Ignition coil & ballast resistor

MdR · Post by **MdR** » Tue Nov 16, 2004 3:03 pm

I've been renewing all the lo-tech ignition service parts on my S. I've replaced the dizzy cap, rotor arm, points, condensor, spark plugs & HT leads. The only item still to be replaced is the coil. I bought all the parts via Autofarm (there is a good reason, honest

) but the coil that arrived has blurb on it saying only to be used with a 0.9ohm ballast resisitor.
What does this do, where does it fit and where do I get one from?
There's nothing between my current (pun

) blue coil & the points/kinglead.
I haven't asked Autofarm yet as I'm looking for an explaination to suit a simpleton
Thanking-you-please.

Nick Moss · Post by **Nick Moss** » Tue Nov 16, 2004 4:26 pm

Hi Martin.

All the pre CDI cars are fitted with a ballast resistor and relay next to the coil on the left side of the motor. Have a look at this wiring diagram

The resistor is number 8

MdR · Post by **MdR** » Tue Nov 16, 2004 4:58 pm

Thanks Nick
I have that wiring diagram (from your site) laminated & stuck to my garage wall

Is it the wire wound around a ceramic block inside a small metal box (that looks like a 2 fuse fuse box?) above the coil in the engine bay?
Cue crappy camera phone & maglight pictures

If so that's been bypassed in my engine bay. The wires in & out have been 'commoned' together using one of the terminals on the small metal box.
Hummmm. Dodgy botched up US repair?!

Nick Moss · Post by **Nick Moss** » Tue Nov 16, 2004 5:03 pm

That's it!

impmad2000 · Post by **impmad2000** » Wed Nov 17, 2004 8:45 am

The theory is this.....
Fit a 6V coil, and a ballast resistor so that it runs off 12V (6V across the resistor + 6V across the coil). Fit a pair of contacts across the ballast resistor that short it out when the starter motor runs. This is so that when the starter motor pulls the voltage down during a cold start, the coil gets max voltage and gives the coils the best chance of starting the engine.

If your ballast resistor was shorted out, perhaps a 12V coil has been fitted. Or it was a temporary bodge as the ballast resistor had gone open circuit. Test this with a meter..... It probably would have worked (For a while) just the coil would get too hot. Check also that the contacts of the relay (51 on the diagram) do still operate.

CDI ignitions don't need a ballast resistor as the spark remains constant even for hard start conditions ( the CDI box just has to work slightly harder!!)

Hope this helps somebody

Nick Moss · Post by **Nick Moss** » Wed Nov 17, 2004 9:10 am

Thanks Tim, very usefull. So if a 12V coil is fitted then the resistor needs to be changed as well?

impmad2000 · Post by **impmad2000** » Wed Nov 17, 2004 9:31 am

nick-moss wrote:So if a 12V coil is fitted then the resistor needs to be changed as well?

The resistor can be removed if a 12V coil is fitted. However, cold starting may become harder.

MdR · Post by **MdR** » Wed Nov 17, 2004 1:37 pm

Thanks!
Now I understand it I'll investigate further with a multimeter.