It's never the coil

[stuffedpike20]

I know 'It's never the coil', but what are the symptoms of a faulty coil?

Will it either work, or not work? Will it work again when it has cooled down?

My 'modern' car (31 years old) died today and I suspect either the coil or ECU.

Thanks. John.

[Bill_qaz]

Just Google ignition coil check, loads of how to with a basic multi meter check primary and secondary coil resistance.

[geoberni]

How 'modern' is your modern with regards it's design?
From the way you're talking, I assume it has a single identifiable Coil?
Most if not all petrol Cars these days have a 'Coil per Plug' system.
There's some useful videos on youtube for coil defects.

The benefit of Coil on Plug is that you can pull the electrical plug from the Coil without risking a Zap, as it's only the 12v input.
This video for example uses simple substitution to check a Coil.
https://youtu.be/XMgGJlaYVvg

But if it's a Single Coil, it's no different to the Minor, the engine fires up or it doesn't; but it's possible to get a joint/break that opens up when warm and then 'resets' when it cools down again..
That's the problem I had with my Minor when I first got him and the RAC blamed the electronic Points. It wasn't, it was the Coil.
See here: viewtopic.php?p=636467#p636467

[stuffedpike20]

Yes it is a single coil geo.

Called the AA who were 'experiencing high demand' and would be with me in about an hour.

After about 20 mins the car started and drove the 2 miles home. Maybe more likely to be a faulty coil than ECU?

I think I will buy a new coil as an insurance policy.

[philipkearney]

Not sure what car you have but I would often run mk3 Astra's of that 30 year ish era. I picked up a very clean low mileage example a few years ago that kept breaking down and then re-starting when cool. Apparently neither the garage or the AA could fix it, blaming a non existent immobiliser. Having picked the car up very cheap, I immediately changed the coil unit and did about another 120k trouble free miles in it. Sometimes it is the coil !

[stuffedpike20]

Thanks all.

A new coil is about £25, so will not break the bank.

The only other thing I noticed was that the fuel pump relay was quite warm. Not sure if relays are supposed to be warm... I have never grabbed one before. The system was fueling OK anyway.

Thanks. John.

[oliver90owner]

I would suspect a Hall Effect device to be the most likely culprit - assuming, of course, that it is an ignition fault.

[stuffedpike20]

It does have a crank sensor that was changed a while ago.

I am hoping that philips experience with the coil might be what is going on. The coil looks like it may be the 31 year old original.

At least parts are fairly cheap compared to a very modern car.

Thanks. John.

[oliver90owner]

It does have a crank sensor that was changed a while ago.

Why was it replaced? Failed , maybe?

I am hoping that philips experience with the coil might be what is going on. The coil looks like it may be the 31 year old original.

If you are happy to live in hope, carry on. Good luck. I suspect the sensor is cheaper than a coil? Certainly cheaper than both a coil and a sensor!

[les]

Buy a new coil, or at least try another, that will eliminate any possible doubt. Diagnosing faults is all well and good but if the problem is intermittent, you might only get the ok on a good day. Sometimes it’s easier to swap out in these cases.
Edit: just realised we’re talking modern here, so maybe not so straightforward!

[stuffedpike20]

The car was fueling, and I suspect the problem was no spark, but did not check this.

The car started after cooling down a while, so I guess this points more to geoberni's thought that there might be a bad connection in the coil, rather then a fault with the crank sensor? Would the car just run badly with a faulty sensor, rather than not start at all?

The car got back up to working temperature for the 2 mile trip home with no problems.

There is nothing worse than an intermittent fault.

[stuffedpike20]

How 'modern' is your modern with regards it's design?
From the way you're talking, I assume it has a single identifiable Coil?
Most if not all petrol Cars these days have a 'Coil per Plug' system.
There's some useful videos on youtube for coil defects.

The benefit of Coil on Plug is that you can pull the electrical plug from the Coil without risking a Zap, as it's only the 12v input.
This video for example uses simple substitution to check a Coil.
https://youtu.be/XMgGJlaYVvg

But if it's a Single Coil, it's no different to the Minor, the engine fires up or it doesn't; but it's possible to get a joint/break that opens up when warm and then 'resets' when it cools down again..
That's the problem I had with my Minor when I first got him and the RAC blamed the electronic Points. It wasn't, it was the Coil.
See here: viewtopic.php?p=636467#p636467

I do not know what the ohms reading should be on my coil, but when I get the new coil I can take readings from both and compare them.

But if the old coil has a brake or dry joint that opens up when it gets hot, will the ohms reading be 'normal' when the coil is cold?

Thanks. John.

[geoberni]

It's going to depend on the car and also whether it's a 'Coil' or a Ballast Resistor Coil.
It might be anything from 1 Ohm to 3 or more.

[stuffedpike20]

Thanks, but will a coil with a break in the winding give a different reading than a good coil, if the bad coil when cold sees the break close up and 'mend' again?

[geoberni]

Thanks, but will a coil with a break in the winding give a different reading than a good coil, if the bad coil when cold sees the break close up and 'mend' again?

It will really depend on the severity of the break.
Back in 2018, Basil cut out on me on the A1 about 6 miles from home.
RAC called, guy said 'its got no spark, must be the electronic points'.
Towed Home.
When I got home, the car started perfectly and I just couldn't fault it, all electrical checks were fine.
I brought a replacement accuspark unit and fitted it.
Several successful local runs in the next few weeks.
Then came a long trip for the Anniversary event at Nuffield Place. An early start, and about 20 miles down the A1, it happened again.
This time when we got home, Basil was still dead as a Dodo, and I soon found the open circuit on the Coil Winding.

So it was a perfectly 'good' connection until hot and perhaps a bit of a road 'bump' at the wrong time.
If the wires are touching it's going to give a good connection, even if the solder is broken. So meter readings will look OK.
After that breakdown, I moved the Coil from the engine back to the bulkhead where they used to be, where there is less severe vibration.

[oliver90owner]

The last time we lost spark while travelling was down to a loose spade connector at the coil. Easy diagnosis and fix at the road-side. Most times, over the last 50 years it has been the ‘pig-tail’ lead which had given up.

But I still check before buying new spares. Diagnosis is generally cheaper than guessing. The usual coil problem, in the longer past, was that of leaving the ignition switched on and seriously over-heating the coil. I’ve not bought a new coil, ever - and have always had a couple of good ones available as spares. Not so easy these days - and modern ones seem to be less reliable than those that were ‘home-grown’.

My commuting (+ social), with spark ignition engines, was over more than twenty years up to 1987 (so about 400k miles) and much less since (compression ignition favoured). Only been ‘playing’ with old cars since about 2005, but still have plenty of spark ignition engines around (mostly magneto ignition, mind).

[stuffedpike20]

Thanks berni and oliver.

One thing that berni said made me think.... my car died as it was going over a speed bump.

It died some weeks ago when I had to drive into a rough layby to allow a lorry to pass....might be significant.

Thanks. John.

[geoberni]

Thanks berni and oliver.

One thing that berni said made me think.... my car died as it was going over a speed bump.

It died some weeks ago when I had to drive into a rough layby to allow a lorry to pass....might be significant.

Thanks. John.

With that snippet, I'd be looking for a loose connection, perhaps one of the terminations onto the Coil low tension (i.e. the 12 connections).

[stuffedpike20]

This morning I started the car, and then wiggled the 12v positive wire. The engine spluttered and almost died.

The connection did not seem very loose; but may have been loose enough to cause the original problem.

I squeezed the connector up a bit with pliers....and on we go.

Thanks everyone.
John.

[geoberni]

It's the simplest things that can cause the biggest electrical headaches.