Fuse Blows w. Left Turn Switch

Having lots of headaches trying to troubleshoot here. Bike is '74 850. Was running well until last week. Had some sudden engine shut-offs that I seem to have resolved by cleaning/tightening connections, but now I'm getting no turn signals on left side only. While testing a few things I discovered flasher unit (modern two prong unit) actually clicking, not flashing turn lamps but is flashing the taillight!? I've fed 12v direct to the left lamp wires at the multi-connector block and they light up fine.
Noticed the white connection to ign switch getting hot. I fitted a lower 15A fuse and she blew when triggering left turn switch (had a 30A in there originally). I have multimeter and LED test lamp but getting blurry eyed trying to figure out what I need to be probing. I have the turn switch apart to see if there is any errant metal or screw loose, but all looks ok.

A step by step would be very helpful at this point.

Cheers.

Unplug the left turn signals at the snap connectors. Take the bulbs out of them. Turn on the key, and left signal. Plug the wire in to one. Then add the bulb. Then plug in the other one. Then add the bulb.
Stop whenever the fuse blows. Inspect the wiring/device. Repair as needed.
Inspect repair connector that is getting hot (loose/high resistance connection/broken strands) Loose rivets holding the male spade terminals to switch, re-peen as needed.

First, a 30 amp amp fuse is too much, unless it is an OE English version (not sure if you even get them anymore, but IDK). IIRC the 35 amp English fuse translates to about 17 amps SAE; your choice of a 15 amp (SAE?) should be fine.

I suggest that you start with a visual examination of the harness; looking for any spots that have worn through. KISS strategy.

Next remove the taillight fairing examine the wires, as above.

Next, remove the headlight examine the wires as 2 above; it is not uncommon for the headlight bulb spring loaded "cap" to abraide wires that are run behind it, or you may have a wire that got caught in the headlight surround or the fastening system that locks (most of the time) the headlight bucket to the shell.

Next, there are 3 wires from the directional switch on the throttle side of the handlebar, unless you swapped switches. Disconnect them, one is power, the other 2 are right side/left side. jump the power to each in turn, if your problem goes away you will need to attend the attendant switch cluster.

Next, reconnect the wires to the switch. Now go to the te tail light and disconnect the running and brake light wires. If this solves your problem attend to the wires belonging to the running/brake lights. IIRC the brake/running and the directionals share a common ground, so check that the wires exiting from the harness are connected to the right wires that emanate from the brake/running and directional appliances.

Last, and probably should have been first. If your harness is original replace the POS.

Best.

If the indicators and tail light worked fine before you cleaned and reconnected wires to fix the ignition problem then it points to you having mixed up some connections when you cleaned them. Especially if the left indicator tries to flash the tail light. That must mean a wires switched over I would think.

It also pays to make sure the battery is well charged when you try to get indicators to work.

RoadScholar said:

IIRC the brake/running and the directionals share a common ground,

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Not the '73-on Lucas 917 tail lamp as the bulb holder is completely isolated inside its rubber mount but the previous 679 lamp unit does.

The rubber-mounted 917 tail and brake light bulb holder has a (red) ground/return wire but there is no common ground connection with the rear direction indicators as standard (although easy enough to add one) so they rely on the bracket, mudguard and frame (as far as one of the harness red frame attachment points, rectifier stud, etc.) for their 'ground'.

If you had a 30 amp fuse in there, the likelhood is the wiring may well be damaged.

At around 20 amps, the insulation melts off the copper - exactly how my bike was when I picked it up.

Most of the original wiring loom is made up from two cable weights - 28/0.12 rated at 14 amps and 14/0.12 rated at 7 amps

gforce said:

Most of the original wiring loom is made up from two cable weights - 28/0.12 rated at 14 amps and 14/0.12 rated at 7 amps

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I can't think of any part of a standard Commando wiring loom that is 28 strand cable.
The original cable was 14/32 SWG (14 strands of 0.0108"/0.274mm diameter).

Modern equivalent standard cable is "14/0.30, 1.0mm², 8.75A - cable OD 2.6mm". 0.30mm is approximately 0.012".

So after blowing through my limited number of mini fuses, testing different components and connections, I've now picked up a 15A "circuit breaker" type self-resetting fuse. I'll use that for further work. My fuse holder has an LED blown signal light which will tell me if it goes open. My bike actually had this type of fuse when I acquired it 4 yrs ago but didn't like it as no way to know when it was blown/waiting to reset itself (guess it has a cool down period of a minute or two?). Don't recall what it was rated for and lost to the ages anyway.

Part of the issue I've having trouble with, a lot of the wiring colours have faded and lighting in the work area is not the greatest so it is sometimes difficult to know with certainty if a line is green with red or green with brown, UW or UY, etc.

If you rig up a 12 volt bulb in place of the fuse, the bulb will light up when you have a short (and fuse would blow) and be off when all is OK. Can be very helpful and save spending on lots of fuses.

Finally nailed down the issue. Two yrs old rubber multi block connector had cross contacted the center double bullet to at least one outer double. Looked fine from outside. Unsure if this happened from some other short event making it over heat or from being squashed 'tween tank and frame.
Luckily when I bought the connector, ordered up a number a new doubles, singles and triple connectors so was able to connect up all lines with them. Makes for a more flexible packing at the frame, so hoping tank won't be pressing up so much.

Putting thing back together now. Found nearly all the lucas turn lamp plastic casings have failed screw mounts. One screw hole had an epoxy blob from previous owner. Are there any recs to repair these fragile fittngs? I"m thinking the QuickSteel type putty epoxy followed by drilling and screw into the blob.