ignition fault.

[kennatt]

ser11 55 pull starter,948 with alternator,standard points,been running fine for years, switch on yesterday ign light on started fine ran for a few minutes switched off and engine continued to run,stalled engine and retried ,same result, Noticed that ign warning light failed to come on.

Checked battery, charging at 14.8volts,so alternator working but still no ign warning light,switching checked ie off position no voltage at +coil,switched on 12 v at coil.

Started up and with switch in off position still running,stalled and checked coil ... no voltage,

Started up again and swtiched off still running,pulled alternator connection and engine stopped so somewhere its picking up voltage to the coil via the alternator.Not yet checked the ignition bulb ,can't see how a blown bulb would effect it,or could it be letting power through to the switch and coil ,because thats what it looks like,Before I pull the speedo and glove boxes to get at the switch and bulb,any ideas would be welcolmed.

[Dean]

Just to do a process of elimination.

Had it been stood a while?

Had you been working on it and changed something. ie give it a service.... ?

Had you taken it out in the rain the day previous? etc.

[bmcecosse]

It does indeed seem that the alternator is somehow backfeeding the ignition circuit......probably a failed diode in the alternator. Just to confirm -with alternator completely disconnected - does everything work correctly?

[MarkyB]

with alternator completely disconnected

This means the fan belt too.

[bmcecosse]

Not necessarily -spinning the alternator with no wires connected won't do it any harm.

[kennatt]

Nothing done to it,used regularly, with alternator disconnected, switching shows power on and off at the coil ,so switch looks ok,but could be faulty and drawing power via warning light feed from alternator,If it was a diode would think that it wouldn't be charging at 14 volts.I've got a spare alternator somewhere will try that first.

[bmcecosse]

Something has failed - you could keep using it by stalling it every time.....

[kennatt]

well sorted it.. After a bit of thought ,worked out that the only way the coil could be powered,was via the switch,so power had to be coming from ignition feed from the alternator to the ignition bulb,to the switch then to the coil. and that if there was NO 12v feed going the other way to the bulb via the switch ,then it wouldn't light,but more importantly there would be no voltage going the other way to balance the flow from the alternator,as it should to put the light out to indicate a charging system, After checking behind the speedo,with the meter ,found a dry/scabby joint,cleaned up and re connected now working fine again. Still can't work out how it was feeding the switch,because you would think that the dry joint would prevent the reverse flow from the alternator,but suppose,it would'nt take much for a good alternator to shove some current past it,enough to power the coil. But there you go another problem stuffed into the back of the brain you learn something every day.

[bmcecosse]

I'm not understanding that one either...but well done sorting it out!

[kennatt]

I think the wirings been got at at some time prior to me getting it,over 20 years ago,because the wiring through the ignition bulb is non standard just a length of black wiring. So god knows what they have done,but it works so leave it alone i think.

[bmcecosse]

As long as there isn't a reverse current drain when the engine is off. May be worth checking.

[kennatt]

thanks roy allready checked that,been having another look at the back of the speedo,still can't see how ,so have given up,and put everything back together.Very strange, oh and pulling the bulb returns the fault.

[bmcecosse]

And there is definitely no 'drain' through the bulb when off... ?

[kennatt]

No,checked at bulb with meter,and ameter on battery,no drain.Nothing on ign feed to alternator,Just working as it should,I also spoke to a reputable auto electrician,and he says he has come accross this before and that the way some older cars were wired through the ignition bulb could show this up .and that the corroded joint would probably be ok with a dynamo since even when new ,at tick over ,they hardly produce 12V so after 30 years would never produce enough to get past a bad joint.Would be interested to find out if anyone else could re produce this by pulling the igntion bulb.