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Thread: ‘85 250sx no spark

  1. #1
    Join Date
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    ‘85 250sx no spark

    85 250sx not getting any spark at the plug. Downloaded the service manual and started doing ignition system checks.
    Checked various continuity readings (ignition and run switch) between black/white and green wires at CDI plug, all checked good.
    Measured resistance between (ignition coil) black/yellow and green wire at CDI plug, 0.4 ohms (standard ie 0.1-0.3). As suggested in manual, check resistance at coil terminals, measured 0.3 ohms (standard is 0.1-0.3)

    Any ideas if it is normal to measure outside the acceptable range at the CDI plug , but measure inside the acceptable range when reading across the coil?
    Since the value is at the upper range, should the coil be replaced?

    Also checked measurements for alternator and pulse generator at CDI plug and measurements were all within range.

    TIA for your input.

  2. #2
    gimmeamidget is offline At The Back Of The Pack Arm chair racerAt the back of the pack
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    Pull the rubber boot off the wires on the back of the key switch and check the connections. I have seen a few where the solder fails and power is not routed through switch.

  3. #3
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    am i right that what you are sayin is when you test the coil from the wires it reads .4 and unplugged it reads .3? if so what is your secondary coil resistance? it should say in the manual what it should be probably like 7.5-11k through the plug cap and 3.5-4.5 with the cap removed. those are general numbers check your manual for exact numbers. the secondary reading should be between the green wire and the spark plug wire with and without the cap

  4. #4
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    Its rare but i have had no spark be as simple as a bad plug cap or wire.

  5. #5
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    Quote Originally Posted by gimmeamidget View Post
    Pull the rubber boot off the wires on the back of the key switch and check the connections. I have seen a few where the solder fails and power is not routed through switch.
    I checked the switch, checked out ok.

  6. #6
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    Quote Originally Posted by lndy650 View Post
    am i right that what you are sayin is when you test the coil from the wires it reads .4 and unplugged it reads .3? if so what is your secondary coil resistance? it should say in the manual what it should be probably like 7.5-11k through the plug cap and 3.5-4.5 with the cap removed. those are general numbers check your manual for exact numbers. the secondary reading should be between the green wire and the spark plug wire with and without the cap

    Correct, .4 measured at CDI plug, with wires connected to coil, .3 measured across the coil.
    Measured the secondary, 24.7k with the spark plug cap, 4.0k with cap removed.

  7. #7
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    Quote Originally Posted by lndy650 View Post
    Its rare but i have had no spark be as simple as a bad plug cap or wire.
    Any idea how to check if the plug cap is bad?

  8. #8
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    testing with a lead on each side of the cap it should be around 5k ohms

  9. #9
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    24.7k with the cap is really high

  10. #10
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    Quote Originally Posted by lndy650 View Post
    24.7k with the cap is really high
    Measured just the spark plug cap and go a reading of 22k.

  11. #11
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    Yup I would try a new cap. not gonna say thats it for sure but its very likely thats way high for a cap

  12. #12
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    Quote Originally Posted by lndy650 View Post
    Yup I would try a new cap. not gonna say thats it for sure but its very likely thats way high for a cap
    Checked the cap on my 85 200x it measured 5.2k. Put it on the sx, still no spark.

  13. #13
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    hmmm... my next guess would be the coil. its hard diagnosing something over the internet. ok so first check your green wire that goes between the cdi and coil and check where its grounded to the chassis. Next the black and white wire that runs from the cdi to your ignition key and kill switch is your kill wire if it contacts any part of the chassis you will lose spark make sure its not grounding out anywhere.

    edit: Stupid question but your neutral light is coming on right?

  14. #14
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    I went down to my shed and looked in my notes this is what i have for testing the wires at the 6 pin connector off the CDI see if this helps at all.
    Blue/Yellow- pulse generator= 290 to 360 ohms
    green/red- neutral switch off= 2 ohms on= infinity
    black/white- kill and key switch off= 2 ohms on= infinity
    green- ground= .1 ohms
    black/yellow- ign. coil= .3 ohms
    black/red- ac voltage= 250-400 ohms


    edit: forgot you have a manual so you probly have all that info... try unplugging the kill switch to rule out a bad switch

  15. #15
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    Quote Originally Posted by lndy650 View Post
    I went down to my shed and looked in my notes this is what i have for testing the wires at the 6 pin connector off the CDI see if this helps at all.
    Blue/Yellow- pulse generator= 290 to 360 ohms
    green/red- neutral switch off= 2 ohms on= infinity
    black/white- kill and key switch off= 2 ohms on= infinity
    green- ground= .1 ohms
    black/yellow- ign. coil= .3 ohms
    black/red- ac voltage= 250-400 ohms


    edit: forgot you have a manual so you probly have all that info... try unplugging the kill switch to rule out a bad switch
    My neutral light is on.
    I do have a manual I downloaded from a link on this site.
    However, I will check the points you mentioned.
    Measure each pin to ground I’m guessing??

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