Electrical problem

jasper · Post by **jasper** » Sun Apr 26, 2009 9:26 pm

The 15 Amp fuse on my 1961 Model 31 (dynamo model) keeps blowing. The bike has a reconditioned dynamo. Am I right in thinking that only a wire shorting to earth could cause this problem? I've looked at the wiring and cannot find any wear or loose connections.
Any pointers to look at would be appreciated.

itma · Post by **itma** » Sun Apr 26, 2009 10:20 pm

a short to earth will certainly cause fuses to blow.
The only way to find it is to check each circuit in turn.
If you suspect the dynamo disconnect it.
does the fuse blow as soon as the battery is connected? or when, for instance the lights are turned on?
have you any other electrical items fitted that may be suspect?
Pure trial and error is the only way.

kernow kid · Post by **kernow kid** » Mon Apr 27, 2009 10:33 pm

Its a nasty prob that pops a 15 amp fuse. As you suggest, a direct short to earth is likely. itma's suggestion about disconnecting bits, 1 at a time is best. I'd take a look at the loom around the headstock and headlight area as thats where there is most movement, when you turn the handlebars. Have a look for any chaffing.
If you fit a clear glass fuse, you can see the filament of the fuse. A dead short will frazzle the filament immediately or create a noticeable gap in the middle. An overload may take longer to pop the fuse and show as a filament that has obviously cooked before parting.
Which have you got?
Regards KK

Eric · Post by **Eric** » Mon Apr 27, 2009 11:07 pm

A weird problem I had a few years ago blew the fuse when the bike was travelling at about 55 mph with the headlight on. This turned out to be a screw that had fallen out out its anchorage point in the dip switch and the vibration at around 55 mph allowed it to jump up and short out to earth. Its that sort of weird you may have to find.

jasper · Post by **jasper** » Mon Apr 27, 2009 11:52 pm

Thank you Eric and Kernow Kidfor your comments.
Only the dynamo and front light bulbs are new. It appears that the problem occurs when the lights are switched on.
I am half way through checking each part of the circuit for continuity and condition and joints condition. So far all is OK.
The wire as seen in the glass fuses doesn't appear to have broken or parted but a blob of solder appears inside the glass. It's almost as though solder in the fuse has been melted. The outside of the brass ends of the fuses also looks cooked. The fuse holder was broken in two on the last occasion - perhaps due to heat generated (??)
On a run recently, with no lights, on the ammeter read 7 amps. When I switched the lights on it showed 0 amps and the lights were very dim. Then after about 10 minutes the lights suddenly brightened and the ammeter read about 1-2 amps. I don't know if the problems are connected.
I'll continue my search tomorrow.

wheel@bothends · Post by **wheel@bothends** » Tue Apr 28, 2009 12:00 am

Try replacing the fuse with a bulb wired across the fuse connections. The bulb will remain lit if there is a path to earth, but will not damage anything [i.e. burn the harness out!] while you waggle the wires about trying to find the fault. The bulb will go out when the problem is isolated.
Robyn

Don Madden · Post by **Don Madden** » Tue Apr 28, 2009 2:46 am

The melted blob & scorched end caps is similar to a problem I had with another bike. Poor contact of the fuse to its holder caused it to heat up & fail. The fuse in this case melted at the end where attached to the cap, not in the middle where they melt from over load.

Cheers, Don. Edited by - Don Madden on 28 Apr 2009 01:47:20 AM

kernow kid · Post by **kernow kid** » Tue Apr 28, 2009 2:28 pm

Hi,
what style of fuse holder is it? There are some abysmally poor quality holders on the market. The plastic type that have a bayonet quarter turn to hold the 2 halfs together can be really rubbish.
On Ebay and via shops like Maplins, you can get clear plastic holders that screw together for about £1.50. These are much better quality and will take 1-1/4 glass cartridge fuse.
Regards KK

jasper · Post by **jasper** » Thu Apr 30, 2009 8:57 pm

quote:
The melted blob & scorched end caps is similar to a problem I had with another bike. Poor contact of the fuse to its holder caused it to heat up & fail. The fuse in this case melted at the end where attached to the cap, not in the middle where they melt from over load.

Cheers, Don. Edited by - Don Madden on 28 Apr 2009 01:47:20 AM

Thank you Don, a really good tip. I'll replace the fuse holder (a quarter turn type) and see what effect it has.
Jasper

jasper · Post by **jasper** » Thu Apr 30, 2009 9:05 pm

Thank you all for your useful comments and tips. I appreciate them very much. Tomorrow I'll try the test with a bulb across the fuse holder connections and also try a different fuse holder.

Jasper