kalel14
Posts: 133
Joined: Sat Jan 09, 2016 4:05 am

Fuse Box- What's Hot What's Not

Tue Apr 21, 2020 7:40 am

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?

Thanks.

Heima
Posts: 106
Joined: Thu Feb 21, 2019 11:46 pm

Re: Fuse Box- What's Hot What's Not

Tue Apr 21, 2020 8:48 pm

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?

Heima
Posts: 106
Joined: Thu Feb 21, 2019 11:46 pm

Re: Fuse Box- What's Hot What's Not

Thu Apr 23, 2020 6:13 pm

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.

kalel14
Posts: 133
Joined: Sat Jan 09, 2016 4:05 am

Re: Fuse Box- What's Hot What's Not

Fri Apr 24, 2020 2:12 am

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.

triangles
Posts: 1108
Joined: Tue Aug 16, 2011 11:40 am
Location: Toledo, Ohio

Re: Fuse Box- What's Hot What's Not

Fri Apr 24, 2020 4:08 pm

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.
2014 Blue Candy FFE
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shniestEV
Posts: 77
Joined: Tue Nov 24, 2015 2:14 pm
Location: Vancouver

Re: Fuse Box- What's Hot What's Not

Thu May 07, 2020 1:52 pm

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.

Keith
2016 Magnetic FFE - Canada

kalel14
Posts: 133
Joined: Sat Jan 09, 2016 4:05 am

Re: Fuse Box- What's Hot What's Not

Sat May 09, 2020 1:28 am

If you could check that for me I'd be very grateful.

Fat Focus
Posts: 53
Joined: Mon Jun 04, 2018 2:58 pm

Re: Fuse Box- What's Hot What's Not

Sat May 09, 2020 10:10 am

kalel14 wrote:
Fri Apr 24, 2020 2:12 am
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.
2018 Focus Electric
2013 GT350 Supercharged Widebody Convertible
1999 F250 7.3 Crew Cab 6-Speed 4x4
1964 Falcon Convertible 289 4-Speed

scottt
Site Moderator
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Re: Fuse Box- What's Hot What's Not

Tue May 12, 2020 9:05 am

When I wired up my auto dimming mirror, I used the seat heaters fuse for the "off when ignition off" connection.
2015 Magnetic FFE (aka Magneto) - Retired
2017 Kona Blue FFE (aka King Kona) - Current

kalel14
Posts: 133
Joined: Sat Jan 09, 2016 4:05 am

Re: Fuse Box- What's Hot What's Not

Tue May 12, 2020 9:23 am

Thanks!

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