95 SN OB GT-40 - no spark

Those flow type diagnosis steps can sometimes point you down the wrong path.

I would back track a bit and start from your low voltage of 5.1V at the coil 3F1-9.

It should be 12V or battery voltage go to the circuit diagram 6-10 The 12V is switched from the EEC relay. Check the voltage there at relay terminal 87 and 30 you may need to make up some jumper wires as it is a enclosed water proof connector. This is with the ECM 60 pin connector plugged in as normal.

While you are there check the voltage at 12.5 amp ECC breaker just above the relays they are normally covered with red liquid insulation so make sure you are getting good contact. You may also still have the large fuse in the fusebox item 14 in the diagram check this connection as it is a known problem area for bad connection and it is the power source for the EEC circuit. There is a modification to remove this if it is the problem.

Get back to us when you have checked these steps out. I did not like the sound of how you had 12V when you unplugged the ECM but lets just get the 12V power source circuit checked first. This is basically the same as steps 3F1-4 to 3F1-6 in the manual.

Good work on isolating the problem as you found the fuel pump relay and the EEC relay are the same. The relay will have high resistance across the contacts this causes the voltage drop you measured.

The other GT40 Fuel Pumps thread going at the moment was just discussing the GT40 relays. As stated it is worth carrying a spare in the boat. They seem to have a decent failure rate.

Ordered 3 from Napa Online today. The local stores had only one in stock and I didn't want to drive to three stores! Will update when installed and test. Thanks

Glad that fixed it. So, it was the EEC Relay then?

I try to keep a running tally of these on the:
Poor Man's gt40 diagnosis thread.