Lights on fine, but turned off the fuse blows

Josh1256 said:

The display shows [1 . ] when not connected. When the leads are touching it shows [00.2]. Hmm... might be worth looking into an analog multimeter

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Your meter has a specified accuracy of .008 ohms on the 200 ohm resistance scale. So zero could be displayed as high as .008 ohms if the meter is in specification. Since you have .2 it is way out of specification. You may have bad leads, poor lead connections or simply be misadjusted. You can pretend that .2 is zero, or you can figure out how to properly set zero - most likely there is a potentiometer on the inside. It's also possible that the battery needs to be replaced but that's a long shot.

Analog meters have a user accessible zero setting.

marshg246 said:

Your meter has a specified accuracy of .008 ohms on the 200 ohm resistance scale. So zero could be displayed as high as .008 ohms if the meter is in specification. Since you have .2 it is way out of specification. You may have bad leads, poor lead connections or simply be misadjusted. You can pretend that .2 is zero, or you can figure out how to properly set zero - most likely there is a potentiometer on the inside. It's also possible that the battery needs to be replaced but that's a long shot.

Analog meters have a user accessible zero setting.

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Got an analog multimeter ordered and on the way so I can get a good reading of the resistance. Just over £10. https://www.ebay.co.uk/itm/165789446913

Deets55 said:

I have the same meter that Greg recommends and have been very happy with it. With that being said, I have used the same Craftsman digital multi-meter for the past 25 years and never had an issue with it on any vehicle I ever used it on, except my golf cart. That required an analog meter. There is a place for each type of meter.
I would avoid those cheap Harbor Freight type meters because they will mess you up when you least expect it. They might work OK for volts, but if you need an accurate resistance measurement they are extremely in accurate. I threw my in the trash because it almost left me stranded one time.

With much regret I had to retire my Craftsman meter because it suddenly became inaccurate. I tested it against my Fluke and found both DC and AC were off. I replaced that with a Fluke 15B I caught on sale.

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DMM has served me well for 40 years. The motorcycle electrics always easily troubleshot.

Josh1256 said:

That'll be another one to check. At the moment I'm going through the wiring harness looking for breaks in wires and replacing old connectors and then will reconnect and see if the fuse blows with the lights off.

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You are looking for a SHORT. Finding a short circuit fast, involves disconnecting many devices at once.

As you plug them back in, one at a time, it will become apparent which device/harness needs further scrutiny.


I do it regularly.

Little update. As advised disconnected everything then reconnected parts of the harness one by one and as soon as the headlight wiring was reconnected there was high resistance: So just kept going through the wires testing resistance through them in the lighting circuit and... That wire with the yellow tape made 0.3 to about 2 ohms resistance.There's a little hole in the top of the bullet where the copper wire can be touched by the probe and when doing that theres no resistance so there's one issue and so if I cut off the bullet head for now and replaced it should be ok. Would be good replace these 3 wires but where to get the spade connectors with locking tabs that go into the plastic headlight junction? Also another place of resistance was the earth from the headlight back to battery earth. Ended up taking the harness out the sleeve along with and pulling off the electrical tape wrap. Soon realised the earth from the headlamp doesn't connect straight to the battery + earth but a bolt that holds the coil on, however the earth going from coil to the battery had an exposed wire Not sure if it would have earthed it was wrapped up in electrical tape and a plastic sleeve but maybe being bent so tight in the loom restricted the erm 'electric flowing'. From the headlamp end of the earth cable to where its attached to the coil theres no resistance. The other things I did was put another headlamp switch in and tested resistance with switch in the 'on' and 'off' positions and didn't make any difference to resistance so I've no idea why the fuse doesn't blow with the lights switched on but does when it's off any ideas? And is it worth wiring the earth directly from the headlamp shell to the battery + earth instead of where the coil bolts to the frame? Thanks sorry its a long waffle

Josh1256 said:

Little update. As advised disconnected everything then reconnected parts of the harness one by one and as soon as the headlight wiring was reconnected there was high resistance:

So just kept going through the wires testing resistance through them in the lighting circuit and...

