Trap for old players.

[Motu]

I had a look at a bike for an old mate today,he's helping his son build up a 1972 Bonnie....mostly all sorted,but there is no charge.My friend is very knowledgeable about Triumphs,and has a better grasp of their electrics than most.They had someone who knows electricals have a look and went over everything with a multimeter....everything works as it should,but still no charge.My mate has had to break into his own precious stock of parts - it has his Boyer ign,his clutch,his stator,rectifier and zener diode,all new.

Surely has to be something basic,so today they rode it in and I put my meters on it.There was 11.9v at the battery,zero at the zener....there is nothing going in alright.I got good AC wave forms on both sides of the rectifier and 7 volts on each at a fast idle - that should give a good DC at the positive terminal,and there was,13V,with 20V at higher revs.All the wiring looks in good order,I'm stumped here,and hook up to do a few more tests.

''Hey,you've got your leads hooked up wrong''...???? how come?....''Positive earth in 1972,Triumph didn't go negative earth until 1978''.I'm not happy with this,I don't see why it should be positive earth,electronics need to be negative earth....but the Boyer can be set for either,and they have it set up correctly,and really don't want to go neg earth.I tried to swap + and - on the rectifier but it blew the fuse straight away.

I could spend no more time on it,so sent them off to find a positive earth rectifier.It wasn't until the son rang up later to talk about it that I twigged to what was wrong.They had the + and - terminals of the rectifier going straight to the battery - so the + was going to the + on the battery....which is earth,not to the - terminal as I thought they had done.So the output was going straight to earth,no way to charge.Swapping the rec wires just shorted the battery and blew the fuse.

Simple really...but it's easy to get fooled with such a simple system....and connecting positive to positive is obvious - the brain just can't cope with hooking the + to the - terminal.

[DemonWolf]

Good on ya Motu... Bet they are now happy campers!

[davereid]

Hell, I'd worked out a much more expensive repair than the one you worked out !

[Motu]

Good on ya Motu... Bet they are now happy campers!

He will be if he gets to Paeroa on sunday!

[What?]

Old Triumphs had electrics????
Mine didn't.
Usually.

[sizzlingbadger]

Quote ''Positive earth in 1972,Triumph didn't go negative earth until 1978''.I'm not happy with this,I don't see why it should be positive earth,electronics need to be negative earth.

+, -, earth it's all in our heads. It doesn't matter which way a bike, car or circuit is designed. Do you know that the electrons actually flow from - to + not the other way around ? So the old + earth was actually more correct (in our heads) than you imagined

[Motu]

The cotroversy has been going on for a long time - GM went to negative earth in the 1930's,but reverted to + earth at some point because it was just too radical.When alternators started to replace generators most manufactorers changed to - earth.In the '70 we were working on both + and - earth systems and were aware of what system we were working on....35 years on it's very seldom I ever come across a + earth vehicle...and all automotive electrical systems are designed for - earth.It took my brain a while to click to postive earth,I was uneasy about the negative earth components but didn't know why.