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Discussion Starter #1
Hi everyone,
I rewired my 3ta/T21 back in 2018 - with considerable input and help from this forum. 12v RM24 alternator & all the trimmings. Main fuse on battery -ve and 7.5amp fuse & relay on each headlight element Everything worked well.

For some reason the fuse on the dip beam element likes to blow. The bulb doesn't blow. I've checked the wtring and all seems ok. Is the fuse too small at 7.5amps. I don't have a problem with the main beam, but then I don't use it very often.

Any clues would be welcome.

Many thanks
 

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What is the specified current draw on your particular bulb? Search the bulb number should find this. Compare to your fuse rating.
 

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Discussion Starter #3 (Edited)
Can't find anything about the bulb. My understanding is that a 12v 55w bulb should draw 4.58 amps. The feed is direct from the battery through the (7.5amp) fuse and a relay. The relay is switched by the standard (1960's) switch gear. As mentioned in my original post the wiring was renewed in 2018.
 

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Discussion Starter #5 (Edited)
That's not so crazy..... in a fit of desperation to get out for a ride, and with not other solution at hand yet, I did just that - fitted a 10amp fuse. Just back from a very pleasant 30mile ride - the headlamp shone brightly throughout!
I would still like to discover what the correct rated fuse should be.
 

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It wasn’t supposed to be crazy.

Headlight bulbs take a burst of current greater than their rated amps before they get properly bright.

I freely admit I’m surprised a 7.5 A fuse didn’t work and I don’t know why the main beam fuse wasn’t affected.
 

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Hi Jeremy,
rewired my 3ta/T21 back in 2018
12v RM24 alternator & all the trimmings. Main fuse on battery -ve and 7.5amp fuse & relay on each headlight element Everything worked well.
For some reason the fuse on the dip beam element likes to blow. The bulb doesn't blow. I've checked the wtring and all seems ok. Is the fuse too small at 7.5amps.
Has this always been a fault since the rewire, or is it a more recent problem?

Does the fuse blow when the lamp is turned on, or just randomly whenever?

My understanding is that a 12v 55w bulb should draw 4.58 amps.
(y) Like TT, I'm surprised 7.5A fuses keeps blowing.

Regards,
 

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Hi Jeremy, The high beam is ok, is that correct?

If yes the problem is in the low beam circuit. It is possible the bulb could have internal short. I'd don't recall seeing internal bulb short before though.

So wire to dip switch is good. Wire to high beam is good.

Problem is between dip switch & low beam.

Very carefully look for exposed wire strands, bullet ends, pinched wires where conductor could be shorted to ground.

Remove bulb from head light. Does fuse still blow? If yes recheck wiring dip switch to bulb socket. If wires are good, may be dip switch has internal short.

If ok, test bulb with jumper wires. Test bulb with the fuse type you are using hooked up with the jumper wires also.

Regarding fuses, what type fuse are you using? Modern flat, or round glass? Vintage glass like Lucas, or modern glass?
Don
 

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Discussion Starter #10
Thanks to you all for your thoughts

TT - the main beam wasn't affected because I very rarely use it. I generally use dip beam during the day. Like you, I'd have thought 7.5 would have enough headroom on a 55w bulb.
Dave - I'll give slo-blow a try.
Stuart - it's a recent problem and would appear to be a random one. I did cahnge to Xenon bulbs late last year but then didn't ride until June this year.
Don - I haven't had a problem yet with main beam. I doubt it's a problem with switching as the main fused power feed is through relays direct from the battery. The switches merely switch the relay on or off. The fuses are in the feed to the relays. I've checked all wiring. I;m using modern mini flat fuses.
Could it be a relay going out?

Regards
Jeremy
 

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Hi Jeremy,
recent problem and would appear to be a random one.
Hmmm ... that the fuse is blowing randomly suggests the cause is most-likely a damaged wire or exposed terminal part; if the fuse was too small for the bulb filament draw when it's turned on, that's when the fuse'd blow every time.

did cahnge to Xenon bulbs late last year but then didn't ride until June this year.
Can you swap back to the previous 60/55 bulb temporarily?

I'll give slo-blow a try.
Mmmm ... bear in mind blowing blade fuses is cheaper than blowing one of the filaments in a twin-filament Xenon bulb ...

The fuses are in the feed to the relays.
Fuse holders separate from the relays or fuses in the relays themselves?

relay going out?
Ime, you'd be unlucky. However, if you're using two separate 4-pin relays, have you tried swapping them?

If you're using two separate 4-pin relays, how is the supply from battery -ve to the relay (30 terminals) arranged? Separate wire from battery terminal to each relay or just one wire most of the way and then a spilt close to the relays?

doubt it's a problem with switching
+1. Two separate circuits.

Hth.

Regards,
 

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Hi Jeremy, Ok I get how you have headlight wired now.

I've seen hundreds of bad relays. Most go open circuit, don't blow fuse. I've seen a few that blew fuse. Removing cover I found part of relay fractured & fell into lower area where terminals rivet on & shorted to ground. The relay didn't work though. I took cover off to see why was blowing fuse. While possible unlikely you have bad relay.

What size is wire from relay to low beam? You mess around with larger fuses you risk smoking the wire if size of wire can't handle current flow if wire is shorted. I've seen customers use too small of fuse & the fuse looks heat damaged before it blows. They do this with cigar light fuse often. Phone charger shorts in socket blows fuse. There are spares in fuse box. Out of 16 so they use 7.5. No problem with phone, but then they use lighter... Works for awhile sometimes.

They make a "Fuse Buddy" small digital amp meter made to test circuit draw. Plugs into fuse socket. Meter has fuse holder to protect circuit. About $30 US. We used them all the time at work. I use it now to test zeners. You can attach jumper wires to it when you're not using fuse socket.


I'm with Stuart on checking wire & socket too. I expect you'll see the fault.
Don
 

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7.5amp fuse definitely is fine for the draw of that headlight bulb. I would not go the slow-blo route, could cause more damage. Problem has to lay somewhere else. Especially since fuse isn't blowing as soon as headlight is turned on.
 

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Hi Jeremy, Not saying this is your problem... But it could be. This is what I mean about visual inspection. This is what I found on my bike recently. See post on wire chafing.
Don
 

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Discussion Starter #15
Hi Don. Excellent thought - I looked at your post on wire chafing. I must admit that the wiring looks good, but a closer inspection tomorrow is waranted.

To answer some of Stuart's questions/points. I've gone back to the original standard halogen bulb at 60/55watts. I've also gone back to the original 7.5amp fuse. The fuseholders are separate from the relays. The power to the relays (via the fuses) is taken from the main battery feed usinga single 28/030 cable for most of the way which is split at the fuses. The feed from each relay to its respective beam is via 14/030 cable.

Having reverted to the "standard" set up that I started with I have tried the lights (without the engine running), operating the switches, moving between dip and main beam without any fuses blowing. Mind you, this was with the wiring hanging out of the headlamp shell. Once everything is fitted back in and the bike on the move I might find a different answer if there is some chafing going on. As I said above, I will check that out tomorrow. I'll report back later.

Thanks for your help.....
Jeremy
 

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Hi Jeremy, I just checked my car fuses, headlight. 2000 E320 Mercedes. Fuse for high beam is 7.5A flat fuse. Bulb is 55w. I pulled bulb & verified. I don't know what mm size wire is without looking it up in manual. Visually looks like British 18g the size a lot of Triumph wire is.

At work I've never seen this fuse blow unless there is a fault. I've had my car several years, never changed the fuse.

So 7.5A should power your 55W bulb fine.
Don
 
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