That wire with the yellow tape made 0.3 to about 2 ohms resistance.There's a little hole in the top of the bullet where the copper wire can be touched by the probe and when doing that theres no resistance so there's one issue and so if I cut off the bullet head for now and replaced it should be ok. Would be good replace these 3 wires but where to get the spade connectors with locking tabs that go into the plastic headlight junction? Also another place of resistance was the earth from the headlight back to battery earth. Ended up taking the harness out the sleeve along with and pulling off the electrical tape wrap. Soon realised the earth from the headlamp doesn't connect straight to the battery + earth but a bolt that holds the coil on, however the earth going from coil to the battery had an exposed wire

Not sure if it would have earthed it was wrapped up in electrical tape and a plastic sleeve but maybe being bent so tight in the loom restricted the erm 'electric flowing'. From the headlamp end of the earth cable to where its attached to the coil theres no resistance. The other things I did was put another headlamp switch in and tested resistance with switch in the 'on' and 'off' positions and didn't make any difference to resistance so I've no idea why the fuse doesn't blow with the lights switched on but does when it's off any ideas? And is it worth wiring the earth directly from the headlamp shell to the battery + earth instead of where the coil bolts to the frame? Thanks sorry its a long waffle

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Leave the headlight socket off and try with the headlight switch in both positions to see if there problem is still there. Post a picture of your master switch and headlight switch from the wiring side. When you state "tested resistance" say where from and to.

Josh1256 said:

Little update. As advised disconnected everything then reconnected parts of the harness one by one and as soon as the headlight wiring was reconnected there was high resistance:

So just kept going through the wires testing resistance through them in the lighting circuit and...

That wire with the yellow tape made 0.3 to about 2 ohms resistance.There's a little hole in the top of the bullet where the copper wire can be touched by the probe and when doing that theres no resistance so there's one issue and so if I cut off the bullet head for now and replaced it should be ok. Would be good replace these 3 wires but where to get the spade connectors with locking tabs that go into the plastic headlight junction? Also another place of resistance was the earth from the headlight back to battery earth. Ended up taking the harness out the sleeve along with and pulling off the electrical tape wrap. Soon realised the earth from the headlamp doesn't connect straight to the battery + earth but a bolt that holds the coil on, however the earth going from coil to the battery had an exposed wire

Not sure if it would have earthed it was wrapped up in electrical tape and a plastic sleeve but maybe being bent so tight in the loom restricted the erm 'electric flowing'. From the headlamp end of the earth cable to where its attached to the coil theres no resistance. The other things I did was put another headlamp switch in and tested resistance with switch in the 'on' and 'off' positions and didn't make any difference to resistance so I've no idea why the fuse doesn't blow with the lights switched on but does when it's off any ideas? And is it worth wiring the earth directly from the headlamp shell to the battery + earth instead of where the coil bolts to the frame? Thanks sorry its a long waffle

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You are mixing two markedly different advice streams.

To quickly find a short, with your minimal electrical knowledge, is what I proposed.

Unplug all the wires in the headlight.

Put the fuse in place.

Plug things in one by one. The "snap" sound will provide audible report when you get the faulty device.

Once you've zero'ed in on it you can either: visually see the defect, or, in the case of the switch being internally shorted, prove it with your DMM.

Repair/replace the fault.

concours said:

You are mixing two markedly different advice streams.

To quickly find a short, with your minimal electrical knowledge, is what I proposed.

Unplug all the wires in the headlight.

Put the fuse in place.

Plug things in one by one. The "snap" sound will provide audible report when you get the faulty device.

Once you've zero'ed in on it you can either: visually see the defect, or, in the case of the switch being internally shorted, prove it with your DMM.

Repair/replace the fault.

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As you said previously... His issue is a shorted connection.

He's either got a misplaced wire connected to his switch that grounds when he turns the switch to the off position, or a skinned wire that connects to ground when he throws the switch. It's also possible that he's got a bare wire touching the inside of his headlight shell, which is a common issue with dead shorts. So he needs to heat shrink everything that's bare inside the headlight shell if there's any wonder that a bare wire might touch the inside of the shell.

He needs to find the problematic connection, which is a misplaced continuity to ground (a short)...... and instead he's measuring resistance of connections

You should be searching for a short..... so reconnect everything, put the tester in place of the battery terminals, set to the continuity setting, turn the key on and the switch off. The tone should be sounding. Now start disconnecting things until the tone goes off... When it goes off, you are on the circuit that is grounding out and need to follow it to double check it's proper connection status, and look for any skinned parts of the wire that might be touching ground.....

At no point does the exact amount of resistance matter.... it's continuity to ground (a short circuit) that is your problem...

*** also, so you turn on the key switch and you'll get a tone from the ignition circuit. Maybe pull the ignition wire from the key switch so you're only left with "switched" circuits left to test.... Those being the light circuits, directionals, brake lights, running lights, headlamps. If the tone goes off when you switch them off then they are not part of the shorting problem... (From what you said originally, the problem is in the main headlight switch circuitry)

Use a light bulb in place of the fuse to test for a short.

