IGNITION SWITCH

[bcmc33]

Over the past 2 years I?ve had four experiences of the engine continuing to run when the ignition key has been removed. It also continues to run with the battery isolator switched off.
My first thought was a faulty battery isolator ? which I changed recently for one bought at Donington. It has now done it again, so I guess it's the Lucas ignition switch. I?ll get a new one over the next couple of weeks.

I?m not really that concerned, but am interested in knowing what?s going on with the ignition switch to allow the engine to be powered from the alternator.

Any ideas?

[paddy]

I think you have to suspect the ignition switch first, but it could also be a fault in any component that is wired both to a permanently live supply and the switched supply - eg the wiper motor.

Paddy

[RotoFlexible]

My original ignition key became sufficiently worn that it could be removed with the switch in the "run" position. Using a replacement key (made long ago, but unworn) solved that problem. Perhaps this is what you are experiencing. (I assume you are not referring to a run-on or dieseling condition.)

[bill308]

My first thought is a dieseling condition too. Possibly too high an idle and one carb open too much?

What's your idle rpm?
Have you checked carb balance recently?

Bill

[bcmc33]

Paddy and I are on the same wavelength......I suspect one of the three permanent live connections is at fault, somehow. I need to trace exactly how they are connected...I suspect I have one on the ignition switch (that can only be described as a big no, no).
A failsafe option would be to fit a diode in each of the permanent live wires - but this is treating symptom and not the cause.
Dieseling is not an option - the throttle bodies are set perfectly.

[Tonyw]

Brian,

If your engine runs on with the battery isolated (your first comment) you have no power so it would not matter if the switch was faulty or not dieseling need not be anything wrong with the way the carbs are set up, if you have a glowing piece of carbon in your cylinder head or a glowing plug this can and will cause running on, especially if you have a vacuum leak or to fast an idle.

Tonyw

[neilsjuke]

Sounds like it is a non electrical problem but to prove it to yourself rig up a temporary warning light earth to the coil switch side , If the light goes off and it runs on then it's engine.
Neil

http://en.wikipedia.org/wiki/Dieseling

[elanmac]

I had a 1967 Mini Cooper which had similar symptoms but only at night. I found it to be faulty stop light bulb, side and stop light filiments were touching. When I stopped with my lights on and my foot on the brake pedal I could remove the ignition key and the engine ran as normal. If I released the brake pedal the engine would stop, likewise if I switched off the lights. The local BMC dealer who supplied the car had no idea what it could be. The same week I bought a Practical Mechanics magazine and found the answer in one of the letters. It looks like Lucas supplied a faulty batch of bulbs to BMC.
Mac

[bcmc33]

Thanks for the thoughts, but as I said at the beginning, the power is coming from the alternator (where else can the power to the fuel pump and ECU come from?)

I will make the effort this weekend to trace exactly where the permanent lives are getting their power - I just hope I've not been that stupid to make them from the ignition switch.

[john.p.clegg]

Brian
If your alternator is putting out power,it must be receiving a voltage on the "input" wire,either from your cars wiring or from itself...

John

[paddy]

If your alternator is putting out power,it must be receiving a voltage on the "input" wire,either from your cars wiring or from itself...

I think that's what you'd expect if there is a wiring fault that causes the ignition circuit to be live after the switch is turned off - if the engine is already running, then the alternator can self-excite above a certain RPM.

Paddy

[bcmc33]

Brian
If your alternator is putting out power,it must be receiving a voltage on the "input" wire,either from your cars wiring or from itself...

Well John, that's the conundrum, and in my mind that's exactly what it's doing - I just don't know how.
I was hoping for some inspired advice from someone that's had the same problem.

Paddy,

I keep the tick-over at 1000rpm where th engine is most happy.

[john.p.clegg]

Brian

When you switch it off but it keeps running what does your IGN light say?

John

[bcmc33]

Brian

When you switch it off but it keeps running what does your IGN light say?

John

A very good question, John. I told myself last time it happened that I should have looked before stalling the engine, but I was in such company that taking the p*ss became the order of the day, and defensive banter took priority. I will do my best to remember to look next time.

[billwill]

Thanks for the thoughts, but as I said at the beginning, the power is coming from the alternator (where else can the power to the fuel pump and ECU come from?)

I will make the effort this weekend to trace exactly where the permanent lives are getting their power - I just hope I've not been that stupid to make them from the ignition switch.

I think you need to positively confirm that the power is coming from the alternator by disconnecting it & seeing if you can get the same symptom, but you won't be able to if it is a sporadic fault of course.

Since the alternator feeds straight back to the battery circuit, your symptoms would be produced by a faulty ignition switch which doesn't actually disconnect the wire to the ignition coil.

The ignition switch can be carefully opened by bending out the tabs and you can clean all the contacts; take care that you don't get springs flying out when you open it. One tip is to do that in a large transparent plastic bag.