AC compressor trouble

Discussion in 'General Automotive Tech' started by Munzel, Jun 6, 2019.

  1. Munzel

    Munzel Member

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    Hello,
    I've got a new compressor (https://www.rockauto.com/de/moreinfo.php?pk=83171&cc=1421626&jsn=453) in my air conditioner. It also cools, but the clutch worries me. At higher ambient temperatures, the clutch grinds before it grips properly.
    I measured the resistance of the coil, 3.5 ohms. I could also measure this on another compressor. Then I cleaned the clutch with brake cleaner, cleaned the ground point of the wiring and retightened it.
    The problem remains.

    Does anyone have any idea what else I could do?

    Best regards
    Munzel
     
  2. Poison_Ivy

    Poison_Ivy Dogzilla Fan

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    I've never heard one grind. They either click on or out. If you could video it in action, we could then better help you out
     
  3. bcschief

    bcschief Member

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    Check for low voltage at the clutch with it engaged.

    Brian
     
  4. Munzel

    Munzel Member

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    Thanks for your answers. I'll have the pressures checked tomorrow, maybe there's too much refrigerant in it. I will also measure the voltage at the coil.
     
  5. Silvertwinkiehobo

    Silvertwinkiehobo "Nothing is foolproof as fools are ingenious."

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    Slippage can occur when the two plates are hot, but the question is: does the compressor hub resist spinning when it's engaged? If so, improper charge can be one cause, too much oil or a mixture of mineral and non-mineral oils on a 134a conversion (because the system was not properly flushed) could cause the drag.
    Also, when you installed the compressor, did you put a couple ounces of oil in the suction port, and turn the hub 15 or more turns after finishing the install?
     
  6. KevinVarnes

    KevinVarnes Well-Known Member

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    Also wouldn't be the worst thing to check the air gap although I don't know why that would cause it to grind. Inline with what bcschief said above, can you apply a clean 12V to the field coil and see if it does the same thing?
     
  7. Munzel

    Munzel Member

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    Now I measured again in the cold garage: the voltage of the coil (engine running) is 10,6V.
    The question is, how much does it matter? Ideal would be 14V, with it the coil would convert a power of 50 watts. Considering the higher resistance by the heating of the coil and the lower voltage, it would still be about 40 watts.

    Tomorrow is supposed to be hot weather, these are good conditions for testing.
    I'll take two wires and take the voltage directly from the battery.

    The gap of the clutch is about 1mm.
     
  8. Poison_Ivy

    Poison_Ivy Dogzilla Fan

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    When revving the engine above idle, there should be a maximum of 13,4 volts available to the compressor. 10,6 volts is way too low
     
  9. Munzel

    Munzel Member

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    That's right. I made a mistake yesterday. The voltage goes into the power quadratic. (P = U² / R)
    With 14V and 3.5 Ohm you get a coil power of 56W. At 10.6V it is only 32W. This power is sufficient to keep the clutch closed. But it is not enough to make the magnetic field so strong that it has enough power over the air gap.
     
  10. Silvertwinkiehobo

    Silvertwinkiehobo "Nothing is foolproof as fools are ingenious."

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    You have to remember, there are other drops in power along the circuit from the battery, through the ignition switch, the A/C control switch, the low pressure switch, then it gets to the main load at the clutch. Just the leg from the battery to the ignition switch, that's a 1 volt drop, and that is in specification. So I agree that 10.6V is a bit low, but still in spec, so going through and cleaning/snugging the connection pins at the controls, the low-pressure switch and the clutch coil might net you 1/2 to one full volt, IF the control head is still good and no damage to any wiring.
     
  11. Poison_Ivy

    Poison_Ivy Dogzilla Fan

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    He still could temporarilly connect a direct jump from the battery, or better yet, from the alternator's output terminal, in case the battery's too old, to the clutch, in order to see if it stops grinding. If grinding persists, he could then rule out everything else in the chain as being faulty.
    Theoretically, all those other components are either off or on. In other words, none of that current is going series through any coil. A massive voltage drop like that could only be due to the percentage that the copper in the wire's diluted. The lighter colored the copper is, the more resistance will be produced. This is how car makers save money on the production and then foot the buyer with a loss, through forcing him to foot the bill for higher amounts of energy needed to push those electrons through alloyed wire
     
  12. kevdupuis

    kevdupuis Membrane

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    While you are under there checking you should have a look at the pulley alignment.
    I had a rattling issue with the new R4 compressor in my 81 300 SD, it may not be the same issue as you're having but it turned out to be pulley alignment. Whether it was due to machining or what ever the fix was to use shims between the compressor and spacers to bring it into alignment.
     
  13. Munzel

    Munzel Member

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    Hello,

    I had no time for a while and the weather was also not warm enough for further attempts.
    The 10.6V may have been a measurement error, but I found a fuse that was slightly deformed by heat. I replaced it and remeasured the voltage. It is now about 12.5V, that should be enough. But the problem still occurs.
     
  14. Poison_Ivy

    Poison_Ivy Dogzilla Fan

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    Then, I'd take Ken's advice and start shimming the compressor towards the front of the car. That way, if there's misalignment, the belt will not pull the clutch against the release springs
     
  15. Munzel

    Munzel Member

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    Here in the video you can see the function.

    At the end of the video, the grinding noise can be heard for a short time, but with my clutch this happens not only when disengaging, but also when engaging.
    At low speed like in the video there is hardly anything to notice. It gets worse at higher rpm and high temperatures, because then the pressure in the system increases.
     

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