Dang switch box keps frying and very irratic idle

[enginepower]

I think I've found my problem but not sure. I have replaced 2 switch boxes already on this 1995 90hp 3cyl. While at idle on the muffs and checking the coil input voltage with DVA, my digital volt meter will sometimes freak out with segments all over the place or shut off only when it's hooked to the #1 cylinder. Other 2 cylinders are reading about 180V. I am suspecting the ignition coil must be feeding back voltage on the control circuit. The first switch box that failed, lost #1 all together but other two were still firing fine. The second time it went out, the whole thing just crapped out. I now have a brand new OEM replacement and it's still doing this weird thing at idle. Will idle fine for a second or two then get real rough and sounds like drive shaft rattling when it does this. Then clear up and do it again but no problems on top end. Am I right by thinking the #1 coil should be replaced?

 

[bgathy402]

Could be a bad trigger. Swap the coils from number 1 and 2 and see if the problem follows the suspected bad coil..

 

[enginepower]

Well, if I understand it right, the trigger just activates the SCR (electronic switch) inside the switch box to tell it to send power stored in a capacitor it collected from the stator to power the coil to produce spark. My head almost hurting thinking so hard on this. Was also basing a theory that maybe the cylinder that fires before #1 could be spiking back and giving that capacitor some extra charge for next coil to fire but I took an ohm reading on the secondary of each coil. Reading from the spark plug end to coil terminal, it's open circuit and the other two are about 850 ohms or so and the problem is inside the coil....guess #1 coil DOES need replacement.

 

[bgathy402]

Yea, sounds like you got it figured out. When my motor had spark issues I was able to trace the problem back to the switchbox side of the circuit because the problem did not follow the coil I suspected was bad. I am not a marine mechanic but I am a master automobile tech and I have always replaced all coils instead of just the one that was bad, when cost allowed of course. My thinking was that if one went bad, how long will it be before the customer is back because the same problem. In thier head it feels like the same problem but it could another coil pack going bad on a different cylinder. Then you are made out to be the bad guy because they think you're trying to take advantage.

 

[enginepower]

OK so I just replaced the #1 coil yesterday with OEM part and it's still making my meter blank out when taking reading on #1 cylinder. I held the meter on the other two cylinders for at least 10 seconds and get a pretty steady reading. I can't hold the meter on #1 for more than 2 seconds or segments will flash all over the place and then it goes blank and have to turn meter off then back on. I hate to fry another switch box. I haven't taken it to the water yet but I have a feeling the problem isn't fixed. Charging system works fine but my tach doesn't seem to work smooth and maybe something is wrong with the regulator/rectifier? It's not directly hooked to the firing circuit but diagnostics say to disconnect it to see if a misfire goes away at higher RPM but my problem is at idle and both times the switch box failed was AFTER I shut the engine off. There were a couple of times I shut it off and had no spark on restart on any cylinder. Waited a few minutes and then it would start.

 

[Texasmark]

Put a .01 uf ceramic disc capacitor across the meter leads at the meter...pull the leads out of the meter just enough to see metal and wrap it around there....obviously the leads will shock you so keep your fingers off them. Your meter is picking up ignition interference and why #1, could be wire routing, doubt the CDI would cause that if it failed.

Get a timing light and put it on the 3 spark plug wires while it's running and when it fails. What do you see?

 

[enginepower]

So I used a different meter and I'm able to keep a reading with it on #1 cylinder. It reads about 10V higher than the other 2? Whatever. I also marked flywheel with numbers for TDC for each cylinder and checked with timing light and have good steady fire on all cylinders at idle and above.. I stumbled upon some interesting info that could cause my problem so I checked into it. Disconnected black/yellow kill switch wire at the switch box and checked for voltage on it while turning key on and off several times. My meter range bar spikes up a bit when first turned on but only reads about .088V steady and .000 volts when off. From my understanding, it should be open circuit when key is on. so checking ohms, I get open circuit when on, and off can be a range of anywhere from 1 to 2000 ohms to ground. Not sure why I'm getting very small voltage when on. So the ignition switch should ground out the switch box to kill it correct?? Wiggling ignition switch makes my dash voltage gauge fluctuate anyway.

 

[enginepower]

Update:

Replaced ignition switch and had the boat out 4 times with a total of probably at least 20 start/stop events. Switchbox still ok. Really think this ignition switch was killing my switchboxes as I read in another forum it could do this. Both times it fried when I shut ignition off. Still have the weird idle thing. I need to bring a timing light out I guess and see if I have any spark dropping out when this happens because if feels like a cylinder is dropping out and get a rattling driveshaft type sound but ONLY at idle. The slightest throttle will make it run perfectly smooth.

 

[gm280]

Update:

Replaced ignition switch and had the boat out 4 times with a total of probably at least 20 start/stop events. Switchbox still ok. Really think this ignition switch was killing my switchboxes as I read in another forum it could do this. Both times it fried when I shut ignition off. Still have the weird idle thing. I need to bring a timing light out I guess and see if I have any spark dropping out when this happens because if feels like a cylinder is dropping out and get a rattling driveshaft type sound but ONLY at idle. The slightest throttle will make it run perfectly smooth.

I have to say this is an interesting problem. Glad to see somebody with the knowledge of electronics troubleshooting their ignition issue. Not many folks understand SCR switching and Capacitor discharge ignition. Knowing how those things work, makes this even more interesting. Not knowing how the manufacture designed their CDI system, it could be possible for the first triggered fire is indeed SCR triggered. But the follow on firing could be SCS (Silicone Controlled Switch) instead of SCR.That way as the first spark pulse drops off, it triggers the SCS on the next cylinder to fire and so on. And that could be the reason for the difference in voltage between them... I can't say for certain unless I deconstructed an old CDI or Switchbox to figure their schematic out. Keep posting your results. I'm really interested in what's cause this... :thumb:

 

[enginepower]

Yes, I am very knowledgeable of how electronics work and I think the electrics are doing fine unless a timing light pulses intermittently and proves me wrong but I had a brain fart...Since my problem only exists intermittently at an idle only, I might have a gasket with a small leak causing engine to ingest a slight amount of water from exhaust side? At idle in gear it's smooth for about 3 seconds and rough and rattly for about 4 then smooth for a few then rough again. Off idle (1k-1200rpm or above) it runs as smooth as can be. Reason I am thinking this is because at idle, my water pressure gauge seems to fluctuate a bit as well. (glad I haven't fried any more switchboxes)