ignition coil resistance

[attwood]

Hello everybody.

I'm currently on holiday in the US, admiring the foliage in New England, the current exchange rate of the dollar and the price of almost everything connected with cars.

I've bought a few of those tools you always want but can never justify the price of (compression tester, timing light etc.) and the most expensive set of HT leads I could find which was $40 (!).

This was because I recently bought a 123 electronic distributor which necessitates changing from screw-in to push-in leads.

I could also buy an uprated (chrome plated) coil for less than $40, but this raised the question of what coil to use for a car with a ballast resistor, such as my Series 2 4.2.

I think the instructions with the 123 distributor said that the coil should have a primary resistance of at least 1.3 ohms, presumably to limit the current flowing through the distributor. The coil in question has 1.4 ohms so should be OK, but it did not say it was suitable for cars with a ballast resistor so I checked with the manufacturer's technical helpline.

He told me that this coil has an extra internal resistor to bring the primary resistance up to 1.4 ohms and that my ballast resistor would no longer be necessary and could be removed. However, he added that his personal opinion was that extra internal resistance generates extra heat within the coil and this was generally not desirable, so it is better to use a lower resistance coil with an external ballast resistor.

Does anyone know the resistance value of the ballast resistor?

Is the 123 distributor likely to be OK with a coil primary resistance greater than 1.4 ohms made up of the ballast plus the coil during normal running, but with a lower primary resistance during starting?

Or should the coil always be at least 1.3 ohms plus whatever the ballast resistor is?

Any opinions will be greatly appreciated.

Dave.

[Heuer]

Dave

A 'standard' S1 Lucas coil has a resistance of 3 ohms. You can use a coil with a lower resistance (typically 1.5 ohms) but you then need an external 1.5 ohm ballast resistor, in series to bring it back up to 3 ohms. So you will need to use a ballast resistor with your new coil to avoid any problems such as misfires or overheating electrics. Your car should have a ballast system as standard with a relay that switches the ballast resistor out during cranking and back in during running so best to stick with that system. Use the 123 but make sure the ballast remains part of the setup.

[attwood]

David, thanks for the info. Maybe I still have time to pick up the coil before I leave the US!

Dave.

[andyp]

If you bump into a small Irish girl called Mary carrying GM parts that will be my sister. She left for New England yesterday - I gave her a shopping list.

Enjoy the fall

Andy