Ammeter burnt out

[peter small]

I have a 1954 Road Rocket which has been running fine but the ammeter went straight over to a max discharge and the face of the meter burnt away, prior to that happening the rear light bulb blew. i am a bit loathed to replace if the cause is unknown.
Any idea`s

Pete

[bsa-bill]

Think maybe worth checking what the Voltage Regulator is putting out, meter over the battery with engine running, think it should be no more than 13.5 but an electrikery expert might like to chime in

[wilko]

Dead short somewhere.

[trevinoz]

Pete,
           As Wilko says, it seems like a short circuit.
Do you have a fuse in the battery circuit? A fuse should blow before damage like this is caused.

Trev.

[peter small]

I have just bought this bike and although it seems rebuilt i have had to strip the engine now it appears the electrics are suspect.
It remains 6 volt
Has a refurbished Dynamo.
It has a mechanical regulator age unknown

The loom is new but no fuses fitted

New battery

I believe it is positive earth it has 2 brown wires going to battery with a single black wire for the other terminal unfortunately in my hast to disconnect the battery i dont recall which went where .

Any info much appreciated i have rewired ac generator bikes but this is my first dc powered bike

Pete

[iansoady]

Mechanical regulator would usually indicate positive earth IME.

The first thing I would do is fit a 15 amp fuse in the main battery lead.

[beezermacc]

If it has a mechanical regulator, i.e. original Lucas type it will have F  A  D   E on the casing (not necessarily in that order. The F and D terminals should have green and yellow leads entering from the corresponding terminals on the dynamo. E (usually black) goes to earth, usually on the frame under the seat, A (usually purple or brown/white) goes to the ammeter. If the ammeter is burnt out you can simply bridge the terminals temporarily, but you must be absolutely certain that the rest of the circuitry is OK. I'm not recommending this other than a 'get you home' device.

[Triton Thrasher]

Mechanical regulator would usually indicate positive earth IME.

They work either way.

[a10 gf]

ammeter went straight over to a max discharge and the face of the meter burnt away

Would be strange if no cables heated up as well, turning up as damaged insulation (unless the short happened in, or very close to, the ammeter). Time to check trough everything, looking for bad cables and what I'd believe was the initial culprit, some direct short to frame.

[BSA_54A10]

And the two could be totally unrelated.
An ameter has a heavy resistor inside which can go short.

[a10 gf]

An ameter has a heavy resistor inside which can go short.

ehhh.... shorted? That would just give a 0 reading on the dial, as current would pass directly trough the ammeter, unhindered.

The ammeter 'dial mechanism' (lol) is getting it's readout from the difference in current flow across the 2 sides of the ammeter 'resistor'.

[trevinoz]

Sorry Trevor,
                      The ammeter doesn't have a resistor at all. It has a very low resistance shunt.
The ammeter is in effect a millivoltmeter and reads the voltage drop across the shunt.

Trev.

[peter small]

As a10 gf quoted a bad short to the frame should have taken his advice. lol.

I  fitted two spade fuses on the two brown wires from negative side of battery.this isolated the rear brake from the rest of the loom this was ok. I would recommend doing this anyway. cost me an Ammeter. plus trying to unbolt the seat while the ammeter is still smoking  is not my idea of fun.

I did suspect the switch being the original but it tested fine and cleaned up for another 60 years.
Lighting showed  a few things up, previous owner didnt own a soldering iron twisted wires onto pilot bulb springs not best practice.

I checked the rear light by substituting the brake light connections.ok

I took the regulator off which isolated the dynamo so they didnt figure in the fuse blowing, again wires pushed into connections.

It had to be a dead short somewhere so i checked every wire and found the culprit a wire from the regulator to switch, should be purple but brown and white in my case.
So off with the tank and there it was a small burnt mark on the cloth braiding near the engine steady, worn through in just over 200 miles as the loom is new.
I re insulated the wire and repaired the loom covering and cable tied it sucurely away from anything that can rub it. check it all and gave it an extra insulation covering.

I know have a better electrical set up than i had.
The lights being 6 volt are still rubbish lol.

Thanks for the advice.

Pete

[a10 gf]

Very nice to find the cause, otherwise things may seem ok, doubts about possible failure remains, and then suddenly the problem is back.

worn through in just over 200 miles

wow, that's a 'short' life. Sharp edges or was the cable squeezed ?

[peter small]

The harness and the cable insulation was rubbed through the copper wire was intact.
The loom was very poorly secured around possible problem items.

I am going to fuse the system 10 amps i can always go up if i need to.
I would sooner have the fuse blow thn the cable cooked.