225 V6 Excel not starting

[Tracer]

Recently bought a Yamaha 225 V6 Excel 1987. Motor does not start if normally wired. When I bypass the Microcomputer as in the emergency mode: plug A to plug B it purs like a kitten.(Pulser directly into CDI) Is the Microcomputer faulty or what could be the problem - sensors?
Normal operation: Plug A from CDI unit into Plug A Microcomputer and plug B from Pulser coil into plug B from Microcomputer.
Any service manuals lying around somewhere?

[boats.net]

Yes you to check the sensors, starting with crank position sensor. You can order a service manual from Yamaha at http://www.yamaha-motor.com, but I'm not sure if they will ship to South Africa.

[dtmackey]

Thers's a complete wiring harness for that year motor with computer on Ebay right now.

D-

[slim]

i know what your problem is. there is a yellow wire in the harness that supplies voltage to the computer. if the voltage in this wire drops below 9 volts. your motor will not start unless you try bypassing the computer as you did. be gratefull there is nothing wrong with your sensors except the crank sensor might need cleaning and adjusting. you can do this easily by taking it off. sanding the metel protruding out of it and replacing it on the motor. when tightening the screws, push it towards the flywheel as far as possible without it touching the flywheel. although i have done this many times and it never does hit even though i pushed it as far as possible. but back to the probable cause. under the starboard head, unwrap the tape from the main harness and find the yellow wire. strip it back to reveal the wire. using a test lead with alligator clips, clip one end to the yellow exposed wire being carefull not to touch to a ground. connect the other end to either the battery(if long enough) or at the starter relay on the hot side(positive battery cable). my bet it it will crank. if not try this. measure the voltage at the exposed point while turning over. remove all the spark plugs so it wont start since you dont want it to while you test. like i said earlier. if you dont have 9 volts, it wont. the problem could be further towards the computer. a few times the harness, after years of use, rubs on the bottom cowling and could be open and corroded. if the motor starts while you have the jumper on it, the fault is most likely in the main harness plug in the motor. you can cut the harness cover carefully and get to the yellow wire. solder and heat shrink and yellow wire the the main boat harness and to the wire that you exposed back by the computer. cut the wire in the middle and install a waterproof quick disconnect incase you ever have to remove the power head or any other reason. this process is very easy and much cheaper then a new engine harness. let us know what you find.

[Tracer]

Thanks guys. Will first clean the crank sensor and try out Slim's advice. This motor has been sitting for about 5 years and no one could solve the problem yet. Some twenty guys have each had a chance before I bought it, and I don't know what they each did. So I started by tracing the wires to see if continuity exists. The Agents say's it is the CDI just by looking at it. Another mechanic say's it is the Microcomputer. I will keep you posted.

[slim]

hey tracer, im glad to know someone finally has the answer your looking for. i am 99.9% sure that the micro computer is just not getting the required voltage. try the crank sensor first since it is a easy check. if it doesnt help. then move on to the harness. like i said. i bet thats it.

[Tracer]

I cleaned the crank sensor. It was not rusted or dirty but I did it anyway. I saw that the positive terminal wire from the battery was darkened by the DC voltage over years, so I cut off the terminals and stripped another piece. I disolved tartaric acid in water to clean the newly exposed blue copper from the blue colour. Then I crimped new lugs on and fitted new terminals. When I measured yellow wire voltage it was 12.45 volts. I primed and guess what--- It purs like a kitten. Took it out to the dam on the weekend and it goes like never before. I bought the harness on e-bay from dtmackey for $200-00, just incase. The biggest problem now is to get it to South Africa by courier or post. UPS Quoted $300-00. I'm still waiting for DHL and Fedex quotations. If you know of cheaper way I would appreciate your advice. Thanks again for your advice.