Heads, Edelbrock Cam & Intake,
    Holley 650, Flowtech Headers, Magnaflow exh.
    Jet Trans 700R4, B&M Ratchet, 4:10 gears,
    3" susp. lift kit "shadetree"
    No rev limiter, No speed limiter lol

  3. #3


    No pullys that I can see on any parts sites.

    I have come across this however:

    The VOV meters based on pressure and temp. It mentions that as head pressure increases, so does the orifice - allowing more flow - hence keeping the low pressure side more steady. In theory, this should largely eliminate the need for the compressor to cycle as much. When the engine is at highway speeds, the VOV should open up more - allowing more pressure to the evap - and that should keep the low side pressure up, which stops (or at least slows) the constant cycling. We are essentially replacing (well, complementing) the low pressure switch + clutch system with a smarter metering device.

    At least thats my theory.

    Orielys has one for $20. At that price, it cannot hurt to pick one up and stick it in.

    I will post back tonight or tomorrow on the results.

  4. #4

    Default Fixed

    Well, since my last post was never approved, let me recap..

    Thursday evening I did some research and came across the "Smart VOV". This is basically like a normal Orifice Tube except that it operates more like a true pressure regulator.

    When head pressures rise (and the low side likewise drops), the Smart VOV adjusts the orifice - it becomes larger. This keeps the high side from getting too high and the low side from dropping too much. Regulates the system. This mostly replaces the function of the low pressure switch needing to cycle the compressor on/off all the time.

    Reading around I got mixed info. Some people claimed it made no difference, others said it works great.

    After installing the thing this morning, I am reporting back to say: it worked a wonder.

    This little bugger keeps the pressure evened out - and stopped the constant cycling at highway speed. Whats even better: since it opens up at high speed and reduces head pressure, there is less load on the compressor at highway speed and I do not "feel" as much slugishness as I normally do.

    End result is: my problem is fixed and I now have (literally) white frost shooting out from the vents.

    I will mention: if you consider replacing your FOT (fixed orifice tube) with a VOV (varialble orifice valve / smart vov) shop around. Murray makes one that is sold at most auto parts stores for $19. Quite a bit cheaper than $35-$45 online.

    - Dean

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