Ignition light stays on

[norfolkngood]

Hi I have had a problem a few times starting had the normal ignition click would try several times then would start and run fine. This has now got worse I came to stopping then turn key off and engine would continue to run. I have cleaned all terminals on starter motor replaced ignition cable but now when I go to start ignition light comes on but stays on even with key removed. Sorry its a long post but any ideas would be gratefully received.

[Kithmo]

Sounds like the diode pack has gone in the alternator allowing a back feed from the alternator to the ignition.

[Phil S]

agree..new alternator should cure the problem

[dasadrew]

if the engine continues to run and the ignition light stays on when the key has been turned and removed, doesn't it sound more like a wiring/contacts fault in the ignition switch?

Drew

[marshman]

As Drew says could be ignition switch. To rule out alternator why not simply disconnect it? If car starts and stops normally (ignore ignition light which will obviously be on - no alternator, no charge) then it was probably the alternator. If the fault still exists then look at the switch.

A bit more analysis/diagnosis is required before condemning the alternator.

Roger

[Kithmo]

if the engine continues to run and the ignition light stays on when the key has been turned and removed, doesn't it sound more like a wiring/contacts fault in the ignition switch?

Drew

Not necessarily, the permanent live feed to the alternator (thick red wire(s)) goes back through the short circuited diode pack and up the ign wire to the switch, feeding the ignition circuit. So the engine may stay on even without the key in. As Roger says, disconnect the alternator, the light will stay on when running and go off when turned off.
Keith.

[dasadrew]

Not necessarily, the permanent live feed to the alternator (thick red wire(s)) goes back through the short circuited diode pack and up the ign wire to the switch, feeding the ignition circuit. So the engine may stay on even without the key in. As Roger says, disconnect the alternator, the light will stay on when running and go off when turned off.
Keith.

You learn something every day! Thanks - had no idea of that sneak circuit!

Drew

[StagJonno]

Not necessarily, the permanent live feed to the alternator (thick red wire(s)) goes back through the short circuited diode pack and up the ign wire to the switch, feeding the ignition circuit. So the engine may stay on even without the key in. As Roger says, disconnect the alternator, the light will stay on when running and go off when turned off.
Keith.

Don't think it's advisable to run an disconnected alternator - would need to check, but any residual magnetism could create a voltage which wouldn't be restrained by the battery and might be enough to exceed the diode voltage rating, then the alternator definitely would be knackered!. But in any case, if it's like my Mk2, the alternator connections are in a single flying socket so both the power and IGN warning connections would be off. With all wires disconnected at the alternator, the IGN light circuit won't be complete so the warning light won't be on under either of the two scenarios.

With your system as is, suggest you switch on the ignition and look at the voltmeter on the dash (allowing time for it to stabilise (say 1 min) - should be 12 - 12.6V dependant on its state of charge. Start the car and hold the revs at 1,000 - 1,500 and look at the voltage again. If the volts are climbing up to 13 - 14.5, then this indicates your alternator is OK. If volts still ~12V, then alternator is suspect. If volts climb up, then it's pointing to a fault in the ignition switch block.

Replacement alternators seem to be available for ~£50, so if you do need to replace it might be worth considering getting an uprated one..

Jonno

PS - just checked the interweb on spinning a disconnected alternator. What I said above seems to be correct, though several folk seem to have got away with it - you feeling lucky?

[marshman]

Good point Jonno, I forgot about that. I ought tto know that as I had two alternators killed on my old escort van when there was an intermittant connection / break in the positive lead to the alternator.
Note to self think things through before giving advice!
I just didn't like the idea of condemming the alternator without further investgation.

Roger

[Kithmo]

As he has the ign light on with the ignition switched off, he can still disconnect the alternator, but not start the engine, to see if the light goes out. FWIW the alternator won't start charging with the ign wire disconnected until it gets to around 3000 rpm so it's ok to run it on tickover.

[StagJonno]

As he has the ign light on with the ignition switched off, he can still disconnect the alternator, but not start the engine, to see if the light goes out. FWIW the alternator won't start charging with the ign wire disconnected until it gets to around 3000 rpm so it's ok to run it on tickover.

That's interesting - very occassionally my Ign light remains on after starting, though extinguishes if I blip the accelerator to above 2,500 rpm - possibly related. Understand from my mech, who works on a number of classics, that what I experience is quite common, though I don't remember it from my '70's daily drivers.

Jonno

[wilf]

That's quite normal. Some alternators need a few revs before they become "Excited" and start to develop volts. Only when the voltage produced by the alternator exceeds the battery voltage will the ignition light go out.