Intermittent ignition problem with 1994 Merc 225-h.p. 2-stroke

[Frank O]

The last time we tried to take my 21-ft Boston Whaler out, the 1994 Mercury Offshore 3.0L 225HP carbureted 2-stroke outboard died a few feet out of the slip. It would crank and crank, but not start. There didn't seem to be any difference if I tried starting it with the built-in gas tank or a portable tank with its own bulb. The sound of the cranking (not even a hint of the engine catching now and then), made me suspect that it was an electrical rather than fuel issue.

This morning I ran some further diagnostics. First I ensured that the batteries were topped up and the connections to them were good. I disconnected the stop circuit from the kill switch and the ignition switch to rule them out as sources of the problem.

I then tried starting the engine with a timing light clamped on one of the spark plug wires. Initially it just cranked and cranked, and there were no flashes on the timing light. Then, however, the engine suddenly caught and ran for a bit, with the timing light flashing away. As the day went on, it seemed as though the problem came and went intermittently. Now and then I could get it to start, but sometimes I was back to the original symptoms.

I'm hoping that the problem is in the stop circuit (the circuit that is normally connected to the kill switch), though at this point I can't rule out that it could be elsewhere. Here are some known facts about the stop circuit:

1) With the kill switch and ignition switch disconnected from it, I got varying impedance readings at various times when I measured continuity between the stop-circuit wire in the engine harness and ground. If everything is working normally, should the impedance on this circuit be infinite, or is it normal for there to be some current flow through the ignition modules where they connect to this circuit?

2) If the problem is in the stop circuit, since I disconnected the kill switch and ignition switch, that leaves fewer failure points. Some of the models covered by my Mercury shop manual have a Shift Interrupt Switch connected to this stop circuit, but apparently not all of the models do. It's difficult for me to discern from the manual whether my engine has this switch or not. It's also hard to trace wire runs around the engine, as most bundles of wire have been tightly wrapped throughout their length in yards of electrical tape (did engines come from the factory this way, or did someone apparently add this electrical tape later?). If the engine has a Shift Interrupt Switch, would it normally be visible in the main engine compartment?

3) If there is no Shift Interrupt Switch, or if it doesn't seem to be the problem, are there any other suggestions on possible failure points to investigate? Can a defective ignition module cause the stop circuit to be tied to ground and cause the starting problem I'm seeing? I've read the suggestion to look for wire chafing that could ground out this circuit, but given all the electrical tape it's hard to inspect the conditions of wires.

Thanks for any possible ideas.

 

[Frank O]

Re: Intermittent ignition problem with 1994 Merc 225-h.p. 2-stroke

Actually, further digging in the Mercury shop manual disclosed more details about my engine. It appears that:

-- Yes, my 1994 225HP does have a shift interrupt switch, on the port side of the engine in the main compartment.

-- Mercury changed the way these switches behave across different model years in the 1990s. In 1995 and later engines, the switch facilitates shifting into neutral by momentarily retarding ignition. In my 1994, however, it achieves the same end by grounding out the capacitative charge from three of the six ignition modules. This is via the same "stop circuit" that the kill switch and ignition switch are wired into. There is a diode in the circuit so that only three ignition modules are grounded by the shift interrupt switch. It appears as though there's a plug connector that should be fairly easy to locate that should allow me to disconnect the shift interrupt switch. By disconnecting the shift interrupt switch, the kill switch and the ignition switch, I will have disconnected all of the routes by which the "stop circuit" is grounded out during normal operations. Doing this and then attempting to start the engine should provide useful information (along with a few impedance tests at various points).

-- I believe the impedance between any point on this "stop circuit" and ground should be infinite when the circuit is not being grounded out by one of the three switches mentioned above. If I again find the impedance to be anything else, that would be a smoking gun of a failure point.

-- All of the tape-wrapped wire bundles I see in the engine look very much like the line drawings of harnesses in the shop manual. So I guess the wiring came this way from the factory.

If you've read this far, thanks for listening!