Electrical diagnosis anyone?

[Peter Naulls]

The Ruby suffered an electrical problem on the way to work this morning. There was an intermittent loss of ignition several times and I noticed the ammeter flicking from small charge (normal) to small discharge. I checked all the obvious terminals - battery, dynamo, ammeter and cut-out terminals but everything was tight. It started and ran but the ammeter was flicking backwards and forwards again. Checked everything more carefully and cleaned the two fuses inside the cut-out.

No problems since so quite possibly I've cured it ... but you know how it is with intermittent faults.

Could it have been the fuses?  Idiot proof answers please - I have never understood the charging system!

Peter.

[bob46320]

Fuses and contact points inside the voltage regulator are a good start, then Dynamo brushes (and of course wiring). BUT why the discharge ? How many amps discharge and what did you have on:-

COIL
LIGHTS Side
LIGHTS Head
INDICATORS On / Off
Brake Lights
Anything else ?

[John Cornforth]

Hi Peter

A loss of charging shouldn't kill the ignition, as the battery will continue to supply it.

It sounds like the supply to the coil is being briefly interrupted.  From the battery, current flows via the ammeter and the ignition switch (part of the PLC switch) to the coil.  No fuses are involved.

I would look at the integrity of the ignition switch, maybe by wiring up a temporary small wattage 6 volt lamp between the coil supply terminal and ground.  With ignition on, this should light up consistently.  Waggling the ignition spade/key may show any flicker or intermittancy. The PLC switches are robust but 80 plus years old and do get slack.

If you don't have a copy, try my Ruby wiring diagram on the Dorset club site: http://www.da7c.co.uk/technical_torque_a...rubies.htm

Cheers, John

[Peter Naulls]

That's very helpful thank you both.

I too was puzzled why the lack of charge was also cutting the ignition. The discharge on the ammeter could I'm guessing be because the brakes were on, but then if the ignition was cut wouldn't the brake light circuit be dead too? I shall study your very helpful diagram - I carry one under the seat!

Peter.

["Slack Alice" Simon]

I don't like the "small discharge" - have you checked any wiring that goes through grommets, for example, for shorting to earth?

I had a silly ten miles the other day because I had changed the coil and forgotten to tighten up the terminals, but I didn't look at the ammeter.

Simon

[Robert Leigh]

You have some useful pointers already, but the wiring could be the problem; has some insulation failed or is there a 'whisker' from a terminal going down to earth intermittently? Obviously the wiring to check is the ignition circuit, the brake light circuit and the horn circuit on the supply side of the horn. I think those are the only things live through the ignition switch (or perhaps not the horn: does it work with the switch off?).
Robert Leigh

[Colin Morgan]

The only time I had such symptoms, the speedo cable (where it runs through the engine bay) had moved across and was touching and providing a direct short to earth from the live part of the starter motor switch.  After a period of ammeter flickering, which I saw but did not immediately understand, everything went dead.  Then there was smoke from under the bonnet as it was was seriously shorting the battery, but once pulled away everything calmed down.  In this case, the short was enough to cause the loss of ignition.  

Colin

[Peter Naulls]

Well the journey home was uneventful.

I was concerned there may have been a short and I really don't like the un-fused ignition circuit, however I think the culprit is a newish battery master switch. Just touching it whilst in the "on" position is enough to cause an intermittent supply. This doesn't really explain what I thought I saw on the ammeter - maybe it was flicking between discharge (brakes lights on) and zero. Quite possible that I mis-read it in my haste to find a place to pull in - and the ammeter needle does have a bit of bounce to it which could have fooled me. That's what I'm hoping anyway! The ignition switch seems sound enough and wiring all looks tidy and intact.

I have a spare switch which I'll swap-over tomorrow.

Thank you again for the suggestions - excellent roadside assistance!

Peter.

[Bob Culver]

If the battery becomes disconnected the system voltage rises and will damage any bulbs, possibly the coil,  and, if persisted, the dynamo.

[Ian Williams]

Sounds like a loose connection to me, not uncommon for terminals behind the instrument panel to come loose