Not an AMC but....

[pecon]

I really need some advice please. My other project is a '59 Triumph Cub and while I was waiting on some parts for the 250CSR I secided to start it up and give it a run, however it would not fire. When I turned the headlight on it glowed bright for a second or two then faded as though the battery was flat but on checking the battery it was showing 6.2 volts. I changed the PRS8 switch for a secondhand one I had but no improvement. I then found that when I disconnected the earth on the headlight that I got a spark as though it was shorting. When I ran a multimeter from the positive earth on the battery to earth on the frame I got a full 6V reading but nothing from the neg side to earth. Thinking that I had reversed charged the battery I checked with another battery but no improvement. The only component changed was the regulator for an AReg six which I double checked is wired correctly. I also found that a 5 amp fuse will blow but not a 7.5 one. Can anyone think why this might suddently have happened and hopefully point me in the right direction?.

[SPRIDDLER]

When I ran a multimeter from the positive earth on the battery to earth on the frame I got a full 6V reading

You wouldn't get a reading when putting a meter across conductors of the same polarity (i.e. battery Earth and frame Earth) when there is no load between the test points since the battery terminal and frame (via the battery lead) are at the same potential. In your case (you say) the Pos battery lead is bolted to the frame and you are putting your meter probes on the Pos battery terminal and the frame so there shouldn't be a reading (as there would be no differential). You need to measure across a Pos and a Neg point to get a voltage. It seems that you are putting the meter across the Pos battery terminal and the frame earth, (the frame actually being Neg not Pos as you think) and that's why you get a 6v reading (Battery terminal and frame are dissimilar polarities).

but nothing from the neg side to earth.

If I'm reading your symptoms correctly it could again be because the frame (earth) is Neg not Pos as you assume/believe, therefore you are again connecting the meter to similar polarities (Neg battery terminal and Neg not Pos frame).

But I may have got the rob end of your stick. It happens

Is the AReg Six a Pos earth unit?


(Hmm, I keep editing to try and put it more clearly but I just seem to complicate my explanation)

[SPRIDDLER]

When I turned the headlight on it glowed bright for a second or two then faded as though the battery was flat but on checking the battery it was showing 6.2 volts............ I then found that when I disconnected the earth on the headlight that I got a spark as though it was shorting.

It probably was a direct short caused by the polarity confusion suggested in my post above.

I also found that a 5 amp fuse will blow but not a 7.5 one. Can anyone think why this might suddently have happened and hopefully point me in the right direction?.

A 5A fuse can blow easily with just the headlight switched on since a 6V 36W headlamp bulb on its own is drawing 6 amps. Why it has 'suddenly happened' is probably/possibly related to something to do with muddling the polarities when fitting the new AReg six.
With my 6V system I use a 10A fuse.

[pecon]

Hi Spridder

Double checked the AReg and it is indeed wired for pos earth (it can be changed to either polarity) and it ran fine for a while with it wired as it is, the only reason I mentioned fitting it was because it was the only electrical work I had done since buying the bike. I also checked in case I has wired the battery wrong when charging (only did that once with another bike) but it is fine. I'm also getting fluctuations in voltage. Sometimes I get a full 6 volts to the coil and sometimes its only 1.4 V or so. The spark when reconnecting the headlight earth leads me to think that there is a major short somewhere but there is nothing obvious. I charged the battery again and when I connected it I could hear a "gurgling" noise from it which stopped when disconnected. Strangely the 7.5 amp fuse had not blown. I think it's time to sit down and start from scratch and examine the wiring from top to bottom.

[SPRIDDLER]

Yes, if your various polarities are correct then it does sound like a short circuit. A common place for this to occur is where the wiring loom enters the headlamp. A wire's insulation can easily chafe due to the constant movement when riding, making the short intermittent depending upon the angle of the handlebars...............
If you do find fuses keep blowing whilst investigating it's a good wheeze to replace the fuse by wiring a low wattage test bulb* in place of the fuse. In that way if you wiggle the wiring (with lights switched off) and a short occurs the test bulb will light instead of the fuse blowing.
Obviously, painstaking tracing with a meter with the lights off (maybe turning the 'bars or operating the brake pedal) is the 'right' way to do it it might be worth checking the dip switch as they are generally unshielded and water gets between the (often corroded) contacts and the 'bars (Earth) causing a short.

* I only suggest a low wattage bulb (parking or rear lamp) rather than say, a headlamp bulb since you only need to see it light up and not draw a higher current through the loom, (or dazzle you!).

The brake light normally has a permanently live supply from the battery and is not fed via the light switch so worth checking for a short circuit within the lamp unit and the brake switch.

[pecon]

Thanks for those suggestions Spriddler. When I bought the bike and tried to run it there was a cloud of smoke in the headlight shell (no fuse at the time) which was traced to the switch (quickly replaced) shorting at the bars. This time I removed it before trying to trace the fault.

Peter

[SPRIDDLER]

When I bought the bike and tried to run it there was a cloud of smoke in the headlight shell

Shame. Replacement isn't easy to get these days, not even from Club Spares.

Smokekit2.jpg

[pecon]

Oh I don't know, I think I have enough to go around