Irritating Points Ignition Failiure - Revisited

That definitely proves that fitting a 'known good used spare' isn't always what it seems.

Did you examine the old cap for signs of tracking? Measure the carbon brush? Check it moved freely?

In article , sweller scribeth thus

So have you replaced the old one to see if it still starts or not, and if not are there any visual differences that might show up when compared with the old one?.

And as to that have you done an ohms test from the king lead to the rotor carbon brush in the cap and compared that with the new one?..

I'd ask that as this doesn't seem to be as logical as it might be, don't want you to have to be towed home again;)...

And just how "wrong" was the replacement you had, in what way was it different?..

Erosion of the internal contact surfaces is also a common failure mode that is sometimes overlooked also.

Chris

I'd expect that to give problems like missing etc before it failed totally?

Yep, possibly.

TBH, the failure the OP has experienced is a bit unusual. Dodgy HT usually shows as poor starting from cold, not a sudden stop when running.

Chris

It was very reluctant to start whilst warm just now (again). I've also fitted new plugs - given all the other components had been changed and the plugs are not that far off their service interval it seemed sensible.

I suspect the problem has not gone away and lies elsewhere.

I didn't want to be the harbinger of doom, but I've suspected that from the beginning. The symptoms didn't point to HT.

Chris

Could be - a king lead finally giving up the ghost or the carbon brush, spring or connection from it to the terminal corroded. But you should be able to prove that by measuring the resistance.

I had hardish starting which eventually became no starting. Severe corrosion between coil and king lead - not really obvious until you looked closely.

Yebbut non-starting was not the OP's original fault. He was driving on the motorway when it stuttered a bit, then stopped.

That's actually more like running out of fuel... :-)

Chris

Or ignition coil shorting internally.

John

Or even externally.

The reluctance to start was difficult to work out as it now had all new consumable ignition components (plugs, points, condenser, coil, rotor and cap), the dwell and timing were spot on. The battery is very new, over-size, fully-charged and spins the engine at a good speed (fastest it ever has!) so what could be the problem?

I had noticed that when it started it tended to as I took my finger off the starter (Aha!). I knew I had a bit of a voltage drop between the battery and the coil. I also knew that I'd had problems with the ignition (key) switch getting hot in the past.

There was evidence it had been getting hot since I last cleaned it up and there was a loose rivet on the live side.

It looked as if there was too much resistance through the key switch and more when the start button was pressed.

I simply fitted a £2 relay worked by the key switch to do the heavy current duties and take the load off the nearly 50 year old key switch and it seems to start more happily with much, much less cranking (two seconds worth, before was a good five plus).

So far it has been fine......

.....but we shall see.

That seems quite plausible the volts supplied to the coil. I take it that this car wasn't of the sort that used to use a dropper resistor or wire for normal running, the coil being a lower voltage one so that when under starting the full volts was supplied to the coil to keep the spark volts up as the starter motah was pulling lots of amps and hence the volts available to the coil were lower then under normal running conditions. And when under normal running the resistor came into circuit to provide the right volts to the coil.

And of course under starting conditions thats when you need the sparks at their best;)

Have you just done a volts check on the Positive side of the coil under normal engine running or when the points are closed and see what the voltage drop is measured with reference to vehicle bodywork to the coil and what the battery's at?..

I once had a Cortina MK 4 it was that was s**te at starting and in the end I discovered with some voltage checking that the battery was going down nowhere near as it ought be with the sluggish cranking. New battery even changed the damm starter motor but it was the lump of earth braid that connected the engine block to the chassis that had come off or broken can't quite remember now so the amps for the starter motor was going back thru the throttle cable, exhaust pipe and other not very low resistance paths;!...

FWIW the 18 year old Audisaurus was not that good at starting a while ago and was doing some other odd things. Changed out the engine management controller and quite a bit else before I noticed that the feed volts to the device were quite low, a dry joint in a relay in a module had gone high resistance as was causing the lack of volts resoldered and all back to life again:)..

It seems a bit too convoluted and doesn't explain why it just cut out with no spark on a motorway.

It's also occurred to me that most of the cranking when fault finding was done using the solenoid button under the bonnet rather than the starter button on the dashboard so if the dashboard starter button can't really have been adding to the problem.

I suppose the ignition components were marginal and the cumulative effect of that plus the voltage drop across the tired key switch was too much for it.

No, straight 12v coil - no ballast resistor.

No. Perhaps I should.

I've never quite understood voltage drop testing - neither have I found a simple explanation of it. Any places to look?

I've had that in the past - but on the Jag slow cranking wasn't the problem at all.

When I raced bangers some years ago I just had two heavy battery cables, one +ve to the starter solenoid, the -ve straight to earth the engine. To simply avoid defective earth straps on unknown scrappers. A basic switch box supplied the coil and starter solenoid as necessary.

I'm still not 100% I've found the fault...

Yes agreed. I suppose you'll just have to try again and risk it happening ;!...

No but the starter solenoid would have been on n extra pair of contacts..

Might be..

OK..

Well if you take a simple car battery and then connect a 12 volt bulb there will be current flowing in the cables connecting that bulb to the battery. Because they have resistance, albeit very little, there will be some voltage and power wasted in that connection because of he resistance of the wire.

If that wire is thin and long then there will be more voltage drop or loss in the cable then if it were shorter and thicker due to the resistance altering.

Try it sometime, switch the headlights on and measure the volts on the battery and the volts on the headlamp it will be slightly less..

Thats what's meant by that but in your instance the resistance of the wires, contacts and the duff rivet, are adding extra resistance in the circuit hence the power is being wasted as such because of that.

That do;?...

Indeed..

Never raced bangers.. drove them on the public road sometimes years 'n years ago..

Yes Joe Lucas!, "The prince of darkness" ;(..

Yes I'd agree with that. Seems not quite a definite diagnoses.

Perhaps you inadvertently corrected the original fault in all the works done but I suppose you'll have to try driving it to see.

Sods are intermittent faults!...

Connect your DVM set to 2 volts or so between coil ignition feed and battery live (not sure if your car is positive or negative earth) Operate the starter and note the reading - a meter with a min/max facility is ideal. That will give you the actual voltage drop in the circuit. Anything more than about 0.5v needs investigating.