Help with ignition problem diagnosis, please?

[Boatleech]

I'm working on a 1994 40HP 4cyl Mercury, s/n OG084586. Symptoms are sudden failure after running for about 40 minutes, then it won't start for days. Found no spark on any cylinder after failure.

Stator resistance values, ground isolation, and cranking DVA at switchbox all check good. Trigger resistance values good, DVA good at switchbox. Zero DVA measured at all coil primaries. #1 coil measures 8Megohms on secondary, #2, #3, and #4 measure within a few ohms of 1K. All coils' primaries 0.3ohms.

#1 coil is obviously bad, but isn't causing the big problem. I strongly suspect the switchbox, but before I spend $225 on a new one, I'd like a confirmation from somebody who knows what he's doing. I'm wondering what sort of switchbox failure would affect all cylinders' output. About all I can suspect is an internal short in the stop circuit, but that seems unlikely, since I get a good DVA value at the input.

Any ideas?

 

[achris]

Re: Help with ignition problem diagnosis, please?

Remove the 'Kill' wire from the switch box and re test. Kill wire is black with yellow trace. It is the wire that grounds the capacitor (in the switch box) when you turn the key to the OFF position. That's the only other thing I can see that you haven't checked. Could the bad coil have killed the switch box? Possibly... but check the kill circuit first.

Chris................

 

[Boatleech]

Re: Help with ignition problem diagnosis, please?

Sorry, I omitted that. Kill wires (there are two of them on this boat) were removed for all tests.

mike

 

[achris]

Re: Help with ignition problem diagnosis, please?

Replace switch box.

 

[Boatleech]

Re: Help with ignition problem diagnosis, please?

I was afraid you were going to say that, but I'm not surprised; thanks...

mike