Fuse Box- What's Hot What's Not

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Well-known member
Jan 9, 2016
Trying to hook up a voltmeter to display only when the ignition is on. I bought what is basically a fused splitter that plugs into the fuse box and provides an additional power lead out. But no matter what fuse I've tried so far in the box under the glove compartment, they are all hot even with the car off and after all the automatic lights etc. go dark. I'd assume, for example, that the wipers would be without power (since I can't turn them on), but nope. It doesn't seem to matter which way I orient the splitter. The closest thing I can get is the front seat heater fuse and the radio fuse, but even those show 0.7V with the car off.

The older cars I'm used to working on had fuses that lost power with the ignition off. Is the wiring concept here that radically different?

Does the voltmeter draw that much power? If not, is there something undesirable by it being left on?

And if the supply voltage to meter is what is displayed on the meter*, most likely the voltmeter isn't going to work on only 0.7 volts.

If the power supplied to power the voltmeter is different than the voltage measured, then you could put a couple of diodes in series with the voltmeter power supply, thereby dropping the 0.7 volts to nothing. When the car is powered up the voltage supplied to power the voltmeter would be 11.4 to 13.6.

You could also do this for the condition indicated by the asterisk*, but your measured voltage would be lower than reality.

I think what you are seeing at those fuses is standby power. The car really doesn't have a power switch or a truly OFF state. The start/stop button is pretty much that, not an on/off switch.

Think of a modern ICE vehicle. if there was no standby power, how would the door locks, the alarm, and telematics work?

I have a hunch that the 0.7 volts radio circuit would be fine for your voltmeter. Have you measured the internal resistance of the voltmeter?
For fun, I plugged a cigarette lighter USB adapter into the 12v power socket. This adapter has a blue led to indicate it is being powered from 12V. After locking the vehicle, the LED remained on. But when I came back a couple of hours later, the LED was off. So I don't know the time delay, but I do know that the voltage will drop sufficiently to not light an led.

I would think the same for the in-socket voltmeter. You lock the car, the voltmeter is on. A hour later, maybe less, the voltmeter is off. Its a voltmeter, it is not going to consume that much power.

UPDATE: Just checked, the 12v seems to cut after after 15-20 minutes. I think I will get one of those voltmeters.
That voltmeter in the lighter is what I have now. I just wanted something that would shut off when the car shut off. I don't get why the cigarette lighter would be engineered to stay on for fifteen minutes.
I don't believe there is a 12V that turns on/off with the car. My dash cam running for 15 min after I turn off the car successfully helped me nab a hit n run neighbor. So it's not all bad that the 12V stays on a while after turning off the car.
Hmm... I hardwired my dash cam and it turns off and on with the car normally. I can check what fuse im using if anyone is still trying to figure that out. I remember just googling what fuse to hardwire dashcam to in a Focus and had no issues.

kalel14 said:
That voltmeter in the lighter is what I have now. I just wanted something that would shut off when the car shut off. I don't get why the cigarette lighter would be engineered to stay on for fifteen minutes.

I'm pretty sure it's not a "cigarette lighter" despite it having similar characteristics. The modern function is simply a power port for accessories, and they probably leave it on for 15 minutes in case you drive somewhere and want to continue using an accessory for a while after parking. If you were to put a lighter element in there, it'd probably burn it up in short order. Just guessing, since I'm not an automotive engineer.
When I wired up my auto dimming mirror, I used the seat heaters fuse for the "off when ignition off" connection.
Okay, new problem. Once the car turns on and the power is on, the main battery is charging the 12V and the meter reads over 14. The goal was to see what the voltage was before the car starts. Looks like I may have to tap into the cigar lighter/powerpoint fuse and have the thing glow for ten minutes or so after every shutdown.
Looks like i used Fuse F85.