Proper voltage from battery to coil?

[CRiordan01]

How much voltage should be running from the battery to the coil? When I have the key in the ignition in the ‘on’ position, I get a voltage reading on the coil of roughly 12 volts (11.8) – is this the proper voltage reading, or is there supposed to be some ballast restor in the system lowering the voltage into the coil? I have a 1974 S3 SWB RHD.

Thanks!

[siiirhd88]

The factory Lucas type ignition coil has internal resistance and does not use an external ballast resistor, so full system voltage to the coil is expected. No telling what is really on your truck, as a previous owner could have installed anything....

Bob

[CRiordan01]

Bob -

Thanks for the feedback. I have an early Lucas distributor and an after market coil installed by a previous owner, so I don't know if that changes the answer any. When I test I get 1.6 ohms on both the positive and negative posts on the coil; based on your comment, it sounds like that would not be a surprising reading?

My truck died on me a week or so back and it looks like some kind of issue with the distributor and possibly the coil. I am in the process of replacing the point set, condensor and low lead tension, but I was also concerned that the coil is getting too much juice through it.

I have only had the vehicle a few months, and I am only just learning how to play with an engine, so hopefully my question makes sense.

[canyonero]

FWIW, I have a similar truck with similar sounding issues. Mine was resolved by cleaning up all the contact points and installing new battery cables and grounding straight to the engine block. Don't know if any of this helps...

[SafeAirOne]

When I have the key in the ignition in the ‘on’ position, I get a voltage reading on the coil of roughly 12 volts (11.8) – is this the proper voltage reading,

Hmm. Presuming that you're taking a reading from the "+" terminal on the coil, I'd think you should be getting a reading of the battery voltage in the above situation (I don't think that there's any resistance in the "upstream" circuit to give you lower readings).

Note that 11.7v to 11.9v is considered to be the voltage of a discharged (dead) battery. This may have nothing to do with your problem, but you've got to at least be aware of it when considerig whether a reading is "good" or not.

[I Leak Oil]

You should be getting battery voltage to the coil with the motor off. Alt. output voltage with the motor running. The resistance internally or externally, as I understand it, is to reduce the voltage to the points\condeser so they don't arc out or burn out (as often anyway). The whole circuit is just like a big relay with a capacitor. The control side is the points side (where you want lower voltage), the load side is the ignition coil.

[mearstrae]

There needs to be more than 12v to crank over and start. There after an alternator will provide 13.8 to 14.2v (a good battery will be close to these voltages when not running). The power to positive side of the coil will show these values. Bad cables and other wiring (or dirty connections) act like a resistors and drop the voltage from the battery to the component being checked.

'95 R.R. Classic LWB
'76 Series III Hybrid 109
'70 Rover 3500S

[albersj51]

[QUOTE=CRiordan01;80286]Bob -

Thanks for the feedback. I have an early Lucas distributor and an after market coil installed by a previous owner, so I don't know if that changes the answer any. When I test I get 1.6 ohms on both the positive and negative posts on the coil; based on your comment, it sounds like that would not be a surprising reading?

Verify 1.6 Ohms of resistance is what you need. Mine is a Lucas 25D dizzy with a Pertronix module with a Flamethrower coil and the coil I have is a 3.0 ohm coil. Not sure if this means anything though.