Ignition coil fuse blows when revving above 3500 RPM

[A.Aeschbury]

Hi All

Finally getting my beetle on the road/rallycross course and having a new and bizarre problem.

I have a 15 amp circuit breaker in line with my ignition coil and fuel pump (wiring is all custom) and as of tonight any time I rev above 3500 rpm the circuit breaker pops. Car will sit there at idle all day it seems but the second you rev it up (loaded or unloaded) it pops that breaker.

I disconnected the fuel pump and ran the carbs on the fuel that was in the lines/bowls and it still popped the fuse. So that leads me to the ignition system. I tried a different coil with the same result. I also tried with the alternator disconnected and that didn't effect anything either.

I didn't see any obvious breaks in any of the wires and the fact that it will idle without popping the breaker leads me to believe there is something more sinister at play. I also replaced the relay for that circuit and tried a 20 amp breaker with the same result.

Any suggestions on what to check next?

Thanks in advance
Alex

[Piledriver]

Welcome to the asylum!

What kind of coil is it?
If it measures less than ~2 ohms it needs a ballast resistor or a *coil driver that controls dwell.
(*Bosch driver as used on late 80s VWs or a Pertronix II)

A traditional Bosch "Blue" coil has the ballast resistor built into the coil, but not all modern "Bosch Blue" coils are made as they should be, or where they say or by whom for that matter.

Kudos for having a fuse, earlier cars didn't from the factory.
Lots of them burned to the ground due to that little omission...

[A.Aeschbury]

Actually I've been in this nut house awhile now. Started a build thread on this guy a few years ago but I let it die.

http://www.shoptalkforums.com/viewtopic.php?t=140298

The coil I have on there now is from Napa and its labeled "no external resistor required" and tested as such when I purchased it.

I've been chasing a ton of weird problems with this engine. I made a few attempts at running in our local rallycross with it last season (even podiumed a few times) but I was plagued with problems all season. So I set to work this off season trying to get it all prepped. I've replaced the following parts:

Rebuilt heads (new valves done at a local shop)
New distributor (009)
New Cap/rotor/condensor
New Plugs
New ignition coil (twice)
Rebuilt carbs
New dual cb performance carb linkage (old one just wouldn't stay synced cause it sucked)
New pistons/cylinders (that was done in my old build thread)
Then new rings a second time because I didn't have good enough filtration and grit mucked up the first set
New fuel pump
And some other things I'm probably forgetting. Basically everything is new since I've owned the car except for the crank, rods, and cam and the case I guess.

Some of my more knowledgable friends are saying it sounds like a grounding issue. I was going to check the transmission ground tonight. I am using the CB coil mount/fuel pump block off so if the coil is supposed to ground through that it might not be getting a good ground due to the gasket sealing the case hole for the mechanical fuel pump.

If someone can tell me how to link images from drop box I can throw some pictures of the car up.

[Piledriver]

The backup lights get power from the coil, maybe something shifts at high rpm?

The next thing would be a std suggestion from me in any case: don't actually power the coil with that wire.
Use it to control a relay running right off the alternator B+ or starter.(with its own sealed inline fuse of course.)
The ignition will work much better on full power, 7-8v is typical running voltage with the stock feed.

This eliminates the power going through ~30 feet of old wiring and takes some load off ye auld fusebox/ignition switch etc.
Works awesome for headlights, too, the power simply bypassing the ign and headlight switch helps.

What exactly does the coil ohm out at?

A 3 ohm coil fed by a 12v line can only pass 4 amps with the points stuck closed.
A low-z coil will do exactly what you describe, reliably.

[Piledriver]

If uploading a pic, it works at 800x600 85% quality jpeg.
For inline displayed links, that is being worked on after a major software upgrade.

Photobucket's working now, and external links should be at least clickable...
May still have image size limits for inline display, its a work in progress.

[A.Aeschbury]

So I ran a wire straight from the battery to the ignition coil as you suggested and it solved the problem. So I started poking around in my engine harness and it turns out at higher RPM's my fan was sucking the wires out of the split loom I was using and into the fan causing all kinds of shorts.

I never noticed the frayed wires because I was only investigating the loom which was completely in tact but once I actually opened it up, frayed wires all over. Cars man, I'll tell ya...

Thanks for your help.

[Piledriver]

Sounds... Electrifrying.
Cool, much mo betta to find it in the driveway.
Buy a lottery ticket tomorrow.

I still strongly recommend permanently powering the ignition (and headlamps too) as I described, huge difference with full power.

...and Adel clamps. Lots of Adel clamps...
And waterproof splices.