1989 Yamaha Excel V6 225 No spark unless computer is bypassed

GG4000

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Oct 22, 2016
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Hello all! New to this great forum.
I have a 1989 Yamaha Excel V6 225 that did run before. Now the no spark issue.
Engine will not start unless we use the A & B connectors to bypass the computer. Any ideas of which interlocks (sensors, etc) would cause this?

Thanks in advance.
 

mjf55

Chief Petty Officer
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Jun 17, 2014
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462
GG4000, Welcome to iBoats.
Looks like you have a tough question here. I cannot answer it directly, but I do have a question.
When you say you use the A & B connectors to by pass the computer, what exactly are you doing. I am not familiar with the A & B connectors. I have the 1989 Service manual ( do you ? ) and in reading it for your Excel model, did not come across any reference to these connectors. Can you enlighten me on what they are?

For an Excel model, you have the YMIS computer systems. Sensors include:
  • Knocking sensor
  • Throttle sensor
  • Therm0-sensor
  • Crank Position sensor
  • Starter relay sensor -
Come on back with some more details.
 

GG4000

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Oct 22, 2016
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8
Hello
I do have the manual. It does not tell which sensor will keep the motor from starting. The A & B plugs are in the manual. Some motors say to disconnect the yellow wire. But not this motor. Tried it. I do show 12 VDC on the yellow wire. The AB plugs have about 5 to 7 black and white wires. You would disconnect the A group and plug it into the B group. It can be used for an emergency start to get you back home. The engine will run fast due to no idle/timing adjustment from the computer. When bypassed it will start up like new. A spark tester will show a bright spark. If I connect the computer, there is no spark.
 

mjf55

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Jun 17, 2014
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462
GG, I read the manual carefully and did indeed find the emergency bypass of the YMIS system on page 1-34.
First, make sure you have 7 volts at the YMIS computer while cranking. (page 1-35
Starting on page 1-25, there are discussions on different aspects of the engine running and what sensors affect the aspect. The first one I would check is the starter relay ( item 1 on 1-25) as it seems to have a direct link to starting and initial timing. The others seem to affect idling, cruising, oil/overheating.

So in the back of the book, there is a YMIS troubleshooting guide. It also suggests the throttle and crank position sensors, along with the YMIS itself. I suggest you follow that trouble shooting guide. It is located just before the wiring diagram for the Excel
 

GG4000

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Oct 22, 2016
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So far we did not get to the the starter relay but it will be checked soon. And the crank sensor seems to be about 500 ohms off. Should be around 138 to 256 ohms and we are getting 700 ohms. Throttle sensor, book says a lower number but we are getting 1k and the sensor label shows it's a 1k sensor. The motor has been running on this. Also the wire colors are not the same as the book. There are no orange wires on the motor.
 

mjf55

Chief Petty Officer
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Jun 17, 2014
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462
CC, from the flow chart I have, it says the Crank sensor should be 740 ohms +/10% ( yours is at 700 ) and the throttle sensor should be 1K, witch yours is. I have attached a screen shot for reference.

Check you PM
 

Attachments

  • YMIS-Flow-Chart.pdf
    198.5 KB · Views: 9

GG4000

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Oct 22, 2016
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Thank you for the info. I will have to take another peek at the manual. I thought the crank sensor was supposed to be about 100 to 250 ohms. So at 700 it might still be OK? Will get on more checks this weekend.
 

mjf55

Chief Petty Officer
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Jun 17, 2014
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462
Near the back. In between the wiring schematics.
 

GG4000

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Oct 22, 2016
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I have the SELOC manual and the ignition system is in section 4. I think we have different books. My book seems wrong. Page numbers and the wire colors are different.
 

99yam40

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Sep 7, 2008
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8,851
get the Yamaha service manual for your motor
after market covering all sorts of motors are not the best
hard to figure out what motor they are talking about and probably do not cover it well
 

mjf55

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Jun 17, 2014
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462
I agree with 99yam40. Get the factory service manual.
 
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