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  1. #1
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    300xs blowing 20amp fuse

    Hoping someone may be able to shed some light on possible causes here?

    Was running yesterday at cruise pace when engine completely died suddenly. Engine would crank but not fire. Found one of the 20 amp blade fuses under cowl in lower fuse holder beside ECM had blown. Replaced with a spare in holder and engine fired no problems. We were towing a skier and left engine idling while we replaced cowl and prepared to start again, after about 1 1/2min engine died again, fuse blown again!
    I had one fuse left so I tried looking for obvious bare wires or anything, tugged on the loom and tried moving all looms slightly in hope of getting issue to go away. Installed last fuse and started again, this time I got straight on plane and headed for shore at cruise pace. Once again lasted about 1 1/2min and died!
    once towed back to bank I had a closer look, can't tell exactly what the circuit is running but it's obviously something to do with ignition?
    Looks to me like a red with yellow stripe wire and a red with white stripe wires coming in and out of that fuse circuit. Only wires I could see with same colours on engine was the coil pack loom has a red with yellow wire going to every coil. Could this be a coil shorting?
    funny that it will run for 1 1/2min before blowing?

    Any help diagnosing the circuit involved and possible causes appreciated.

  2. #2
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    Very likely you have a coil shorting out. See if any of them are real hot if you run again. Don't burn yourself! You can check coils for resistance to see if they are within spec, you'll have to reference a manual for that.

  3. #3
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    Post a picture of the fuse block with the blown fuse location and I can check my Merc manual to see what it powers. I'm going to say it's for the coils,
    Checkmate 2002 Convincor 270 496 MAG-HO

    2003 Cougar 22MTR w/300xs SOLD
    90 21Skater w/300xs - sold
    98 STV Euroski w/280 - sold


    2006 Trailblazer SS 6.0l 395hp

  4. #4
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    red/yellow is your coil fuse, if you have a mulimeter check for short to ground on the red/yellow. you should see continuity, disconnect one coil at a time.. if you see the short go away when you pull the 4 pin plug , you have found your bad coil.. if no change after all are disconnected the you need to look at the harness or coil harness.. hope that helps.. Rick

  5. #5
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    Yea, thanks rick.
    that was going to be my next step.
    unfortunately I'm out of town for the next week so can't get a photo or play with it until I'm back.

  6. #6
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    What year motor is it?

  7. #7
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    2009.

  8. #8
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    Just ran the engine on the flusher. Had a heap of 20amp fuses on hand and was going to just start pulling the plugs on coil packs one at a time until it stopped blowing fuses.
    So it fired and ran for over 10 mins at idle and never popped a fuse!! Grrrrrrrr. I hate when things won't play up at home on the flushers for diagnosis!
    I guess I'll have to get the boat wet and load it up a bit to get it blowing the fuses. got no time this week so will have to wait till next week again.

  9. #9
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    On the hose try trimming up and down, shaking the wheel, shaking the wire looms. Sounds like it's shorting which can be frustrating to find.

    pointer

  10. #10
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    Yea, did all that. I'm pretty confident it's a coil pack, just gotta find which one!
    I've had coil packs fail before, most start a slow demise and only drop out at high rpm or load (again making it bloody hard to work out which one), then slowly they get to the point they die completely.
    Only strange difference this time is it's popping the fuse.

  11. #11
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    Pull the coils off and look for cracking on the back side. My bad coil had what looked like a white goo coming out.
    Checkmate 2002 Convincor 270 496 MAG-HO

    2003 Cougar 22MTR w/300xs SOLD
    90 21Skater w/300xs - sold
    98 STV Euroski w/280 - sold


    2006 Trailblazer SS 6.0l 395hp

  12. #12
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    Quote Originally Posted by Euroski View Post
    Pull the coils off and look for cracking on the back side. My bad coil had what looked like a white goo coming out.
    Mine did the same.

  13. #13
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    Ran boat for 45min hard yesterday, never blew 1 fuse!?
    F@$&ed if I know?

  14. #14
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    It will, give it time! Just buy an extra coil along with having a 1/4inch ratchet and 8mm socket in your tool kit.
    Checkmate 2002 Convincor 270 496 MAG-HO

    2003 Cougar 22MTR w/300xs SOLD
    90 21Skater w/300xs - sold
    98 STV Euroski w/280 - sold


    2006 Trailblazer SS 6.0l 395hp

  15. #15
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    Well, raced last weekend. Motor never missed a beat all weekend.
    However had a bit of misfortune nearing the end of the day when conditions got real rough. Stuffed the boat hard in a following sea and busted sections of the deck! Very lucky actually as we could easily have sunk considering how deep she went in! Shape of the deck and windscreen saved me, deflecting most the water over the boat and my head.
    will be off the water for a while I think!

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