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Discussion Starter · #1 ·
Went for a 30 mile ride yesterday and all was going well until I jump on the freeway. I just got it up to 70 mph when it blew the fuse. Looked at all the wires I could get to without taking the tank off and couldn't see any problem. I had a spare fuse in my pocket but as soon as I put it in it blew that fuse too.
Called my AAA membership to get a tow but found out the towing my wife signed up for didn't cover motorcycles! So had to fork out for a tow bill. Apparently, I need to have the AAA "premium" tow card for motorcycles!
Hooked up my test meter to locate the short but, (of course), wasn't shorting anymore. Looked at all wires and can't find anything wrong. I did notice that my right coil was very hot but think that is because I had the key on and the points were closed that the coil fed. Wouldn't think a coil going bad would cause a fuse to blow.
I have 300 miles on the bike since putting it together with a new wiring harness. Have had no troubles until now. Appreciate if anybody has a good idea for something to check.

Gary
 

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You need AAA RV coverage rider. I found out the hard way too but it only added $15 a year to my premium coverage. Used it once afterwards and I think it was worth it, otherwise don't drive further away than your buddy or wife/girlfriend will drive their truck.
 

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Are you using a fuse rated too low?

Likely places for a short circuit are inside the headlight and under the tank. Or wires get nipped at the steering head.

If your bike has the coils under the seat, the seat base sometimes touches an LT terminal on a coil.
 

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I had a similar problem with random fuse blowing. Turned out to be the wire to the stop light switch popped out & was shorting.
 

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Discussion Starter · #5 ·
Are you using a fuse rated too low?

Likely places for a short circuit are inside the headlight and under the tank. Or wires get nipped at the steering head.

If your bike has the coils under the seat, the seat base sometimes touches an LT terminal on a coil.
Coils are under the gas tank (70 Bonnie).

Wire at steering head appear okay.

Thanks.....Gary
 

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Discussion Starter · #6 ·
I had a similar problem with random fuse blowing. Turned out to be the wire to the stop light switch popped out & was shorting.
Looked at those wires also and they appear to be okay. I'm kind of concerned about the wires coming from the stator. Not sure if they are being hit by the primary chain and maybe that's causing the short. However, not sure if that would cause the fuse to blow.
 

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Discussion Starter · #7 ·
Are you using a fuse rated too low?

Likely places for a short circuit are inside the headlight and under the tank. Or wires get nipped at the steering head.

If your bike has the coils under the seat, the seat base sometimes touches an LT terminal on a coil.
Hi TT,
Fuses are 15 Amp (US type). I pulled the headlight and the tank and no sign of bare wire but did see some areas where wire has been rubbing or pinched causing a dent in the insulation. Wrapped them with electrical tape in those areas but still don't think that is the problem.
Just finished putting tank back on and headlight back together but haven't tried to fire it up. I'll do that in a little while after dinner. Can't go too far because haven't got the towing insurance figured out yet. Thanks for your help.....Gary
 

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Discussion Starter · #8 ·
Just got back from a short trip up the street and back. All went well and runs fine --- for now!
 

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If it only blows when lights are on, a headlamp bulb terminal can be maybe shorting out against the shell.

I've had problems with too much copper wire showing inside/under a dipswitch and touching the handlebar.
 

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Hi Gary,

concerned about the wires coming from the stator. Not sure if they are being hit by the primary chain and maybe that's causing the short. However, not sure if that would cause the fuse to blow.
It wouldn't. If in the standard position, the fuse is in the Brown/Blue wire attached to battery -ve. As such, it can only blow if the short-circuit involves the battery, which the alternator wires do not.

Just to worry you a little, the fuse doesn't always blow if the engine's running (alternator supplying the bike's electrical consumers) and a short starts to melt the wires (standard rated for only 7.5A) ...

Hth.

Regards,
 

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Discussion Starter · #12 ·
If it only blows when lights are on, a headlamp bulb terminal can be maybe shorting out against the shell.

I've had problems with too much copper wire showing inside/under a dipswitch and touching the handlebar.
Lights are all working and lights were not on when the fuse blew. I was accelerating and had just reached 70 mph when the fuse blew.

That dip switch is a good idea. I'll check that today.....thanks!
 

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Discussion Starter · #13 ·
Are you using the Zener Diode and points or other?
I have a 76 T140V engine in my 70 Bonneville with standard ignition - points,condensers, and zener diode. I was wondering if, (since I was accelerating at and at high rpm's when the fuse blew), it's possible for the zener to go bad and short out if it got hot and then work okay when it cooled down? ---- The wire terminal connections were good and tight at both the ground and the hot side so didn't see a problem there. The diode is original from way back in the 70's but tested okay when I put it on. I actually put it in my bead blaster and took all the oxidation off it before testing it and putting it on the bike so I know it has good connections. Have you ever heard of one going bad and causing a short?.....Gary
 

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I could be wrong about this since I'm getting old and the memory is fading fast. But..... my 71 Daytona has an AC alternator, and selenium full-wave rectifier mounted under the seat, and a zener diode out front for voltage regulation. I know that at one point the selenium rectifier failed but it failed open instead of shorted. I replaced it with a little 25 amp silicon rectifier bridge which is still in there and still works. My only point here is that the zener diode gets a lot of blame for blown fuses but a bridge rectifier could also fail shorted which would be likely to blow your system fuse. You might have a look at that.
 

