Merc. 470 Coil Burns Out 3 times

[Chuck Tribolet]

I installed a Pertronix high performance coil which works with a full 12 volt input. It will produce 40,000 volts for greater performance with the Pertronix Ignitor, Electronic Ignition. All I know is there was a bare spot on the power feed to the coil that was up against the manifold. Since this was the only short in the system that I could find, I rewired it to the coil direct and bypassed the resistor as stated in the installation manual.

That makes sense. With electronic ignition, current regulation is done by the electronics
box.

 

[2Dive4fun]

Took the boat out on Sunday for an afternoon dive. The motor ran great. Time will tell all, but I think I have fixed the problem.

 

[Hank49]

Took the boat out on Sunday for an afternoon dive. The motor ran great. Time will tell all, but I think I have fixed the problem.

You must be stoked. As Chuck asked, I don't know the properties of electricity well enough to explain why a shorted wire heats up the component it connects to. We have a maintenance guy here on the farm who worked for Northern Lights generators and he installed and repaired these in yachts all over the world. He tried to explain it to me....
But I had an experience in my boat where the wire from the trim switch on the shifter had worn through due to incorrect routing on installation. It would short a fuse and shut down the whole power system. Like I had a dead battery.

 

[Chuck Tribolet]

That fuse was doing EXACTLY what it was supposed to do: keep the wiring from going
up in smoke. It only takes a second or two of dead short from a 12V battery to ground
to fry wiring. With no load, there's nothing to resist the battery dumping everything it's
got in nearly zero time.

 

[Hank49]

That fuse was doing EXACTLY what it was supposed to do: keep the wiring from going
up in smoke. It only takes a second or two of dead short from a 12V battery to ground
to fry wiring. With no load, there's nothing to resist the battery dumping everything it's
got in nearly zero time.

Right. But I don't understand this part. If the short took current away from the system and dumped it into the "ground", how does that heat up the fuse....or the coil? If you test all those connections while they fuction properly, you get a 12 volt reading. So why does jumping to ground heat it up? Low voltage (equal to pressure in water pipes) makes for higher amprage (flow rate in water pipes)? And is this like now pumping water that was flowing through an 8 inch pipe into a 1 inch pipe so it blows?

 

[Chuck Tribolet]

Right. But I don't understand this part. If the short took current away from the system and dumped it into the "ground", how does that heat up the fuse....or the coil? If you test all those connections while they fuction properly, you get a 12 volt reading. So why does jumping to ground heat it up? Low voltage (equal to pressure in water pipes) makes for higher amprage (flow rate in water pipes)? And is this like now pumping water that was flowing through an 8 inch pipe into a 1 inch pipe so it blows?

The fuse has a very little bit of resistance. The heat generated in
a resistance is proportional to the square of the current. When the
short happens, the current through the fuse goes sky-high, the
fuse gets HOT and melts, and opens so no more current flows.

The voltage is still 12V, back at the inards of the battery. The
high current causes voltage drop happen all along the wire, and
even a bit inside the battery (batteries have some internal
resistance, though lead-acid is pretty low).

Or to continue your water analogy -- the water pressure in the
shower is real low when the sprinkler system is on. ;-)

 

[Hank49]

The fuse has a very little bit of resistance. The heat generated in
a resistance is proportional to the square of the current. When the
short happens, the current through the fuse goes sky-high, the
fuse gets HOT and melts, and opens so no more current flows.

The voltage is still 12V, back at the inards of the battery. The
high current causes voltage drop happen all along the wire, and
even a bit inside the battery (batteries have some internal
resistance, though lead-acid is pretty low).

Or to continue your water analogy -- the water pressure in the
shower is real low when the sprinkler system is on. ;-)

This is what has confused me. I always associate "high" with hot and "low" with cooler. But in voltage, low makes for hot....also...? As I was hooking up my new fishfinder (the wiring in my Belizean made boat is ....well, not so good) I found connections that read 12 volt. But when I hooked it up and turned on the 'Finder', the voltage at that connection dropped immediatley to about 9 volts. The machine would sense this and shut down immediatley. (and I blew the fuse, but luckily found some better connection points). I've worked with pipes and water for a long time and electricians tell me it's the same as electrical current, just different terminology. Like "open" in current is "closed" with water. Plus I can see water. Thanks Chuck, Hank

 

[2Dive4fun]

Ran the boat this past four days. Day two my coil went out. I installed a new one and dis-connected the Tac. It ran great for the last half of the trip. The Tac was working fine until the RPM went past 3000 and then the TAC would spike all the way over. I figure there was a short somewhere with the TAC wire that is connected to the positive side of the coil. Time again will tell.