t120rv - fuses

[thunderace1]

Dear All,

My 1972 t120rv eats 35Amp fuses - with other owners experience what would be the most frequent reason why my bike is doing this and how can I check what is causing this problem?
I am using a sparx 21A electronic igintion kit, sparx rectifier, motobatt MBTX9U battery, ordinary spark plugs and standard everything else. The earth wires all seem to be okay and are connected to the rear frame that holds the rear mudguard in place. When I replace a fuse the bike will start quite happily but then a fuse will go and I am scuppered and this is happening on every ride out.
 

[L.A.B.]

There would certainly appear to be an intermittent short in the system somewhere, so it should only be a matter checking through the wiring carefully, however if it only occurs when the engine is running or the bike is moving or vibrating then it does become more difficult.

You could try fitting some temporary lower Amp fuses (10A) to each individual circuit (ignition, headlamp, brake light etc.) as that may help you to pinpoint the actual circuit that has the short as only the lower rated fuse on the affected circuit should blow.       

[thunderace1]

Dear L.A.B.

Thanks for your advice.

Going around the whole of the electrics I came up with two problems:

In the sparx left hand replacement switch internals some of the soldering was very poor leading to one wire being able to touch the metal case - I have now taken this apart and resoldered all the connections, and

In the rear light unit the small length of earth wire between the light fitment and the metal support had corroded. I have purchased a patent new unit but the wiring does not correspond to my old one. That had a brown and a brown/green whereas the new one has a black and red wire. Trying various combinations nothing works unless the red wire connects with the earth wire from the rest of the bike, fair enough but connecting the black wire I either get the rear light permanently on or only works with the brakes. How do I organise the connections so that the rear light comes on when the lights are put on and also shine (brighter) when the brakes are engaged?

[L.A.B.]

I have purchased a patent new unit but the wiring does not correspond to my old one. That had a brown and a brown/green whereas the new one has a black and red wire. Trying various combinations nothing works unless the red wire connects with the earth wire from the rest of the bike, fair enough but connecting the black wire I either get the rear light permanently on or only works with the brakes. How do I organise the connections so that the rear light comes on when the lights are put on and also shine (brighter) when the brakes are engaged?

The later type bulb holders are usually enclosed within a rubber mounting with a third (often black/white) earth wire which is intended for connection to the harness earth wiring rather than a strap from the bulb holder to the support, therefore it would seem your pattern lamp assembly may not have any earth connection? 

If so, then you need to run an additional earth wire from the bulb holder to a good earth point, or prefrerably connect it to the harness earth wiring (normally red).

The tail lamp red wire will probably be for the brake light filament (so connect to harness brown) and the black for the tail lamp filament (connect to brown/green) but if wrong, simply swap red and black over.           

[thunderace1]

Dear L.A.B.

Thanks for your comments ,everything seems fine now (touch wood).