Blowing the main fuse

[NobleHops]

Man, this last 5 yards is kicking my ass.

OK, suddenly my bike is blowing the main fuse. Unfortunately there are several non-stock variables that could be contributing.

For starters, it is blowing the fuse usually at or just after startup. It'll start, run fine, then I'll shut it off, turn the key to restart it and bzzt, burns the main fuse. SOMETIMES when I turn the key I hear a tiny squeak sound of something getting energized. Soleoid?

The non-stock items are as follows:

I ran a wire from the white/red wire leg up to the coils to power the Dyna ignition.

I had a break somewhere in the brown (brown/white?) wire from the ignition to the taillight, resulting in no taillight, so I butt-spliced in a new wire from the headlight connector to the taillight connector to bypass it, which fixed the taillight issue. This leg seems like a likely candidate for a short that could cause the fuse to blow.

There is a Tim's 4-brush starter on the bike and the aforementioned dyna ignition and coils.

Plus I bypassed the stock fuse and put an inline-ATC fuseholder.

At my brother's suggestion I swapped-in a 40-amp fuse and it survived several startings and stoppings so far. He theorized that it's drawing too much current somehow, seems unlikely to me though, and I'm a little leery of doing that for fear that whatever is causing this will now build up more heat in some connector somewhere. Is that a bad idea?

I'm inclined to trace-out that brown power leg and see if there is a short to ground somewhere. Any other ideas for troubleshooting?

N.

[Rick Pope]

Nils,

Unplug everything not required to start the engine, and repeat. See if the (proper) fuse blows. Add one curcuit at a time until you isolate the problem.

Or, you could leave that 40 amp fuse in and just watch for smoke. But keep a fire extinguisher handy.

[letxbruce]

I experienced the same problem on my 79, except that it usually happened several miles from my house. It turned out that the Dyna Coils terminals had a slightly different angle than the stock coils. This caused the left outboard coil terminal to sporadically tickle the bottom of the fuel tank as the rubber mounts moved. The effect ranged from intermittant loss of two cylinders to a hard short that blew the main fuse. To fix it I bent the terminals and wraped them with tape.

[daves79x]

Nils:
I think the easiest thing would be to measure the current drawn by just your starter and compare it to the current drawn by the stock starter. I can provide you with the stock current value. My first thought was the 4-brush starter was drawing more current than the stocker, but I guess we can find out by measuring.

Also, if your solenoid is not making real good contact inside, that could lead to higher current draw, although I recall it is new. Let em know if you want me to measure the current on a stock starter.

Dave

[NobleHops]

Thank you Dave, will do. Back to you in a day or so when I dig back into it.

[spencer]

If you are suspecting that the starter motor is drawing too much current, that is not the problem. The current for the starter does not go through a fuse. At least not on my schematic. I'm not sure where the problem is, but there must be some kind of intermittent short somewhere else.

[daves79x]

Spencer:
Thanks for pointing that out - now that I think about it, you are probably right. I'll dig into the diagragm and see what IS fused when you push the starter button vs just the key being on. Nils - maybe check again that right switch connector in the headlight? These things can drive you nuts!

Dave

[NobleHops]

I experienced the same problem on my 79, except that it usually happened several miles from my house. It turned out that the Dyna Coils terminals had a slightly different angle than the stock coils. This caused the left outboard coil terminal to sporadically tickle the bottom of the fuel tank as the rubber mounts moved. The effect ranged from intermittant loss of two cylinders to a hard short that blew the main fuse. To fix it I bent the terminals and wraped them with tape.

Bruce, you get an A+ with your first post: This is exactly what happened. I will edit to post pics later, but I'm headed back out to the shop now to continue installing the new wiring harness Spencer and Dave and Rick, you both were spot-on too. I am learning a lot of lessons the hard way, as usual.

In the interest of full disclosure of my folly, unfortunately I took my brother's advice and upped the amperage of the main fuse. I only confess this idiocy in public in the hopes that it might prevent somebody in the future from doing the same. The result of that was that the partial short through the gas tank drew enough current through the black/white leg of the wiring harness before the fuse blew to melt the tape and insulation from the loom in two spots. The advice to trace by subsystems was also good: when I pulled it apart to see what leg had the problem and saw it was the black/white wire melted I knew it was in the ignition circuit. Melting the insulation on the wiring with a higher amperage fuse made diagnosis easy, although lets just say I do not recommend this method

451ZR might rightly bust me here because he went out of his way to send me (free) terminal adapters that would have allowed me to reuse the stock coil wiring, and those being insulated, if I had installed them right away this might not have happened. That is another vote for installing those screw-to-spade terminal adapters when converting to Dyna coils, and reusing the stock wiring. I had already made up a wiring loom per Tim's' instructions when I got those, and was too lazy to go back and redo it right then.

The good news is, I got a main wiring harness that is actually in better shape than mine from eBay for net $45, although it needs a skosh of repair. It will take me another hour to install the new harness, and I have some some soldering and shrink-tubing to do to repair some wiring to the right control pod, but I have new sheathing and terminals from Vintage Connections that just arrived, so I have everything I need to repair it, and good reason to believe I have indeed found the problem and this is the end of it.

As I said, my lessons come hard, but I am learning a ton of course, and will finish this bike thanks in large part to you guys.

N.

[letxbruce]

It's been so long since I had the problem, I don't remember exactly what the terminals looked like and how I rigged them. I'm going to pull the tank when the fuel gets low and check to see if my fix is still OK or needs a little "re-engineering".

[SteveG]

Nils -

Sorry for your pain, thanks for the info.

Steve