Coil voltage

[FloydP]

Does anyone no what voltage I should see across the low tension side of a coil, it's the type with a ballast in the wire feeding it (stag if it makes any difference) I was expecting 12v on start up, dropping to 6 when running, but my voltmeter is all over the place, showing 1.5v even when only the positive probe is on the terminal and when the negative probe is on the other side it still fluctuates but around the 3.5-4v. I'm thinking my meter is on the way out, so I suppose really my question is should the voltage be 12v across the coil when starting and 6 when running.
Cheers,
Floyd.

[piman]

Hello Floyd,

no, the voltage across the coil fluctuates when running as the points or electronics are opening and closing. Do your voltage check between the + ve coil terminal and earth.
If you do a check with the engine not runningr, you'll see 12 v with the points open and 6 - 8 volts with the points closed. (No idea what it will show with electronic ignition module?)

Alec

[Pete Lewis]

with a ballast feed yes 6-8volts between the feed and earth, but  when youre  cranking  it gets 12v from the starter solenoid to boost coil voltage to aid cold battery starts

8v coil has 1,5 ohm primery
12v coil has 3 ohm primery

if you ever fit a electronic unit it will be better to feed that with 12v  to avoid any miss fires
if you ever bypass the  ballast wire   make sure you fit a 3 ohm coil or the HT will blow rotors and the high current will burn points
some elec units will fail if you use a 6vcoil on 12v

some coils are designed for ballast use but are marked up as 12v just to make some more confusion.

if that makes sense   Pete