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CIRCUIT FAULT... please help

Discussion in 'Troubleshooting and Repair' started by NT15MCP, Mar 22, 2011.

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  1. NT15MCP

    NT15MCP

    5
    0
    Mar 22, 2011
    I have a small circuit issue. I've been working with electronics for almost 10 years and this fault makes no sense to me. If you can explain the indications I have I would appreciate it.

    Background: This is a split-phase A/C motor control circuit. There is an automatic control for the motor that turns it left or right and a manual control switch to turn it left or right if another switch is selected to manual. The motor will not turn in either direction and both auto and manual power sources have been verified good. The motor windings are verified good, the resistor is verified good.

    Troubleshooting: First we checked the resistances of the switch positions for the switch S-1 since we were initially informed that the motor worked only in auto control. Swith S-2 was placed in Manual for these checks. The red arrows show the contacts checked with S-1 in LEFT position. These contacts read an OL resistance condition. Contacts 3 and 4 while in LEFT read 0 ohms as expected. In the RIGHT position contacts 1 and 2, as well as 5 and 6 also read 0 ohms. We replaced the switch with a new switch verified to be good and received the same indications in circuit.
    The motor windings and resistor were verified individually to be the proper resistances via terminal connections at the motor. Leads were lifted on S-2 contact 5 for both wafers to isolate the capacitor and the capacitor was checked with a Huntron Tracker and a Capacitance meter. The Huntron Tracker image is reproduced on the schematic.
    The LEFT and RIGHT paths were verified individually via resistance measurements with S-2 as shown. RIGHT verified with the blue arrows and LEFT with the Green arrows. RIGHT path resistance: 30 ohms as expected. LEFT path resistance: OL resistance condition. This path if the two switches were repositioned to manual on S-2 and LEFT on S-1 is electrically the same points measured with red arrows.

    The Question: What fault could cause a switch to have an open resistance across internally shorted contacts (7 and 8) when a circuit is connected, but a short resistance across the same internally shorted contacts with the external circuit removed? I was fairly certain that the resistance there should read 0 ohms if the switch is intact regardless of the external circuit.

    (Circuit images are attached)
     

    Attached Files:

  2. NT15MCP

    NT15MCP

    5
    0
    Mar 22, 2011
    Found the fault

    Okay, so the fault was a poor wire lug that resulted in an open at the orange dot... the problem is it still doesn't explain how I got an open reading across two switch contacts that should have been internally shorted... anybody have any idea?
     

    Attached Files:

  3. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,387
    2,772
    Jan 21, 2010
    I'm afraid I don't see how the connections are supposed to work in the lower diagram. The connections shown for S1 and S2 do not indicate (as they do in the upper diagram) the way the switch operates.
     
  4. NT15MCP

    NT15MCP

    5
    0
    Mar 22, 2011
    Okay, for better understanding... the drawings have been modified to show the switch positions during testing vice the default switch positions. The numbers in the circles are the contacts on the switches themselves (there are 2 sets for one of the switches because it has 2 layers of contacts commonly referred to as wafers). Inside the circles of numbers there are lines connecting the contact numbers that the switch should short together internally.
     

    Attached Files:

  5. NT15MCP

    NT15MCP

    5
    0
    Mar 22, 2011
    Please Delete

    I stopped caring.. chalking it all up to PFM (Pure F'n Magic). Please delete this thread if you are an administrator. Thanks.
     
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