Thanks for the assistance everyone its now fixed, it was the light switch that was broken. By pure luck found a spare light switch exactly the same in the garage, set DMM to ohms and put a probe on the spade where the power comes in and the other probe on the spade where it comes out to the headlight. When comparing the two light switches with 3 different settings: 'On', 'Parking light' and 'Off', the old switch had more resistance and was always changing whereas the spare switch did not. As said by a few I could have used a continuity buzzer setting to check the circuits but my DMM must be the only one made not to have that feature! So anyway, tried the new switch and all is fine the fuse does not blow when the headlight switch is off now. Wiring definitely needs a tidy up but apart from that its working. Cheers

Josh1256 said:

Thanks for the assistance everyone its now fixed, it was the light switch that was broken. By pure luck found a spare light switch exactly the same in the garage, set DMM to ohms and put a probe on the spade where the power comes in and the other probe on the spade where it comes out to the headlight. When comparing the two light switches with 3 different settings: 'On', 'Parking light' and 'Off', the old switch had more resistance and was always changing whereas the spare switch did not. As said by a few I could have used a continuity buzzer setting to check the circuits but my DMM must be the only one made not to have that feature! So anyway, tried the new switch and all is fine the fuse does not blow when the headlight switch is off now. Wiring definitely needs a tidy up but apart from that its working. Cheers

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Always a good feeling to figure it out. What's next?

Josh1256 said:

Thanks for the assistance everyone its now fixed, it was the light switch that was broken. By pure luck found a spare light switch exactly the same in the garage, set DMM to ohms and put a probe on the spade where the power comes in and the other probe on the spade where it comes out to the headlight. When comparing the two light switches with 3 different settings: 'On', 'Parking light' and 'Off', the old switch had more resistance and was always changing whereas the spare switch did not. As said by a few I could have used a continuity buzzer setting to check the circuits but my DMM must be the only one made not to have that feature! So anyway, tried the new switch and all is fine the fuse does not blow when the headlight switch is off now. Wiring definitely needs a tidy up but apart from that its working. Cheers

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Thank you for posting a conclusion. It will help others with a similar problem.

Great job being persistant and patient.

Can you post clear close up images of the faulty switch?
Including maker markings if any. I'm curious if this is OEM (Lucas sourced or ?) part, or "pattern" switch? Have any long time riders here seen this type of failure (I'm talking the headlamp switch shorting to ground) before?

Saber said:

Always a good feeling to figure it out. What's next?

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The wiring looms had a lot of repairs over the years so ideally replace the heat shrinks and butt connectors with more complete wires. The indicators need sorting, it's '69 made to look like a roadster so never had indicators but just for safety at night riding they're going on and then ride as much as possible as when it stops raining

concours said:

Can you post clear close up images of the faulty switch?
Including maker markings if any. I'm curious if this is OEM (Lucas sourced or ?) part, or "pattern" switch? Have any long time riders here seen this type of failure (I'm talking the headlamp switch shorting to ground) before?

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Yeah sure. This is the old switch:
This is the flow of power (white wire from ignition switch, blue to headlight):
Where the probes went for testing (Top left at the back and Bottom right):
Testing old headlight switch resistance in the 'ON' postion (numbers were jumping around):
Testing new headlight switch resistance in the 'ON' position (DMM is out by 00.2 anyway, so no real resistance) :

When putting either the old or new headlight switches in 'Parking Light' and 'OFF' positions it doesn't show any resistance reading.

Just to be clear, if the switch was in either the 'OFF' or 'Parking Light' position the fuse would blow. If the light was in the 'ON' position when I reconnected the wiring the fuse would NOT blow. Thanks

Josh1256 said:

Yeah sure. This is the old switch:

This is the flow of power (white wire from ignition switch, blue to headlight):

Where the probes went for testing (Top left at the back and Bottom right):

Testing old headlight switch resistance in the 'ON' postion (numbers were jumping around):

Testing new headlight switch resistance in the 'ON' position (DMM is out by 00.2 anyway, so no real resistance) :

When putting either the old or new headlight switches in 'Parking Light' and 'OFF' positions it doesn't show any resistance reading.

Just to be clear, if the switch was in either the 'OFF' or 'Parking Light' position the fuse would blow. If the light was in the 'ON' position when I reconnected the wiring the fuse would NOT blow. Thanks

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Thank you.
Date code & all.
If you gently pry the sheet metal latches away, we can see how it failed.

concours said:

If you gently pry the sheet metal latches away, we can see how it failed.

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Maybe just dirt and corrosion?

Josh1256 said:

Maybe just dirt and corrosion?

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Great closure for future searchers.