Auto coil tester

Discussion in 'The Projects Forum' started by Topgearx, Sep 29, 2007.

  1. Topgearx

    Thread Starter New Member

    Sep 29, 2007
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    I am trying to build a automotive coil tester/driver. I had a schematic but have lost it. I remember some of the components,but am not shure how to assemble them.
     
  2. Topgearx

    Thread Starter New Member

    Sep 29, 2007
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    I had the schematic, started gathering up the parts,then i lost the diagram, it seemed vague anyway. It consists of a big capicitor,22mf 600v mabey(dont know where i got that value)if that sounds like one that would be in a garadge door opener or from a large airconditioning unit, mabey for capicitor start motor.An automotive relay with five contacts(not shure if they are all used),a automotive capicitor(mabey for absorbing the spark from inside relay when it switches to break the primary circut in the coil and it had two spark plugs. There were four wires coming out of it with alligator clips on them two went to a 12v battery and the other two went to the primary coil leads, may be used for a single or double high tenson leaded coil. You isolate the coil for testing. For a single high tenson leaded coil you connect the tester to the battery then connect one of the other two leads of the tester to the primary coil contact and connect the high tenson lead from the coil to one of the spark plugs that are mounted on the tester with a window to view the spark. The battery charges the coil or the big capicitor or both. Then the relay switches somehow breaking the primary circut and sparking the plug then I think it repeats.
     
  3. thingmaker3

    Retired Moderator

    May 16, 2005
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    Where did the schematic originally come from?
     
  4. Topgearx

    Thread Starter New Member

    Sep 29, 2007
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    The plans are from a Kawasaki Technical Training manual. Troubleshooting Kawasaki Ignition Systems Video Guide section.
     
  5. SgtWookie

    Expert

    Jul 17, 2007
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    Well, you COULD rig up a circuit with that stuff - which would work fine if you were testing coils designed to work with points. But except for some big-block Chevy motors in RV's, all the ignitions I know of went to electronic back in '75 - they stopped using points, and the current being dumped to the coils was increased, causing much higher secondary voltages.

    Using just points & condenser on a coil designed for electronic ignition systems might not point out defects at the higher currents being supplied by the electronic ignition; ie: primary getting warm and shorting out; secondary getting carbon tracks in the tower, etc.

    But yeah, if you were going to test one that was DESIGNED for points, then sure.

    You should also add a current limiting resistor to the mix though - otherwise, your relay will get fried contacts pretty quickly - along with your coil getting excessively warm due to the high current. Check the "Help" section at your auto parts store - they should have an ignition resistor for a Ford, Chrysler or Chevy hanging up. Doesn't really matter which one; they should all be about the same value.

    Your relay is double-pole double-throw (DPDT), or double-pole single-throw normally-closed (DPST,NC), right? That's what you'll need.

    Take your automotive point condenser (cap) and hook it up across one set of NC points. The side of the cap that is supposed to screw on to the distributor's breaker mounting plate also gets a wire to hook to the vehicle's ground -or- battery negative terminal, and also a wire to one side of the relay's coil. The points side of the cap goes to the minus (-) terminal of the ignition coil, after disconnecting the existing connection to the distributor or wiring harness.

    Back to the relay - connect the other side of the relay's coil to one terminal of the remaining NC contacts, The other side of that NC contact should get wired to a clip that goes to the battery - and also to the positive (+) side of the coil via the ignition load resistor (after disconnecting the lead to the wiring harness.) The relay's coil should also have a diode connected across it (in reverse polarity to normal current flow) to absorb the back-EMF when the points open. Connecting your .22 uF 600v cap across the contacts for the coil will help preserve the relay's contacts and make it cycle more slowly.

    You obviously want to connect the positive clip to the battery as the very last step. As soon as the clip makes contact, the relay will start buzzing like mad, and the coil will be sparking like crazy (if it's any good.) Make sure you have a properly-gapped spark plug on a known-good piece of secondary ignition cable and that it's properly grounded, or you'll subject the coil's secondary windings to VERY high voltage.

    Rule of thumb for radio-noise surpression secondary ignition wiring is no more than 1K Ohm per foot, no more than 20K Ohms per wire.

    Some vehicles use pure copper secondary ignition - in that case, the plugs should be resistor-type; otherwise the vehicle will be a rolling Tesla-field generator.

    Make any sense?
     
  6. SgtWookie

    Expert

    Jul 17, 2007
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    Here, I threw a schematic together. Make sense?

    Had to use a tube for a spark plug - didn't have a spark gap symbol.

    Edited to fix inverted diode & misc - file has been renamed. C1 is the condenser (cap) from a vehicle's ignition system; just about any will do. For best results, use the same type replacement cap for the unit under test. You could then compare your results by swapping the replacement cap with the one in this circuit. 1uF is the default for the tool I was using; ignore that.
     
  7. Topgearx

    Thread Starter New Member

    Sep 29, 2007
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    This is the relay i have.
     
  8. Topgearx

    Thread Starter New Member

    Sep 29, 2007
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    Wondering if a Panasonic GAA 20VDC capicitor will work? It's out of a late model Ski-Doo
     
  9. SgtWookie

    Expert

    Jul 17, 2007
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    Argh, I just realized I put the diode in upside-down. :rolleyes: I'm not having a good diode day. ;)

    Well, you really want to keep the coil of the relay out of the circuit with the ignition coil primary; otherwise they'll interact. Also, since your relay schematic indicates it is NO (Normally Open), it wouldn't work for the schematic I've drawn. You would need some way of energizing the relay's coil.

    I suppose you COULD simply leave C2 and the upper set of relay contacts out, and just zap the relay coil wire to the battery positive terminal - that would close the relay, causing the coil to fire. However, you wouldn't have any kind of endurance test available. That was the idea of using the 2nd NC contact to energize the relay coil; it would cause the relay to make & break the "points" connection as fast as it could go.

    Oh, that cap you have should work fine. Doesn't it have a clamp on it for the breaker plate attaching screw?


    Radio Shack carries a relay that would work just fine with the original schematic:
    12VDC/10A DPDT Plug-in Relay, $8.49 Model: 275-218

    You might find something at an electrical supply house that would be cheaper. I've already checked a few online auto supply sites, and it seems that most of them carry something similar to what you already have.

    It's up to you what you want to do; get another relay, go ahead and try the mod using just the relay coil wire to the battery positive, or see if something else surfaces. I just don't see a way offhand of using one set of contacts without the relay's coil interfering.

    I've attached a mod of the schematic, putting changes in for what you have. No, you don't have the switch; that's just representing you manually touching the battery + terminal with the end of the wire.
     
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