Ignition fuse blows

[Slice of Life]

I have a 92 Mercruiser 5.7 350 MAG with Thunderbolt IV ignition that recently had the 15A ignition fuse blow when I tried to abruptly go to WOT from cruise. After getting it towed by a friend back to the slip, I replaced the fuse to find that as soon as I turn the ignition key the fuse blows immediately...most of the time. I'm getting electrical to all of the other switches (blower, bilge pump, lights, horn) but none of the gauges will come on. The kicker seems to be that it will not blow immediately every time I replace the fuse, and I even was able to start it once before the fuse blew again within a couple of minutes. The wire that goes to the fuse that is blowing is pink (faded red I guess) with the black line on it. Battery has sufficient voltage at 12.55.

Possibly a related problem is that it has been very stubborn on wanting to start lately, and that on one of the troubleshooting fuses I put in that stayed on going before it blew again, the engine did not want to start at all, but kept cranking and cranking. I even had a 10A that lasted a few mins before it blew when I was out of 15A fuses. I have a multimeter and see that I'm getting below .20 V on each of the 3 wires that go to the ignition. The starter is so damned hard to get to in this engine compartment I haven't messed with it yet. Any ideas? Thanks a bunch.

 

[glenn property of pam]

Its never a easy task to find the bad item or wire especially when its not dead to ground. If it was mine I would splice a lamp across the fuse holder (I have a car spot light that I use for this task). The lamp takes the load and will glow bright due to what ever is grounded completing the circuit. Then start up-plugging items until the lamp either goes off or goes dull. Download a diagram of the wiring and try everything that's connected.

 

[Slice of Life]

Ok, so after trying to find the electrical gremlin I think I found the problem to be in the ignition switch. I cleaned the 3 contacts that connect to the purple, red with black line, and yellow with red line wires with contact cleaner really well. Fuse has not blown since. I left the engine at idle for about 25 minutes after starting to make sure the fuse did not blow, and also charge the batteries. I even had a 10A fuse in without it blowing, and will put a 15A fuse back in. I still feel uneasy about taking it out really far, but for the time being it seems ok. Will definitely have spare fuses on board. Is it possible that this could have been the culprit? As said before, even when the fuse did not blow immediately the engine would refuse to start, only crank. Now it starts right up.

I also cleaned the contacts to the engine kill switch. Is it possible that dirty contacts on the kill switch would cause the engine to crank but never start? Thanks.

 

[bruceb58]

I really doubt your ignition switch is the problem blowing fuses unless your ignition switch is mounted to a metal panel.

Dirty contacts on your kill switch could cause your engine to not start.