# How much current should my car draw with the ignition on?

With the ignition fully off, my current draw is pretty much zero, but with the ignition on (to be clear - engine not spinning, starter motor not engaged) it increases to 2.5 amps (at about 11.5v, since I've run the battery a bit flat testing this). I've turned off everything I could find that might be drawing power - now I'm wondering if the ignition coil would perhaps draw that when on? It's a 1982 Ford Capri MK3, so it hasn't exactly got a lot of electrical gizmos.

I can't seem to find any information on what I should expect as the base draw in this scenario, and obviously I'm wondering if I've got a partial short circuit somewhere.

• Does the ignition system have points? If it does, the amount of current flowing will depend on whether the points were left in the open or closed position when you last turned the engine off. If the points are closed, 2.5A doesn't sound excessive. Commented Feb 20, 2016 at 22:06
• @HandyHowie - would that be the same thing as the distributor?
– tjbp
Commented Feb 21, 2016 at 2:34
• The points are the little pair of contacts that are inside the distributor. You set the gap between the contacts with a feeler gauge. Commented Feb 21, 2016 at 8:28
• If you take the distributor cap off and do your current test again, it the points are closed you can gently open them and see if the current drops. Commented Feb 21, 2016 at 8:30
• @HandyHowie tried just unplugging the main lead to the distributor, unfortunately didn't have an impact on the draw at all.
– tjbp
Commented Feb 21, 2016 at 13:43

## 2 Answers

I presume you car has the original distributor with points. If during your testing of current drain with the ignition on, the points happened to be closed, then the ignition coil will be getting powered from the battery via the ballast resistor.

A rough calculation R=V/I would show the resistance of the circuit = 12/2.5 = 4.8 ohms

This sounds a reasonable resistance for the ballast resistor and HT coil, so I would say that the current draw is roughly correct. You need to try doing your test with the points open to check that the current then drops, to nearly zero.

• Thanks, it was this after all, I disconnected the power source from the ignition coil and it dropped to half an amp.
– tjbp
Commented Feb 24, 2016 at 19:28

2.3A seems a little high? However though, you may have a really good battery to compensate for that. My Dodge Magnum with KEY ON draws around 1.9A. It doesn't sound too excessive. If you have a spare battery, grab it, hook it up, and see how long it takes to kill the battery from start to finish. Most cars at KEY OFF draw around 15 - 50mA. I think your car is fairly ok considering that when you actually activate the KEY ON position, you're most likely activating the fuel pump, the A/C, The engine computer, the radio and dash, lights, and any solenoid mechanisms that draw power on at startup (injectors, sensors, etc etc etc...).

My Dodge uses a HUGE battery, it looks like something that would start an F-250. It's somewhere around 1000CCA, so if your car draws a lot of power, it may just have a beefy battery.

• You'd be absolutely right, but this is a 1982 Ford Capri - the fuel pump is mechanical, there is no A/C, no engine computer and the radio is off. I can't think of anything besides the ignition coil and the dash lights that might be drawing power, and I'm not sure they'd draw 30W...
– tjbp
Commented Feb 20, 2016 at 20:00
• Given the year, and circumstances; Thats extremely excessive. I would check the ignition system. You'd need to pull apart the column to test your ignition and if it has a secondary coil/ relay you'd need to test it. If something small is shorting out, it can also cause this symptom. Older vehicles can be both easy and difficult to troubleshoot. Mainly due to the lack of computers and the miles of wires they use lol Commented Feb 20, 2016 at 20:35
• Some older cars hive bimetallic guages .I dont know if joseph lucas prince of darkness did these.Anyway those guages could explain things . Commented Feb 21, 2016 at 11:04