Fading ignition light/battery power

Hi, having ignition/battery power problem and looking for some advice before trying to run it down. I recently moved and trailered my 1974 Norton Mk. IIa about 200 miles. Although it was strapped down tight, the Uhaul trailer was light and bounced a lot on the road. Had no problems before transporting bike, so I was thinking something came loose, but I have checked most connections and didn't find any loose connections.

When the key is moved into position for starting, the headlamp ignition light does not always come on. When it does come on, it doesn't stay on long. Sometimes it will just flicker and go out. Sometimes it will stay on when the trickle charger is plugged into the electrical connection on the side of the bike. When bike has been sitting for a while and light stays on, it will kick start ok, but then light goes out and it will not start again. I have a TriSpark ignition so it won't idle well when light goes out. When I try to kick start it, I can see light come on when motor is turning over.

The battery is new and has good charge.

After doing some testing with a continuity light, I found the battery power will go off before hitting the ignition switch. I can hold probe on connection for wire coming from battery at back of ignition switch, and everything will just go off on its own. So, I don't believe its the switch since power shuts off before then.

Thank you for your help,
James Prichard

A battery cell can go gunnysack from such bouncing. It will show 12 volts at rest but when any load is put on it voltage will drop according to how many cells are bad. 2 volts per cell. I had it happen on a one week old AGM Exide battery a few years back. Kind of a mind screw trying to use a VOM meter along the side of the road deep inside the Apache Nation. Disconnected it and ran jumpers from a car battery and Varoom! All was well except that the nearest battery was 70 mile away. The guy with the car that stopped did a 140 mile round trip and wouldn't even take gas money

Yes, my battery died the same way. When this problem started, I took the old battery and had it tested. It had a bad cell. I replaced it and still have the same problem. Thanks for your reply.

I believe there is usually a fuse between the battery and the switch. Perhaps you cd try a different fuse?
Or short (temporarily for testing only) right through the fuse holder, to see if maybe the holder itself is acting up?

You may have done so already but it may be worth just checking that the + and - connections to the battery terminals are really tight....a lose connection may just be the problem!

Mr. Rick said:

I believe there is usually a fuse between the battery and the switch. Perhaps you cd try a different fuse?
Or short (temporarily for testing only) right through the fuse holder, to see if maybe the holder itself is acting up?

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I took the fuse holder apart and cleaned the contacts for the fuse. The fuse looked ok. It appeared to be a 15 amp. I will try a new fuse and see if that helps. Thank you for the suggestion.

Lincolnshire Poacher said:

You may have done so already but it may be worth just checking that the + and - connections to the battery terminals are really tight....a lose connection may just be the problem!

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I have checked them, and cleaned the wiring contacts that bolt to the battery. The battery is new and has good voltage. Looking at the Old Brits simplified wiring diagram, it appears the fuse is the only thing in between the battery and switch on the negative side. Is it possible that something on the plus side is defective and weakening the entire circuit? Fixing electrical problems is not one of my strong suits, so thought I would ask. Thank you for your reply.

It is definitely possible to have a poor (positive) ground connection. Or several. Look at every place you see a red wire going to engine or frame or coil (assuming a stock harness).

A jumper wire to circumvent entire circuits comes in handy at times like this.

Mr. Rick said:

It is definitely possible to have a poor (positive) ground connection. Or several. Look at every place you see a red wire going to engine or frame or coil (assuming a stock harness).

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ok, will do. Thank you.

Dutch said:

A jumper wire to circumvent entire circuits comes in handy at times like this.

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ok, got one. Plan to start checking soon. Thanks for reply.

Got it! It was the main fuse! Danged, it looked good, too. However, when I jumped it, it was easy to see that it had been the fuse all along. At least I was able to clean up some wiring and make sure all the connections were tight. It was after going over the wiring that I went back to the fuse. I should have checked it better. Thank you to all who made suggestions. It helped. I appreciate it.

Crummy Fuseholders and corroded grounds are our banes.

Mr. Rick said:

It is definitely possible to have a poor (positive) ground connection. Or several. Look at every place you see a red wire going to engine or frame or coil (assuming a stock harness).

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You may like to check the fuse holder. The original fuse holder with an internal spring to provide tension to the fuse may deteriorate due to vibration. My 71 Commando fuse suffered from 'arcing' that eventually made it impossible to start the machine. I replaced the fuse holder with a modern bayonet type fuse that provides a much more robust connection. I fully agree with Rick regarding the positive earths - there are several attached to the frame and it maybe that the one has weak connection. It is bound to be a simple defect...when you have identified it...have fun!

jamesp said:

Got it! It was the main fuse! Danged, it looked good, too. However, when I jumped it, it was easy to see that it had been the fuse all along. At least I was able to clean up some wiring and make sure all the connections were tight. It was after going over the wiring that I went back to the fuse. I should have checked it better. Thank you to all who made suggestions. It helped. I appreciate it.

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Just spotted this ...well done!

jamesp said:

I took the fuse holder apart and cleaned the contacts for the fuse. The fuse looked ok. It appeared to be a 15 amp. I will try a new fuse and see if that helps. Thank you for the suggestion.

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I think you should have a 30 amp fuse...but check

jamesp said:

I took the fuse holder apart and cleaned the contacts for the fuse. The fuse looked ok. It appeared to be a 15 amp. I will try a new fuse and see if that helps. Thank you for the suggestion.

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The 850 manual recommends that a fuse no greater than 35 amps is used....a 15 amp fuse may be a bit vulnerable?

Lincolnshire Poacher said:

I think you should have a 30 amp fuse...but check

The 850 manual recommends that a fuse no greater than 35 amps is used....a 15 amp fuse may be a bit vulnerable?

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There are 2 ways to rate a fuse, one is the blow rating and the other is the slow burn one. The original fuse will be marked 35A blow 17.5A continuous, modern fuses are only marked with the continuous rating so 15A is correct if modern.

kommando said:

There are 2 ways to rate a fuse, one is the blow rating and the other is the slow burn one. The original fuse will be marked 35A blow 17.5A continuous, modern fuses are only marked with the continuous rating so 15A is correct if modern.

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Great thanks - every day is a learning day!