1989 Yamaha Excel V6 225 No spark unless computer is bypassed

[Gary4000]

UPDATE 5/31/17

It was a bad computer. Replaced and the boat started up fine. Thank you all for the advice, between diagnosis and purchasing an official Yamaha service manual, and a special thanks to bondobill who provided the replacement YMIS unit.

https://www.yamahapubs.com/ Quick link to where I got the official manual.


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Hello
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

[99yam40]

I have no idea, but maybe the kill switch could be bypassed that way.

hopefully Rodbolt , Boscoe, or someone else that knows will chime in

[ausnoelm]

Exactly what "computer" are you bypassing, and how?

[rodbolt17]

the early excel motors used a unique ECU.
it also used a separate CDI assy inside the ECU.
by bypassing the ECU you lock the pulser coils directly to the CDI and ign timing is fixed.
test for good 12V and good grounds to the ECU.

[Gary4000]

The computer is bypassed by swapping the A and B plugs. And the motor starts in seconds. With the computer attached, there is no spark.
There is 12 volts on the yellow wire going into the computer. The black wire is connected to a ground terminal on the engine. It does ring out to ground. The motor has run before with the computer.

[Gary4000]

I'll I tried some resistance tests with the CDI box from this troubleshooting guide http://www.cdielectronics.com/wp-con...side-Pages.pdf (page 100)

When I have the computer bypassed, i'm reading 350 Ohms for the white/colored wire trigger resistance tests. Once i plug the computer in, it spikes to 42k ohms.

[rodbolt17]

that is because when you hook it up there are many parallel circuits.
you don't do resistance tests on the pulser coils with them hooked up.
output voltage tests? yes you can hook them up.
have you tested the crank position sensor and the TPS?
we know the pulser coils are working if you have spark on all 6.
if you have spark on all 6 we know the charge coils are working.
if you have spark on all 6 we know the ign coils are working.
what we have is the spark PRODUCING side is working.
the spark CONTROLING side is not.

[Gary4000]

We did test the crank PS and TPS and they seemed OK.
We just ordered the real Yamaha manual and it will be here soon.
Any other ideas? Any way to test the computer?

Thanks

[Gary4000]

ICU-15 9128 is the number on the micro computer. Are there any code readers or something that can tell us why the computer will not allow a spark?

[ausnoelm]

Best follow tips on post 4 before you go any further, and remember, connections must be good, not just look good!

[Gary4000]

OK Then

Well it turns out the computer was bad according to Tuppens. This YMIS?? is 30 years old and hard to find and if found $$$$. Wondering if we can use the motor without the computer.

[99yam40]

probably not with a very good ending unless you want to run slowly.

I am not sure what timing it gets locked to, but I bet it is not where it should be at regular operation.
maybe just to get you out of the way of a bad situation

[Gary4000]

Options

Find a computer, New motor, Ditch the boat

[boscoe99]

probably not with a very good ending unless you want to run slowly.

I am not sure what timing it gets locked to, but I bet it is not where it should be at regular operation.
maybe just to get you out of the way of a bad situation

What? No happy ending?

[bondobill]

I have a computer if your looking
Came off my 89 excel 225. Engine had 900 hours on it.

Bill