Voltage across coil

ocarros

Member
I want to check the voltage going in to the coil. If I turn ignition on, and put multimeter on positive end of the coil and ground the other lead, should multimeter read a constant 12v? Or am I checking it all incorrectly?
 
If you have a ballasted coil, you should see ~9V, 12v while cranking. Unballasted, 12V. Remove the connections, measure the resistance in the coil - ~3 ohms should be unballasted (for 12v, eg Lucas DLB101); ~1.5 ohms needs a ballast, usuaully hidden in the loom somewhere.
 
Thanks JP. I wasn't cranking nor did I check the ohms. The multimeter was flickering around 10/11v for a few seconds but then would drop down to 1v and flicker around 2/3v for about 4 / 5 seconds before jumping back up to 10/11v. Would this show coil is on the way out or should I also do a resistance test? New battery was fitted to the car about 6 months ago and this shows a constant 12v
 
Thanks JP. I wasn't cranking nor did I check the ohms. The multimeter was flickering around 10/11v for a few seconds but then would drop down to 1v and flicker around 2/3v for about 4 / 5 seconds before jumping back up to 10/11v. Would this show coil is on the way out or should I also do a resistance test? New battery was fitted to the car about 6 months ago and this shows a constant 12v

Depends if the points are opened or closed
 
I dont follow the first part of that note - why would you see 12V if there was a ballast resistor in line? You should only see 12V when cranking if it has a ballast resistor.
Anyway, disconnect both sides of the coil to test its resistance - my DLB101 is 3 Ohm, and I see 12V at the +ve connection. Also disconnect +ve side and test it against earth to see if you have 12V or ~9V. Bad condition points, or capacitor could give odd voltage readings at the +ve connection if everything is connected.
 
The current taken by the measuring instrument is too low to create a voltage drop so with points open and the ignition on there is virtually no voltage drop so you will see 12 V at the coil input ( and indeed the output) , but with the points closed, and thus the circuit complete, one should see the 9V as the coil will pull enough current to create the voltage drop across the ballast
I have just checked my P6 2200 with the engine running and I am getting just under 10V at the coil , so the ballast is working. NB the alternator is creating about 14V at the battery.
 
Thanks all. I think my coil is kaput which is why I was checking the voltage across it. Wasn't sure if I was doing it correctly (I was doing it per point 1 in Per Anders post). The voltage all over the place prob does mean I need a new coil
 
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