20amp ignition fuse blows when trying to start

[Ahende7]

i have a mercruiser 5.7l with the Thunderbolt V ignition (serial number 0k020401). Boat started and ran perfectly until we were switching skiers- when I tried starting the engine there was a click from the starter then the 20amp ignition fuse blew on the lead coming from the battery. I have replaced the battery, starter, solenoid, and ignition switch with no luck. The ground wires to the blower and bilge pump rocker switches were also melted, I replaced those. I have tested every wire/circuit I can think of for continuity and everything seems fine. Every time I turn the ignition key to start the fuse immediately blows. I removed the spark plugs and hand cranked the engine to see if it was hydro locked, but i could crank it over without a problem and did not see any water come out of any cylinders. I’m thoroughly confused since it literally went from running perfectly one second to not even turning the next. Any ideas?

 

[achris]

You need to separate the 'engine' wiring from the 'boat' wiring... Run a new (fused) red and black from the battery to the back of the dash, install a red and a black terminal block and connect the new cables, then remove anything that isn't engine from the engine harness and connect it to the new terminal blocks.

Now you're ready to find out what's blowing the fuse...

Chris

 

[scoflaw]

Bypass the fuse for 10 second intervals and see what's getting hot.

 

[Ahende7]

You need to separate the 'engine' wiring from the 'boat' wiring... Run a new (fused) red and black from the battery to the back of the dash, install a red and a black terminal block and connect the new cables, then remove anything that isn't engine from the engine harness and connect it to the new terminal blocks.

Now you're ready to find out what's blowing the fuse...

Chris

I tried to use a remote starter on the starter solenoid (on top of the engine by the 50amp breaker) and the fuse blew immediately- could it still be a problem up by the dash or would this indicate that the problem is somewhere in the engine compartment?

thanks,
Andy

 

[achris]

The 20 amp fuse on the lead coming from the battery.

Where exactly does that lead with the fuse in it go? And what colour is it?

 

[Ahende7]

The 20 amp fuse on the lead coming from the battery.

Where exactly does that lead with the fuse in it go? And what colour is it?

The lead from the battery with the 20amp fuse is red with a black stripe and goes to the switch panel first (blower, bilge, lights,horn) and then goes to the ignition switch. From there the ignition switch powers the gauges.

 

[achris]

and then goes to the ignition switch

There's your problem. The only wire going TO the ignition switch should be the red that comes up from the engine harness, via the kill switch (if you have one) and then a fuse... Coming off the ignition switch should be a purple that powers the instruments and also goes back down the harness to the engine (to power the ignition). The other wire will be a yellow/red, which is your START wire.

Get that (foreign) wire from the switch panel off the ignition switch and reinstate the original wire that should be there.

Chris.....

 

[Ahende7]

There's your problem. The only wire going TO the ignition switch should be the red that comes up from the engine harness, via the kill switch (if you have one) and then a fuse... Coming off the ignition switch should be a purple that powers the instruments and also goes back down the harness to the engine (to power the ignition). The other wire will be a yellow/red, which is your START wire.

Get that (foreign) wire from the switch panel off the ignition switch and reinstate the original wire that should be there.

Chris.....

I probably described it poorly. It is a purple wire from the ignition switch that powers the gauges. There aren’t any foreign wires- I have had the boat for 17 years and haven’t added any accessories or had any problems until now.

 

[achris]

Ok, so the power is running the other way, you're powering the switch panel from the ignition supply. I repeat, get boat wiring off the engine system, supply it independently...

When you use the remote starter switch, power of flowing into the 'instrument' circuit, via the fuel pump oil pressure switch, and whatever is causing the problem is instantly being powered. Until you separate the systems, you're going to have trouble working out where the problem lies.

Chris...

 

[Ahende7]

Thanks- it’ll be a few days before I can get to it, but I will give it a try.