K6 750 cranks but won't start. SOLVED - GSXR.com
  • 3 Post By rv6john
  • 1 Post By apiazza
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post #1 of 5 (permalink) Old 07-13-2019, 09:50 AM Thread Starter
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K6 750 cranks but won't start. SOLVED

A guy posted up on FB about his K6 750 that just stopped running while riding. The starter cranks the engine but it does not run.

He had a previous issue with a fried ECM due to an overvoltage caused by an R/R installed by the previous owner. Boiled out battery as usual. But that was all fixed and bike ran well for months.

A shop had the bike for 2 1/2 months and had no success finally telling him that the ECM was probably bad again.

I offered to swap his ECM into my K6 to test but since he is about 50 miles away, we went through some tests over the phone first.

I had him double check for spark and there was none at any coil.

The cluster was flashing FI due to a C28 (secondaries removed) and a C46 fault but neither would cause the no run issue. Also temperature showed between the flashing FI. This confirms that the ECM is talking to the cluster and that is a good sign of ECM health.
The fuel also "primed" with key on which is also a good sign of ECM health.

I had him check for battery voltage at the spark coils and it was present so ignition and kill switch were good.

He makes the drive to my shop and we pop his ECM into my bike and it starts instantly. At least we now know it's not the ECM.

I reconfirm that there is voltage to the spark coil and also note that my test light does not flash on the ground side while the engine is cranked with the starter.

The conclusion is that the ECM is not triggering the coils and the usual reason is that it is not getting a signal from the crank position sensor (CKP).

I tested the CKP for resistance and peak voltage and both are within the service manual limits. Recalling several hot engine no run issues with no FI codes we pulled the clutch cover and found that one of the CKP mount screws was totally loose and stuck to the magnet near the coil sensor.

Loctited both screws, cover back on and the bike fired up instantly.

So this is another situation of a CKP problem not sending a good signal to the ECM but the ECM is not unhappy enough to throw a C12 code.

BTW, we did check with the CKP unplugged and after a few seconds of cranking, the C12 code popped up. I'm guessing that the ECM is only looking at coil resistance or some other low checks and not actually for a pulse.

Originally Posted by Chuckster View Post
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post #2 of 5 (permalink) Old 07-15-2019, 01:55 PM
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Well I'm glad I didnt chase the crank position sensor that long. The 675's are a bit known for the issue and as soon as I posted about the issue several jumped in and said crank shaft position sensor was the likely culprit. Mine though was the sensor actually bad. My 200 ohm sensor was measuring about 150x higher than it should.

Good job figuring it out and helping someone get back on the road. Too bad he wasted so much time and money.

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post #3 of 5 (permalink) Old 07-15-2019, 03:10 PM
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Good write up John, thanks!

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post #4 of 5 (permalink) Old 07-15-2019, 05:35 PM
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You have a nice gift John. Strong work

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post #5 of 5 (permalink) Old 07-17-2019, 04:06 PM
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Yep, thats pretty cool. Good info to know as well for everyone.

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