(Another) elusive electrical problem... FOLLOW-UP

Hi All and Thanks again for your input.

With your suggestions in hand, I spent some hours this weekend going over every inch of the harness and electrical system on the bike. I don't know much about electrics but I found nothing I recognized that would account for my blown-fuse problem, EXCCEPT that both battery terminals were quite loose. Any thoughts of whether that could pop a 30A fuse? And how?

I tightened the battery leads (and a few bolts here and there), installed a 15A fuse and fired it up. In about 5 miles of mixed riding with and without lights I had NO problems with the fuse.

I did find a few anomalies; The system is partially hot with the key OFF so the horn works and there is a 4.2± V draw across the (-) lead even with the key switch removed. Measured by removing the fuse and putting my VoM in line). The diode & regulator have been removed in favor of a Tympanium and there's no capacitor in the system.

Could loose connections at the batter terminals cause the system to blow a 30A fuse after 10+ miles on the road?

Thanks again and have fun out there.

-Danny
Oakland CA, USA

It is normal for a lot of bikes to have a live horn and brake light when the ignition is switched off.

It would be interesting to know what the amperage draw from the battery is when everything is switched off - that may help you track things down.

You measure it by inserting your multimeter inline (plus or minus terminal), but make sure your meter is rated to take that kind of current, otherwise you'll blow your meter.

GTCarlos said:

The system is partially hot with the key OFF so the horn works

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Are you sure your "1971" isn't actually a late 1970 model as that would be normal (inc. working brake light) for pre-71 models.

GTCarlos said:

and there is a 4.2± V draw across the (-) lead even with the key switch removed.

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"Draw" (current) is measured in Amps, not Volts.

Hi Guys,

My meter isn't the sensitive enough to show actual draw. It reads an erratic 0.0, 0.1 or 0.2A with everything off. With luck I'll borrow a meter this weekend that can resolve that current. That said I expected there should NO voltage with everything off and the key out. The 4.2V reading is steady.

As to year I'm just going by what the none-too-knowledgeable P.O. claimed, corroborated by the none-too-reliable DMV with the former probably informed pretty much exclusively by the latter. I'm much more familiar with old Italian bikes. Only a few stray Brits have come and gone through the collection over the years. My last Norton was a White 1975 Mk.III I bought new in '76. This one was just too pretty to pass up and it looked like a good candidate for re-hab. It's S/N 145654. Does that make it a '70 or "71?

Thanks as always.

-Danny

GTCarlos said:

It's S/N 145654. Does that make it a '70 or "71?

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That is a '71 model.

It should read ZERO amp draw with the key off, unless brake light is lit or horn button pressed.

If it's drawing amps, you have a short somewhere that will drain your battery and eventually deplete it.

GTCarlos said:

I did find a few anomalies; The system is partially hot with the key OFF so the horn works and there is a 4.2± V draw across the (-) lead even with the key switch removed. Measured by removing the fuse and putting my VoM in line). The diode & regulator have been removed in favor of a Tympanium and there's no capacitor in the system.

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The 4.2 volts that you read from your inline fuse holder is coming from your Tympanium voltage regulator. I would consider that to be a normal reading. I happen to have one as a spare and hooked it up to a power supply and read the voltage as you described and got a reading of 4.5 volts on my digital voltmeter. However with an analog meter I read 0 volts. Also you may have a very small amount of amperage leakage through your voltage regulator as well.

When checking for a short circuit use an ohm meter set to its lowest scale or the audible setting. With the battery wires disconnected, connect one meter probe to the negative battery wire and the other to ground. If you have a short you will hear the beep of the continuity tester or read close to 0 ohms. It's too bad that you don't know if the switch was in the on or off position when the fuse blew. It could have helped to isolate the circuit problem.

It's interesting that you noted the horn is hot all of the time. Originally on the 1971 and newer Norton Commandos the horn only works when the ignition switch is on. I can't imagine why anybody would want to change that circuit to make it hot all of the time unless you don't have the original Lucas handle bar switch. You may have a horn switch that goes directly to ground and if so, the horn would have needed to be rewired. Another possible reason would be if the bike had an ammeter installed. If that is the case the horn would have needed to be wired directly to the main battery wire.

Speaking of horn wires, the original Lucas horn fits in a tight area under the battery tray. The original Lucas wiring to the horn uses a special push on terminal that is shorter than the standard push on terminals. The reason why short terminals are used is that clearance is needed between terminals and the engine mount cradle. You may be able to see the horn terminals by using a flashlight and determine whether or not this could be a problem.

Seeing how you are using an aftermarket voltage regulator and not using the blue 2MC capacitor, I would be inclined to make a new wire from the battery fuse and run it directly to the voltage regulator. From the voltage regulator run another wire directly to the ignition switch at the number 1 position and remove the original brown/blue wires. This effectively bypasses all of the original brown/blue wires which could be a source of intermittent short circuiting. If you feel as though you need the accessory receptacle you can run a separate wire up to the number 1 switch position.

GTCarlos said:

Could loose connections at the batter terminals cause the system to blow a 30A fuse after 10+ miles on the road?

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Loose/dirty connections in your electrical system can cause problems but it definitely will not cause your fuses to blow. Only a short circuit will do that.

I hope this is of some help,
Peter Joe