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Thread: MC-20 GARD-PAK

  1. #1
    Join Date
    Feb 2005
    Posts
    26
    Does anyone know how to jump out the Robertshaw MC-20 Gard-Pak/Motor protector? My Trane compressors use this *&$% junk as a safety to protect the compressor. Its an oil pressure/ motor winding combo deal. Is their a direct replacement?

  2. #2
    Join Date
    Mar 2003
    Location
    NY
    Posts
    2,164
    Is the GardPak bad? Usually this problem is in the oil switch.

  3. #3
    Join Date
    Dec 2003
    Location
    45th Parallel
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    957
    There should still be a direct replacement, can't say for sure haven't seen one in years.
    What is it tripping on? Oil Pressure or motor temp. Have you ohmed out your motor sensors?

    Have you checked comp oil pressure?

    Does it trip immideatly or after about 90 seconds?

    I am skeptical that you have checked any of these things, since you asked how to jumper it out. A tech that new to check all the above would be able to read a diagram and by-pass control . Although I would not recommend doing so, unless an absolute emergency and customer signed off knowing the risks.

    edit: sorry just read your profile, guess your not a service tech, you are the customer. Don't by-pass you could scramble the comp real quick. These controls are actually pretty reliable. in 20 years I have only found 1-2 that were actually bad and not tripping due to an actual abnormal condition.

    [Edited by drivewizard on 04-11-2005 at 11:39 PM]

  4. #4
    Join Date
    Feb 2005
    Posts
    26
    Mr. wizard...I have 36 of these units on my Trane compressors and for the most part they work ok. However when the motor winding resistors on the compressors fail (ie. have a resistance value over 100 ohms)the MC20 prevents the compressor from starting. True I do not know why the resistors in the compressors are failing. However The Trane manual suggests Jumping out the MC20 when one of these resistors fails: Trane Service Bulletin HCOM-SB-20
    Dated 10/7/77,"Once the open sensor is located by taking resistance readings, it can be bridged out of the control circuit with a resistor. this will allow the motor to run with two winding protection instaed of three."
    Maybe you just disagree with my terminalology "Jumper" instaed of the politically correct "Bridge".
    I was wondering if someone made a part for this instead of going out to Radioshack and buying a 82 Ohm Resistor and wiring it into the circuit. I've also noticed that often times more then one sensor fails leaving you with only one sensor remaining. Would you recommend replacing a $4,000 compressor because a resistor fails? Maybe that's why your a Tech. and not an owner!

    Okay know that I'm done ranting, has anyone run into this situation? Sorry Mr. Wizard...

  5. #5
    Join Date
    Nov 2003
    Location
    Boston
    Posts
    600
    As you may know the sensors are embedded in the motor windings and are not field replacable.
    The only way to "jump them out" is to go to Radio Shack and shell out 99 cents (unless you buy them from Trane) for a pack of resistors and "bridge" terminal c to s1 or s2 or s3.
    If one sensor is failed and your unit is from 1977 be happy.
    If three sensors are failed and the unit is from 1977 then bridge all three sensors and figure to change out a compressor soon, but still be happy!
    If the unit is a newer unit then you have to start thinking "What would make these sensors burn out so soon? Compressor running hot? Superheat OK? Oil pressure low? etc...........

  6. #6
    Join Date
    Apr 2005
    Location
    Inland Empire,CA
    Posts
    56
    use a resistor and wire in-line with c terminal connected to terminals s1, s2, and s3

  7. #7
    Join Date
    Dec 2003
    Location
    45th Parallel
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    957
    jvrba, I don't think I recommended changing out the comp. I read my post twice and still didn't see that, maybe I missed ssomthing.
    The internal sensor is sometimes not the reason it trips on motor temp. I have found the sensor wires rubbed thru, due to comp powere wires laying on them in conduit and vibration wears thru sensor wire. Have seen completely worn thru to were only one strand of sensor wire was making circuit, and it was broken also causing an intermittent open which dropped off comp on motor temp. By the time I would get there and reset/check it had made the circuit again and ohmed out fine.
    Point: don't assume you have a bad sensor, unless you check directly at the comp term. and it is still open. Even then the wire may have come off the back side of the motor term plate.

    Oh, and no I don't disagree with your use of the term "jumper" when used in the proper context with more info. Have "jumpered out a few myself.

    [Edited by drivewizard on 04-12-2005 at 08:08 PM]

  8. #8
    Can anyone tell me if the resistance of the Gard-Pak V thermistors would be the same? I have a compressor experiencing random trips and I suspect it is bad thermistors. I have swapped modules from this comp to a known good one and the problem stayed. Resistances are 1363, 1394, and 1030 on the comp that runs fine and 1243, 915, and 956 on the one that trips with the comps off... not that far off from one another but I suspect the resistance drops as the comp runs... How would you test resistance of them on the fly to find the bad one(s)? Does this module have a delay built in? The comp that trips sometimes runs for days before tripping, sometimes just hours.

  9. #9
    Anyone? Anyone? Bueller?

  10. #10
    Join Date
    Dec 2003
    Location
    45th Parallel
    Posts
    957
    Not familiar with the GardPak V, did you mean Gardpak IV?
    What is it used on?

  11. #11
    OEM on a Multistack chiller, has a hermetic recip, I think Bristol. If I knew a good default resistor value I could run the thing and find the bad resistor(s) quickly and jump out only whats bad. Gard Pak V is a good number, I think because it is OEM. Thanks for any help.

  12. #12
    Join Date
    Dec 2003
    Location
    45th Parallel
    Posts
    957
    I dunno, you could try Multistack #608-786-3400 tech support is usually pretty good.


  13. #13
    Join Date
    Jan 2004
    Location
    New Jersey
    Posts
    534
    Originally posted by jvrba
    " Would you recommend replacing a $4,000 compressor because a resistor fails"? Maybe that's why your a Tech. and not an owner!

    Okay know that I'm done ranting, has anyone run into this situation? Sorry Mr. Wizard... [/B]
    jvrba--

    Would you change the compressor if all the sensors failed? I would not. Now of course the customer could make that decision and probably should. I feel if the mfgr suggests bypassing one or two sensors why not bypass all the defective sensors?

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