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T140 blowing fuse

6K views 19 replies 9 participants last post by  StuartMac 
#1 ·
On a test ride after a carb fettle....well if you can call a quarter of a mile a test ride...my T140 blew its 25A fuse. Its the first time this has happened so I am out on a limb. As I am new to the Meriden Triumphs and their electrics, mine has a Boyer system. The problem is that the fuse blows as soon as I turn the ignition key. Following the blue/brown wires from the ignition it goes to the battery and fuse, to a block (not sure what it Is) near the battery and down , I assume to the rectifier? I cannot as yet see any broken wire etc. but today I have bought a new multi-meter with continuity function to hopefully expose any broken wires.
Can anyone suggest my path as the black art of electrics are an alien word to me at least.
Thanks Bry
 
#5 ·
It's usually something live touching the frame or engine. A wire pinched or rubbed through the insulation, so the copper touches something.

As Rambo says, the coil low tension terminals and battery post can touch the steel seat base. Sometimes there's a bare live terminal inside the headlight and it touches the shell.

If it only blows when you switch on, it's not a fault between the battery and the switch. It is a fault in something that becomes live when you switch on.
 
#7 ·
All of the white wires are common, i.e. connected to each other.
When the ignition switch is turned on, they connect to the battery through the switch.
You need to find which of these is the culprit.

If you intend to use a meter, use the Ohms range.
Remove the fuse or disconnect the battery.
Light/indicator switches normal, OFF.
Disconnect the oil/ignition light, because this is a low resistance path, when the motor is not running.

If the fault points to the handlebar switches, it could be anywhere after the switch, or maybe even in the switch itself.
 
#9 · (Edited)
These type problems are best solved by logic. If you think logically, you can guide yourself right to the fault. We must start with the conclusion that one, and only one, item is shorting. The fastest way to find the short is by segregating the system into smaller parts and ruling out the items that don't blow the fuse.

► First check Ranbo's point on the battery touching the bottom of the seat. That's always Issue Number 1 because the newer batteries are slightly taller than the OEM.

► Go to the rear of the ignition switch. Power enters the switch on Brn/Blu. As Caulky pointed out, your problem is either with the daytime wiring (Wht wires), or night time lighting circuit (Brn/Grn). I see you are in the UK, but it is not clear if you run daytime headlamps. If you do not, then the problem is not in the lighting circuits. You can therefore rule out anything wired with Brn/Grn. If you do run 24/7 headlamps, then you can segregate the systems by unplugging either of the 2 output wires (either Brn/Grn or Wht) one at a time.

White only goes to ignition kill sw*, front brake light sw*, rear brake light sw, and oil pressure indicator lamp*.

Brn/Grn only goes to headlamp, tail lamp, and instrument lamps*.

(* All these items make their connection inside the headlamp shell. So that's the best place to start unplugging.)

By unplugging and re-connecting these wires 1 at a time you can make a seemly complex task very simple.

Hope this helps.
 
#10 ·
Thanks for all your help, we have cracked it. By elimination it turned out to be the rear brake light switch. To cap it all the fettling I was doing on the carbs was high float height causing flooding on the left carb. A test ride gave the carburation pretty good, not spot on but close to work with. Hopefully I can fine tune them in tomorrow when I return from work.
This is my longest ride on the bike since I bought it 5 months ago....I,ve had a lot of other stuff and other bikes to work on, but I came back with a grin from ear to ear. So much so that I gave my open faced helmet to my 29 year old son ...we work a lot together on his and my bikes... and said "take it up the road". Well as you probably guessed he came back with that same grin, this was his first ride on an old Brit, you never know we might have a future convert.
Thanks again for all your help

Bry
 
#11 ·
This is my longest ride on the bike since I bought it 5 months ago....but I came back with a grin from ear to ear. So much so that I gave my open faced helmet to my 29 year old son ...and said "take it up the road". Well as you probably guessed he came back with that same grin, this was his first ride on an old Brit, you never know we might have a future convert.

Congrats !!


As you probably have guessed by now. Triumphs came equipped with a grin as "standard equipment". If you ever loose it, you can buy another by part number !!

:beerchug
 
#14 ·
Hi Trybry, When you say the brake light switch, what went wrong with it? Could you see the fault or did it just go ok with unplugging switch? Did you have to replace the switch to correct fault? Thanks.

On a side note, don't mess around with the old plastic floats & needles. Get new genuine Amal stay up floats & viton tip aluminum needle. Amal website explains about them.
Don
 
#15 ·
Don, at first I unplugged the rear brake light switch as an elimination process and that cured the blowing fuse. On inspection the switch was stuck in the compressed position, I took it apart freed it up and re-set it on the bike and all was well. Probably just a fluke on my part but I was happy as a lark when it all worked.
On the floats I agree with you on replacing the original plastic stuff and new needles, its on the to-do later list.
Bry
 
#16 ·
The brake light switch should not be able to short circuit the wiring so i would suspect there is a problem in the rear lamp holder or wiring to it.Leave alone for now but keep it in the memory for when it blows again.
 
#17 ·
Hi Bry,

blew its 25A fuse.
If this wasn't finger trouble, this isn't the correct rating for a T140 fuse:-

. If the electrics still have the tubular white plastic fuse holder that takes a fuse that's a metal strip inside a glass tube with metal end caps, the fuse should be rated 35A "blow", 17.5A 'continuous'.

. However, be aware that such fuses from Japanese bikes and some other applications, the stamped rating (usually on at least one end cap) is continuous - i.e. it's actually a 70A-blow fuse! :eek: Look at the middle of the strip inside the glass tube; if it's obviously fat, it's one of these and not correct.

. Otoh, if the bike uses a modern automotive blade fuse, they're all rated 'continuous' so the bike needs a 15A one.

Hth.

Regards,
 
#20 ·
Hi Bry,

Mine has the newer blade style 25A, apart from the Boyer ignition the bike is stock.
As I say:-

. Meriden originally fitted a 35A blow/17.5A continuous fuse to your bike;

. because of how blade fuses are rated, the "25A" is "continuous", so it's a 50A "blow" fuse.

A single high-rated fuse on its own isn't anything even close to an ideal way to protect individual wires and other electrical components, so its rating is a compromise between protection and convenience; ime and mho, a 25A blade fuse is too much of the latter and too little of the former.

Fwiw, even my T160's - with standard electric-start and additional electric farkles - are quite happy with 15A blade main fuses.

Hth.

Regards,
 
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