Shift interrupter blow ignition fuse

[mrsaundep]

Mercruiser 3.0, MCM 140, No. 5568367, 1982, Alpha one (Boat maker Sealine)

Every time the shift interrupter switch closes (either when shifting or by manually switching the arm on the switch) the engine stalls (as it should) but the 20A fuse in the ignition circuit (starting key) under the steering console blows instantly. I loose all power (nothing to starter motor) until I replace the fuse. The engine then starts and will run fine until the switch is activated which then blows the fuse. (With the switch disconnected it will shaft into ahead and astern o.k - its a little bit crunchy going into astern). Any ideas on why the fuse keeps blowing? Are there any checks to make on the switch?

ideally I would like to replace the interrupter switch as the lugs where the spade terminals connect are broken and short I can get the connections nice and tight with a couple of zip ties), however I understand these switches are obsolete. Is there any options for changing / upgrading the interrupter switch on this unit?

Many thanks
Paul

 

[Bt Doctur]

If moving the arm to operate the interupter is blowing the fuse the interupter is wired to the wrong side of the coil or the interupter is wired incorrectly. 1 screw goes directly to ground and the other screw is isolated from ground

 

[Bt Doctur]

click to enlarge

 

[nola mike]

Yeah, guessing that the interrupter is wired to the (+) side of the coil, which will short to ground when activated.

 

[mrsaundep]

Thank you for your replies. I replaced the original distributer with Delco EST over 6 years ago (self installed), wired everything as per instructions and everything including the switch has always worked fine for several hundred running hours. Last year I had various troubles including water in fuel tank and then carb issues - (no idle - see seperate post, ultimately resolved by a reman carb). So boat was not run for a good 6 months over a UK winter. Since the new carb everything runs fine except for this issue of the fuse blowing everytime the int switch closes. The wiring to the switch has NOT been touched at all during the above troubles so I cannot figure out how it could possibly now be wired up wrong (and the wires are run in such a way that there is not enough length in 1 wire to put it on the opposite terminal). I will have another look and trace the wires from switch onwards next time I'm down the staithe.

 

[nola mike]

That shift interrupter is a switch to ground. Unless it's allowing voltage to ground somehow, I don't see how it can blow the fuse. Check your wiring carefully, might have 2 wires running together that rubbed through the insulation or something. Try disconnecting the switch at the distributor, see if it still blows a fuse when tripped. Also maybe when disconnected see what happens if you ground the (-) of the distributor (should kill the engine, but not blow a fuse). As far as replacement, last I looked merc only sold the whole shift plate assembly as a unit.

 

[yes-i-canada]

Hey @mrsaundep sorry to revive this thread, but I currently have the *exact* same problem as you describe! Did you ever find out what the problem was? I would greatly appreciate any thoughts...