Voltage across coil terminals

[SimonBrown]

I’ve just fitted my Series 2 with a correct E Type distributor, but found the voltage between the positive and negative terminals on the coil is 10.9volts. That is ignition on, engine not running and points closed.
There is no ballast resistor fitted.
Battery resting voltage is 12.7 volts.
Is this satisfactory or am I losing too much through the circuit/ignition switch?
Thank you
Simon

[johnetype]

It may be fine but I'd also measure from the coil negative to ground and from coil positive to battery positive to understand where the nearly 2 volts is going.

Once you know where and how much all the voltage drops are in the circuit can you make a better judgement on what's alright and what isn't.

[bitsobrits]

I believe the "lost"voltage is powering the coil (which is basically a transformer). Disconnect the negative wire from the coil (keeping points closed) and measure the voltage between the coil positive and the negative wire. Should be nominally the same as battery voltage. If that is not the case, you may have a parasitic load somewhere in the circuit as you conjectured.

[40GT]

You need to measure positive to earth, not positive to trigger.

[Tommd]

Go with post #2 above, being sure the points are closed when you make the measurements.
Tom

[Stevemorris]

What Voltage do you have at the + wire going into the coil with it disconnected? It should be battery Voltage. If it is then the coil is consuming which is normal. Does the car start and run OK?

[SimonBrown]

Hi Steve
12.28v at the +ve disconnected and 12.53v at the battery directly.
The car starts and runs fine.
So it appears my concerns are unfounded ?
Thanks
Simon

[johnetype]

No, Steve's test tells you nothing except that you've got continuity between the coil +ve terminal and the battery +ve terminal - and you knew that already because the car starts and runs.

There's a long length of wire with several connectors in it between the coli +ve terminal and the battery +ve which passes through the rev counter, one side of fuse 6 & 7, through the ignition switch and on to the battery.

If any or some of those connectors have become a higher resistance and you're loosing 2 volts across them, that needs to be corrected.

[SimonBrown]

Hi John,
Please correct me if I’m wrong, but if I have 12.28v at the coil +ve and 12.53v at the battery, I am only losing 0.25v between the battery and the coil?
The major loss is across the coil, which may be normal?
Simon

[johnetype]

Once you disconnect the wire to the coil +ve all voltage measurements are meaningless because you've broken the circuit and no current can flow. It's not the normal operating condition. It's why Tom added his caveat in post #5 because you need the points closed for current to flow.

[mgcjag]

Hi Simon.....yes your only loosing 0.25v between the battery and the coil....dosnt sound like you have issues to me....Steve ps ..typically voltage across individual components in a series circuit will be less than the supply voltage

[johnetype]

Steve, this is just not correct.

If you measure a voltage without any current flowing you will get zero voltage drop and hence it's pointless.

If you do what the other Steve has suggested and disconnect the wire at the coil +ve and measure the voltage what you're actually doing is measuring the voltage at fuse 6 and 7 terminals which is the last point at which any current is flowing through the circuit.

In the measurements Simon did he had 12.53 at the battery and 12.28 at fuses 6 and 7. That means he's lost 0.25 volt between the battery and through the wiring to the ignition key and then fuses 6 and 7. BUT there's no current flowing through the coil under this test so it's not a real world measurement of the true voltage drop PLUS any voltage drop in the wiring through the rev counter hasn't been measured either.

It really isn't hard to do the real world measurements Tom and I suggested. With those measurements you now have the long series circuit that is the ignition feed through the rev counter and on to the coil broken into three parts. First part is from the battery to the coil, second is across the coil and the third is from the coil across the points down to ground/battery -ve.

The voltage drops across those three elements have to add up to the battery voltage. Ideally, first is zero, second is battery voltage and third is zero as well but in reality if second (across the coil) has already measured 11ish volts , first will be ~1.5 volts and third will be ~ 0.5 volts BUT that's speculating; do the measurements correctly and then it's possible to discuss if there's a problem or not.