Electronic Ignition - Problem

[RobM]

HI there. I'm afraid this is a bit 'off topic', as its not my 7 that's playing up, but my racing Metro..

 

We've just put the engine back in after its winter rebuild, during which time we converted the ignition from the standard points to electronic ignition using an Alden kit. The engine runs fine, but it refuses to run with the radiator fan on. The fan works well when the engine's off, but if you turn it on when the engine's running the engine instantly dies.

 

We have a theory that it might be some sort of magnetic induction from the fan motor interfering with the electronic ignition; the two are seperated by only about 5 inches. The engine bay's so crammed there's no other way of organising it really.

 

The next thing to try is obviously converting the dist. back to points and seeing if that works. Prior to that though any ideas would be gratefully recieved!

 

thanks

 

- How can such a cute looking car sound so ferocious! -

[Chris W]

I doubt if it's magnetic induction causing the problem.... the electronic ignition would be well shielded and anyway 5" is a good length (or so I keep telling my wife).

 

More likely the fan is creating a ton of electrical sh*t on the power supply lines thus causing the electronic unit to malfunction. If you have no access to a scope you could try fitting the capacitor from the old points unit across the fan terminals and seeing if that cures it.

 

Failing that, get a 470uF or 1000uF (microfarad) electrolytic capacitor from Maplins et al (pence) with a max working voltage of say 25v and fit it across the electronic ignition supply lines. Watch the polarity as these capacitors are polarity sensitive. One way they will work and the other way they will cause a nice sharp bang as the capacitor goes west (and east all at the same time). Fit it as close to the electronic unit as you can.

 

1.8K SV 140hp see it here

 

Edited by - Chris W on 1 Mar 2003 14:18:13

 

Edited by - Chris W on 1 Mar 2003 14:19:14

[RobM]

thanks for the reply, but interestingly, the problem has now gone away...

 

I had to fiddle with the alternator in conjunction with another problem (charge light not working), so maybe that has cured it and it was alternator based?

 

Your capacitor solution? Could you explain further as I'm interested - thanks.

[Chris W]

Well, if the fan is causing electrical noise on the supply lines it could happen that the voltage on the line to the electronic ignition unit falls below the minimum acceptable value for a fraction of a second, but long enough for the unit to drop out and therefore kill the ignition.

 

Putting a capacitor across the supply lines acts analogously to using a water tank to smooth out a water supply. So for example if you had a supply of water from a hose that was jerky and intermittent, you could smooth out the water supply by using the hose to fill a tank and take the water supply from the tank, which would be a smooth flow. Anything downstream of the tank wouldn't know about the hosepipe. The capacitor similarly stores an electrical charge and delivers it when the "hosepipe" gets intermittent.

 

1.8K SV 140hp see it here

[RobM]

Great, that's very clear, thanks very much.