Ignition spark after negative earth change

[Concrete-crusher]

Hi I have fitted a dynalite alternator and in doing so changed from positive to negative earth. I have also rewired the tachograph and reversed the ignition coil.

Now it won't start. I have checked the coil electron flow with a graphite pencil between the ht lead and plug and it looks okay, but I cannot see the plug sparking. If I measure the voltage at the coil its slowly dropping from 12.8 down to 11.5

It was running ok before the above changes, any advice welcome

Steve

[billwill]

Did it have ANY electronic ignition parts, including simple contact breaker replacement.

or Perhap it needs a new capacitor in the distributor.

[Concrete-crusher]

No electronic parts , I had thought either the coil or condenser , it's just odd that a week ago before switching to the dynalite alternator it started and ran ok , just was,nt charging the battery.

Can a swap from positive earth to negative earth have any detrimental effect on the coil or capacitor. Btw the coil may well be original although I changed the capacitor about 12 years ago. But she's only done about 50 miles in that time.

Steve

[Quart Meg Miles]

Hi I have fitted a dynalite alternator and in doing so changed from positive to negative earth. I have also rewired the tachograph and reversed the ignition coil.

It was running ok before the above changes, any advice welcome

Steve

Please explain how you "reversed the ignition coil." If you left it alone it should have worked as it is not polarity sensitive (and neither is the condenser).

[paddy]

If you left it alone it should have worked as it is not polarity sensitive.

Not sure I agree with that:

http://mgaguru.com/mgtech/ignition/ig104.htm

Paddy

[billwill]

Hi I have fitted a dynalite alternator and in doing so changed from positive to negative earth. I have also rewired the tachograph and reversed the ignition coil.

It was running ok before the above changes, any advice welcome

Steve

Please explain how you "reversed the ignition coil." If you left it alone it should have worked as it is not polarity sensitive (and neither is the condenser).

Um, I have read that they are polarity sensitive, not in the usual way but because spark plugs work better when the centre is one polarity (I forget which) relative to the earthed casing.

I don't understand it either... but I think the primary coil is marked with a Plus and Minus sign.

[billwill]

If you left it alone it should have worked as it is not polarity sensitive.

Not sure I agree with that:

http://mgaguru.com/mgtech/ignition/ig104.htm

Paddy

That's the one.

[billwill]

The type of capacitor used in a distributor should not be polarity sensitive, but I wonder if when used for a long time chemicals in the capacitor tend to form in part an electrolytic capacitance, which is polarity sensitive.

It's a long shot, but trying a new capacitor won't cost too much.

All things and symptoms considered though, the most likely thing is that a wire somewhere is connected to the wrong place.

[Quart Meg Miles]

Seems I is getting out of date. Thanks for the education, Paddy, I'd never heard an explanation as clear as that one.

Also, I had forgotten that modern coils don't use the case as a terminal any longer. I guess that strap to chassis is more for radio suppression than anything else.

So apologies, Steve, for muddying the water. Lots of information in Paddy's link.

[Chancer]

Do you possibly have a radio suppression capacitor fitted to the ignition coil?

Decades ago I fitted one incorrectly to the CB coil terminal, the vehicle ran alright for a while and then there was tremendous arcing across the points and I only just managed to struggle home at around 1200 RPM, I think I then had to replace the condensor, of course it could just have been coincidence.

My thinking was if you have swopped the Ign and CB leads around on the coil but not moved the capacitor...
.....................................................

[bob_rich]

Hi Steve

My suggestion would be to leave the coil as it was SW or +ve on coil to the ignition (via the tacho loop if you have that type of tacho--direction of loop through tacho may need to change but that should only effect the tacho not the ignition) and CB or -ve on coil to contact points. Swapping from +ve to -ve earth will change the polarity of the spark but if all is in good condition it should make little or no difference--provided plugs are in a good condition.

High voltage stress up to 300V on the capacitor ( condenser) and 20kV on the output of the coil can under odd circumstances result in breakdown if the polarity is reversed but this is a very long shot and would need the parts to be on there last legs anyway.

It is quite common for certain accessories to be tagged onto the coil SW terminal so if these were moved to the CB this could kill the spark

As others say check the wiring carefully. If still a problem run a lead from the battery to the coil SW +ve and a another lead from the coil CB or -ve to the points. Then the wiring loom should be removed from the possible source of the problem.

If it still does not produce a spark then coil and capacitor swap should find the problem,

best of luck

Bob

[john.p.clegg]

Is your coil 6V or 12V (should be stamped on the bottom)

John

The bottom of the coil that is....not yours!!!

[billwill]

It couldn't be a 6 incher... oops I mean 6 volt coil, but it might be a 9 volt one if the car is fitted with a ballast resistor or ballast resistive feed wire.


A ballasted coil runs at about 9 volts under normal conditions, with the extra 3 volts dropped across the ballast resistor.

Why you might ask... Well its because when you are starting the car the starter motor take so much current that the available voltage from the battery & wiring drops to about 9 volts, so during starting the starter switch takes the ballast resistor out of circuit so that the coil is still getting a full 9 volts to ensure healthy sparks at start time.

~~~~

If you have a ballasted ignition coil circuit and your wiring swapover mixed it up, that could be the cause of your problem.

[paddy]

Do you have an electric fuel pump?

[Concrete-crusher]

Thanks for some useful suggestions. I will check my coil voltage but I'm pretty sure it's a 12v non ballast system. I do have an electric twin su fuel pump that sounds okay as I can hear the usual click click sound as it pumps.

There are no suppressors fitted, there does seem to be a shunt ammeter and a battery condition gauge which are extras. I have swoped the wiring to the ammeter and the battery meter seems okay, also the fuel meter seems okay. I have reversed the feed and earth on the tachometer and reversed the white pick up wire direction.

There is voltage on the switch side wire so the white wire reversal should be okay.

Maybe the condenser is on its way out as suggested I will replace it and see.

Steve