Ammeter pegs to negative

Went out to start the Fastback, which is always a 1-2 kickstart bike and noticed when the ammeter pegged to the minus side when the ignition switch was turn to "on". The bike refused to start. Got lights, horn; battery seems fine. No smoke anywhere. Started to try to find the cause by isolating things that may be grounding like the Boyer, coils, ingition switch, rear stop light; everything that works off the ingition switch. Still no luck; meter still pegs with ignition on.
Did notice that the battery goes dead within a minute of having the ignition on.
Am planning on getting a new battery tonight.
Question: Would a bad battery cause the ammeter to peg to full (-) when the is ignition turned on? I've never experienced this before.

Tom

Sounds to me like you have a short from - to +, if you have pos ground. Start disconnecting things to see if you can find the short. I doubt if it's the battery but stranger things have happened. Does it charge up OK? I'm assuming you have a proper fuse in the battery?

Dave
69S

Dead battery would read 0 amps needle centered. Discharge via short would peg to neg side. Its possible the gauge is shorted inside headlamp and also possible the tail light ground strap is shorting so keep looking and prove you are man enough to possess a Commando. Sunday on a ride Wes and I got about 5 miles out when Trixie stalled out no power. I found battery had broken its zip tie down and flipped sideways. I'd hit a hole in pavement like 60 mph I though might of pinched the tube it bottoms to hard. Put it back up and got started by a swift charging kick instead of usual step on. Got 20 miles further off beaten path exploring and she died again with no power anywhere. Though it was dead battery dragging the juice down but while Wes was taking his out to try on 'Trixie I got my test light out and finally found the intact looking glass fuse had internal connection issues that a spare fuse fixed. Don't apply to your case though, just example can't assume anything that looks right is.

Find the short circuit by disconnecting things.

Finally figured out the problem. Although not shown on the electrical schematic, the front brake cable has an in-line stoplight switch. Constant hot to one side / continuity to brake light when switch closed. Somehow, the hot side is shorting to the switch internals. Not enough to fry the insulation but enough to peg the ammeter. I disconnected the terminals from the switch and all is well. I did find it interesting that the factory schematic didn't indicate a front brake light switch.

Thanks Icky for another fault in manual to watch out for. Harley has identical hydraulic brake switches if a disc on yours, just pick bullet or flat blade terminal connection version. If inline cable, oh well, my buddy bypassed that with clothes pin switch.

Yeah, I've noticed there's a few things missing on the early schematic, but nothing that can't be figured. I pretty much drew out my own harness with the handle bar/switch harness separate, and the headlamp wiring. All these things help when things go wrong.

Dave
69S