Re: Multiple sparkplug spark when key is turned to on position, 1973 Merc 1500 Thunde
The switchbox aka electronic ignition module mounts flat on the inside of the front cowl support. It's retangular in shape and has terminal boards on either side. Wires from the distributor hook to the Port side of the box and power wires/coil wires connect to the Stbd side.
The trigger is built into the distributor housing and is non-repairable. If you have a bad trigger you must replace the entire housing.
Here's a wiring diagram which will help you relate the different parts of the ign:
http://www.maxrules.com/oldmercs/Wiring/1966ona/35.jpg
And here's instructions on how to do some tests to your ign system which may help you narrow down the problem:
"MERCURY BATTERY-POWERED CDI TEST FOR DISTRIBUTOR MODELS
This test is for the 332-2986 switchbox used from 1967-1978 on all the inlines.
This test assumes your coil is good (problems with CDI coils are rare).
DISCONNECT BATTERY
1. Turn off ignition;
2. Disconnect all 3 distributor wires on the Port side of the switchbox (and the ?mercury switch? if present);
3. Remove the High Voltage lead from the ign coil to the center of the dist. cap (remember it unplugs from the coil and unscrews from the cap);
4. Reconnect the H.V. lead to the COIL only;
5. Position the free end of the HV lead approx. 3/8" from ground (block, shrouds etc), and find a way to hold it there;
6. Jumper the brown and white terminals on the distributor side of the switchbox to each other.
RECONNECT BATTERY
7. Check that you have +12V at the red terminal (even with the ign off);
8. Turn on ignition and verify +12V at the white terminal (same side as the red terminal);
9. Ground the black terminal on the distributor side of the switchbox - this should cause a spark each time you touch ground.
If you get spark with the distributor bypassed, and it won't fire with the distributor connected, the trigger is bad and the entire distributor housing assy must be replaced.
If you get no spark using the test, the switchbox is probably bad. In that case, be sure to check for correct power on the switchbox, check all connections, and check the coil's resistance to make sure it's OK."
In your case, you're getting those spurious ign firings. If you still get crazy sparkings after you disconnect the distributor, you've got some sort of issue with the switchbox. As per the above test, you should only get spark every time you ground the black terminal on the distributor side of the switchbox.
If you find that the multiple firings do in fact go away with the dist disconnected, most likely the issue is with the trigger. A CDI Marine replacement trigger assembly is the way to go and will likely last the remaining life of the motor and then some. I've never had a CDI replacement part blow out.
Here's hoping it's the switchbox, there's a lot of used ones kicking around and it'll be considerably less work and expense to replace that part, than to tear apart the distributor.
HTH & let us know what you find...........ed