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Discussion Starter · #16 ·
I could be wrong about this since I'm getting old and the memory is fading fast. But..... my 71 Daytona has an AC alternator, and selenium full-wave rectifier mounted under the seat, and a zener diode out front for voltage regulation. I know that at one point the selenium rectifier failed but it failed open instead of shorted. I replaced it with a little 25 amp silicon rectifier bridge which is still in there and still works. My only point here is that the zener diode gets a lot of blame for blown fuses but a bridge rectifier could also fail shorted which would be likely to blow your system fuse. You might have a look at that.
Hello Doberperson,
First of all, thanks for your input! Do you think the bridge rectifier could possibly be intermittent? It is brand new minus 300 miles on the bike since I installed it.....Gary
 

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It is brand new minus 300 miles on the bike since I installed it.....Gary
Brand New Old Stock Lucas? or brand new Lucas current manufacture? or brand new reproduction Hong Kong Harry?

I have been loath to purchase any of the green box Lucas products OR the Hong Kong Harry parts.

That being said I needed a brake switch and couldn't find NOS at a reasonably price so I bought a HKH switch.

Someone will help you out with testing your rectifier with a DMM but they will want to know which of the 3 options you went with. :wink2:

Any of them will fail if you do not support the center lug properly while installing. All it takes it the tiniest of twists internally to break the connections inside.
 

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Hi,

I was wondering if, (since I was accelerating at and at high rpm's when the fuse blew), it's possible for the zener to go bad and short out if it got hot and then work okay when it cooled down?
I'd never say it's impossible but it's very unlikely - Zeners don't get hot and cold instantly. The symptoms you've described are more likely a vibration-related short-circuit (metal-to-metal contact at certain rpm-related vibration) or failure.

Btw, as your bike's a mix of oif and dry-frame years, where's the Zener located - attached to the air-filter box (oif) or to the finned heatsink under the headlamp (dry-frame)?

my 71 Daytona has
selenium full-wave rectifier mounted under the seat,
If it really did, the bike rectifier was non-standard and the seat was about four feet off the ground ... :eek: While original Lucas rectifiers are a bank of plates separated on a central mounting, looking not unlike search engine pictures of selenium rectifiers, they are silicon-diode bridge full- or half-wave rectifiers. The square after-market rectifiers with four or five male tab terminals protruding from one large side are still full-wave silicon-diode, just encapsulated for vibration protection.

a bridge rectifier could also fail shorted which would be likely to blow your system fuse.
If you look at Lucas or Triumph wiring diagrams, you'll see a full-wave rectifier has four separate diodes. Afaict, both diodes rectifying the same half of the AC wave have to fail closed for there to be even a possible short-circuit path to the central mounting. So, again, while I'd never say it's impossible, it's also very unlikely.

Any of them will fail if you do not support the center lug properly while installing. All it takes it the tiniest of twists internally to break the connections inside.
In principle, yes, but certainly original Lucas (as opposed to Wassell "Genuine Lucas" :rofl) tried to make them tolerate at least some unintentional abuse. :) And it's the soldered connections between the plates that break if the plates rotate independently.

Ime, best is to avoid the Lucas - whether original or "Genuine" :D - altogether and go for an encapsulated one from a supplier you can take it back to if it goes phut - your local parts dealer, Maplin, etc.; unfortunately, while they all look the same, the quality varies ...

Hth.

Regards,
 

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Discussion Starter · #19 ·
Cal30M1:Brand New Old Stock Lucas? or brand new Lucas current manufacture? or brand new reproduction Hong Kong Harry?
Hey Cal30M1,
Thanks for responding. I don't remember what kind of rectifier I bought. It was 3 years ago that I bought it but I do remember grinding down an open end wrench so that it would fit and hold the nut under the mounting bracket so I could tighten the top nut without damaging the rectifier. The wrench was to thick to slip under the bracket. All I can tell you is the rectifier looked just like the original rectifier....Gary
 

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Discussion Starter · #20 ·
StuartMac:
Btw, as your bike's a mix of oif and dry-frame years, where's the Zener located - attached to the air-filter box (oif) or to the finned heatsink under the headlamp (dry-frame)?
Hello Stuart!,
I have a 70 Bonneville with 76 T140V engine in it. The zener diode is located in the original spot on the finned heatsink under the headlight.
Do you think it's possible to be intermittent and cause a short and then cool off and be okay?
For what it's worth in this discussion, the battery has been staying charged after long rides.

Always appreciate your help. Thanks, Gary
 
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