Mercruiser 4.3 EFI - No Spark

[scrimshank]

My boat is fitted with a Mercruiser 4.3 EFI with what I believe is a Thunderbolt IV ignition system. The motor consistently starts promptly and runs smoothly at all speeds.
On the water today I had just got up on plane when the motor sputtered, died, and has since refused to start.
The twin batteries are strong, and there is most of a tank of gas.
With the flame arrestor removed I can see that the injectors are pumping gas as the motor cranks over, however there does not appear to be a spark.

With the ignition switch on there is 12.45 V at both terminals of the coil (this seems odd to me).
In the course of searching this forum, I found the attached troubleshooting procedure for my system (Thanks Don S!).

In working through this procedure:
*With key in Run position I have 12V at the positive side of the coil (and at the negative side)
*I have 12V at the Wht/Red terminal on the sensor inside the distributor
*I have some confusion over the next step:
I disconnect the coil high tension lead from it's central position in the distributor cap and connect to ground through a Spark Gap Tester.
I disconnect the Wht/Grn lead (from the sensor in the distributor) from it's connection to the harness (In my case, a Purple/Wht wire).
With the ignition key ON, I strike the Wht/Grn lead to ground.
This is where my confusion arises:
If I strike the disconnected Wht/Grn lead from the sensor to ground, I get no spark.
If I strike the connector from the harness (Purple/Wht) that connects with the Wht/Grn lead, I do get a spark.

I think this means that the coil is working, and that the sensor is bad - but I wonder if I'm misinterpreting the troubleshooting guide.

Thank you for any insight.

 

[dubs283]

Re: Mercruiser 4.3 EFI - No Spark

you performed the troubleshooting correctly

the pur/wht wire you have is coming from the module (so to speak) - the ecm controls the spark really, but you did troubleshoot that the sensor is indeed faulty - it happens quite often with the older style sensors

one of the sensors jobs in an efi set-up is to perform the job of the crank sensor, it tells the ecm that there is engine rpm and that it should begin providing signal to spark

 

[scrimshank]

Re: Mercruiser 4.3 EFI - No Spark

Thanks for your prompt and reassuring reply.
Should I be concerned that I have 12V at both terminals on the coil?
In trying to rationalize this observation, I suspect that the connection to the negative side of the coil must be interrupted to induce a higher voltage in the high tension portion of the coil.....?

 

[dubs283]

Re: Mercruiser 4.3 EFI - No Spark

Should I be concerned that I have 12V at both terminals on the coil?

that is normal

the coil is resisted internally unlike a conventional system which relies on an external resistor before the coil