Engine runs on with key out

[Iain]

Had my engine running for the first time on carbs with no alternator and it was fine

Just had it running now and gone to turn it off and it carried on, had to use the killswitch to kill it off

Since the last run it's had the alternator fitted and the auxiliary items properly fused. It's on R1 carbs and H&H dizzy. The coil and fuel pump are wired into a relay that takes +ve from the killswitch and is triggered by an existing ignition live off the Nova loom.

The dizzy gets 12v with the key on, 0v with the key off, start it up and it's at 14v, key out after running and it stays at 14v

What's happening? Alternator backfeeding something? Wired up the relay wrong?

[Iain]

Couple of dash lights on really low too

[therealnovaboy]

Yeah I had this. The alternator back feeds the ignition. I ended up putting a diode in to stop it. It's been fine ever since.

[mowgli]

time to rewire it as a carb engine.

[Iain]

I've done quite a substantial rewire on it... I may have used a different existing ignition live to trigger the relay so maybe I'll try swapping that.... I do recall having this issue when I first built it and finding a different ignition live source sorted it.

Not sure about a diode it feels like avoiding the problem. Would that be on the white/blue?

[therealnovaboy]

It's not really avoiding a problem it's engineering a solution. It was a while a go I did it and can't remember what I'd need to look at a diagram again. if you need to know I'll find have a look.

[therealnovaboy]

I think it was the wire to the battery light wire I put the diode.

[Iain]

Yeah the blue/white wire... I'm sure its the black wire I've used the more I think about it... There must be some ignition lives that aren't cut off by the ignition switch

[torquey]

I had this due to my clocks the pcb was broke causing one of the track to touch and would keep the engine running with no key.

[Walshysr]

I've recently had a similar problem in my van it was all down to a cheap light I had plugged into the cigarette lighter and was back feeding so my van would not shut off haha had my puzzles for about 10 minutes out side the shop:roll:

[Iain]

I wired the fuel pump and coil directly to the car ignition live and it behaves fine. Run it through my relay and fuse box and it misbehaves.... It sticks at 5v when the keys out

[therealnovaboy]

I have a theory but may be questionable. The battery light goes out when the alt is charging when there is 12v on both sides of the ignition and therefore no current flow energising the lamp. In reality you have for example 12v from the batt and 14v from the alternator. Because you are using the switched live from the ignition to only power the relay you are getting the difference in voltage eg 2v back feeding the ignition and pulling in the relay. 2v (your 5v) is enough to pull in the relays as there is no load on it.

Your options are. Fit a resistor or resistor wire from the switched live to the coil of the relay or a diode on the alternator to batt light wire. If you had more resistance on the switched live you would loose the 5v and not have enough power to energise the relay. The diode will stop the 5v being back fed to the switched live.

Im in sure that's right but I can't remember where I fitted the diode so my theory might be wrong.

[meritlover]

I think you've pretty much nailed it.
The charging circuit doesnt switch on or off against ground. The regulator raises the potential at the excitation coil as it charges so the potential across the lamp is equal, no current flows and it goes out.

To stop the back-feed, the diode needs to go in series with the excitation wire.

[Iain]

It worked fine like this with the injection set up and same alternator and relay configuration though ;(

[meritlover]

It worked fine like this with the injection set up and same alternator and relay configuration though ;(

Yes, but you also said it was fine until you fitted the alternator so it must not be a current path on the rest of the loom.

To test it, turn the key off so that its running on and pull temhe excitation wire off the alternator. If the engine stops, thats the problem.

The inj system probably had enough load that the voltage collapsed and the relays dropped out. Without seeing a diagram of your loom before and after, its up to you to work out the reason. But hopefully doing the above test will diagnose it.

[Iain]

Ok thanks. Sounds like a potential solution would be to run the coil and fuel pump off the biggest existing ignition live (via a small fusebox) that I'm currently using to trigger the relay and use another spare ignition live to trigger the relay for the rest of the gear (epas, gauges, etc etc)

That way it'll kill the coil and pump and thus the engine which will flick the relay for the rest of my gear.

[therealnovaboy]

You must have had more load on the switched live with the injection setup.

You could probably wire the battery light to a switched live your relay is not on so that the battery light is completely separate from the relay that remains energised.

[Iain]

Done what I said above and all is working well. Coil and fuel pump run with no relay off the ignition live. Rest is fused that gets killed as the engine actually dies now. Thanks all.