'02 Merc 4 Stroke - Diagnosing bad ECU?

[Thirsty Endgrain]

s/n 0T733923

Hi all,
Coming in the other day it started acting erratic, would idle and then bog down hard as soon as I put it in gear. Limped home at 6 knots, admittedly longer than I should have with a bogging engine but it was getting late/dark.
Assumed carbs since it's a Mercaha known for plugged jets/gumming issues, though I've always run Quickleen and the other additives to keep fuel stable since buying it and rescuing it from someone who couldn't get the carbs cleaned. That was a couple years ago and I figured it was time for a cleaning again, which I did. Put it all back together and realized it only has spark on cyl's 1/4. 2/3 had no spark, so I replaced the coil. No change, so I traced wires back to the ECU, which is providing only intermittent signal to the 2/3 coil. Does this confidently condemn the ECU? Are there any other lines of troubleshooting I can do? I already ordered a used one off of ebay to get it on the way, but if I'm missing something that someone knows of I'd appreciate any advice. Thanks.

Oh, I made a video of it surging in and out of spark, not sure it would help troubleshooting, but figured it couldn't hurt. I was poking around at the wires to see if it was a loose connection somewhere, but couldn't confidently reproduce the surging traced to any one wire/harness or the other.

 

[dingbat]

s/n 0T733923

Hi all,
Coming in the other day it started acting erratic, would idle and then bog down hard as soon as I put it in gear. Limped home at 6 knots, admittedly longer than I should have with a bogging engine but it was getting late/dark.
Assumed carbs since it's a Mercaha known for plugged jets/gumming issues, though I've always run Quickleen and the other additives to keep fuel stable since buying it and rescuing it from someone who couldn't get the carbs cleaned. That was a couple years ago and I figured it was time for a cleaning again, which I did. Put it all back together and realized it only has spark on cyl's 1/4. 2/3 had no spark, so I replaced the coil. No change, so I traced wires back to the ECU, which is providing only intermittent signal to the 2/3 coil. Does this confidently condemn the ECU? Are there any other lines of troubleshooting I can do? I already ordered a used one off of ebay to get it on the way, but if I'm missing something that someone knows of I'd appreciate any advice. Thanks.

Oh, I made a video of it surging in and out of spark, not sure it would help troubleshooting, but figured it couldn't hurt. I was poking around at the wires to see if it was a loose connection somewhere, but couldn't confidently reproduce the surging traced to any one wire/harness or the other.

two cylinders out points to stator
Starts on page 112

https://www.cdielectronics.com/wp-content/uploads/2016/12/7th-Edition-Troubleshooting-Guide_RS.pdf

 

[boscoe99]

75 or 90 HP model?

The Yamaha F75 and F90's of that vintage did not have an ECU. A CDI assembly was used. Capacitor discharge ignition system.

 

[boscoe99]

 

[Thirsty Endgrain]

two cylinders out points to stator
Starts on page 112

https://www.cdielectronics.com/wp-content/uploads/2016/12/7th-Edition-Troubleshooting-Guide_RS.pdf

I am as of yet unfamiliar with stators. They can cause this kind of erratic behavior/surging? I'll look into that manual now...

Edit: my understanding is that stators would not affect ignition on this particular motor, but I'm here to be schooled not to know things.

 

[Thirsty Endgrain]

75 or 90 HP model?

The Yamaha F75 and F90's of that vintage did not have an ECU. A CDI assembly was used. Capacitor discharge ignition system.

Right thanks for the correction, CDI. It's a 90hp. Would the erratic signal from the pinout of the CDI to the coil possibly be caused by the stator, or is it more likely to be the CDI? Is there a way to test this?

 

[dingbat]

Would the erratic signal from the pinout of the CDI to the coil possibly be caused by the stator, or is it more likely to be the CDI? Is there a way to test this?

From the above attached manual...........pages 111 and 112

If (#1 and/or #2) or (#3 and/or #4) is not sparking, swap the White/Green and Green/White stator wires and retest.
If the problem moves to the other cylinders, the stator is likely bad.
If no change, replace all CDMs.
A continued no spark condition on the same cylinders indicates a bad trigger.

 

[boscoe99]

Do you see any markings on the CDI box? If so, what are they?

If a Yamaha box is installed, the output of the CDI to the ignition coils can (should be) be tested. As shown below. If the CDI output is normal then perhaps it is the ignition leads. If below normal then test for the inputs to the CDI.