That's Right: Another Gauge Wiring Question

[XanRules]

Hi y’all,

There are a lot of different threads out there on different forums for tapping into different places on different cars for different purposes, but I’m having a hell of a time figuring out which is which so I figured I would just ask. Some stuff involves adding circuits and fuses, some doesn't, some is only applicable to certain trim levels or countries (the fuses are labeled totally differently in the euro cars, for example). So here's the deal.

I am installing a boost gauge in my USDM 2017 Fiesta ST. I have heated seats and a sunroof if that matters (I know those are 12V ignition powered). I need to know what to tap for the following:

• 12V Constant Power
• Ground
• Headlights or Taillights (powered on only when headlights are on)
• 12V Ignition Power (power only with key on)

I would like to tap into the headlight switch below the vent as much as possible so I don’t have to mess around with running wires and fuses across the car into the fuse box in the passenger kick panel, but can do that if needed.

Here is the disclaimer - When it comes to this specific issue, I am stupid. If you link me to another thread that is not specifically about tapping into those wires or fuses and requires me to figure anything out or measure anything with a multimeter, it won't register in my smooth, soft little brain. I need it spelled out for me.

Thanks in advance!

 

[slopoke]

For me, my preference is to use a fuse tap. That way the load that you are adding won't cause the tapped into circuits fuse to blow, in the event the component malfunctioned. You don't need to have a critical car function disabled because the added component caused a short. Plus the added component has a fuse for protection. Lastly, I hate cutting into factory wiring and avoid it unless that's the only option.

 

[XanRules]

For me, my preference is to use a fuse tap. That way the load that you are adding won't cause the tapped into circuits fuse to blow, in the event the component malfunctioned. You don't need to have a critical car function disabled because the added component caused a short. Plus the added component has a fuse for protection. Lastly, I hate cutting into factory wiring and avoid it unless that's the only option.

Cool, yeah, that seems to be the consensus so I'll probably just do that. I know where the ground bolt is, but I have no idea what to tap into (fuse or otherwise) for the other three. I guess tap into 17 (heated seats) for 12V ignition power? Which of these is 12v constant and which is for the headlights?

 

[slopoke]

When I installed my gauges, I just used a fuse position that would normally be "Hot" when the ignition is on. I can't remember which I used. My AEM gauges did not have a backlight, so I did not have to use a separate power source that is only "Hot" when the lights are on. If yours has a separate wire for night back lighting, I don't think you'd notice it during the day and should not lessen the life of the backlighting lamps. Good luck.

 

[XanRules]

Found another thread saying the brake light fuse, F18, is constant 12v. So that’s 3/4…

 

[slopoke]

Found another thread saying the brake light fuse, F18, is constant 12v. So that’s 3/4…

I'd use an audio circuit fuse. That's because I'd want to tap into a non critical circuit, just in case the added circuit had a failure of some kind that may damage the circuit you tapped into. I'm kinda anal in that respect. Must be from my 40 years of aviation related experience.

 

[XanRules]

Okay, yeah, it looks like the radio fuse may also be a constant 12v to the degree that there are any in this car (apparently most of them will switch off after a little while but whatever) - crucially after a lot of research it appears that there is no headlight fuse in the car. The headlight fuse is a constant 12v, and the dimming od the interior lights is controlled by the BCM, so you have to actually tap into the headlight switch. Annoying!

 

[XanRules]

Ah crap. There’s a bunch of “radio” fuses. Which is constant?

https://fuse-box.info/ford/ford-fiesta-2014-2018-fuses-and-relay/amp

 

[XanRules]

Alright so it looks like the final answer is:

Ground - ground.
Constant 12v - Lighter Socket (F33, 20A) OR brake lights (F18, 10A) - whether or not the lighter sockets are a constant 12v seems to vary with model year.
Ignition 12v - Windshield Wipers (F11, 20A)
Headlights - blue wire coming from headlight switch. There is no fuse for the headlights, even if you use the "headlights" fuses in the under-hood fuse box. This appears to be the only wire that gets a signal ONLY when the headlights are on. You have to tap into this wire.

I'll give it a try soon and report back.

 

[Bull Run]

Alright so it looks like the final answer is:

Ground - ground.
Constant 12v - Lighter Socket (F33, 20A) OR brake lights (F18, 10A) - whether or not the lighter sockets are a constant 12v seems to vary with model year.
Ignition 12v - Windshield Wipers (F11, 20A)
Headlights - blue wire coming from headlight switch. There is no fuse for the headlights, even if you use the "headlights" fuses in the under-hood fuse box. This appears to be the only wire that gets a signal ONLY when the headlights are on. You have to tap into this wire.

I'll give it a try soon and report back.

Did the blue wire coming from the headlight switch work?

 

[XanRules]

Did the blue wire coming from the headlight switch work?

I confess I haven’t gotten around to trying yet. I’m going to give it a shot in the next few weeks and I will let you know.

 

[XanRules]

Did the blue wire coming from the headlight switch work?

Hey bud, I was out there for two fuckin’ hours with a multimeter today and gave up. I tapped into the above mentioned fuses and then just combined the dimmer wire and the power wire (red and orange iirc). The gauge doesn’t dim with the headlights but it works and after two hours in the rain that’s what’s important to me.

 

[Bull Run]

Hey bud, I was out there for two fuckin’ hours with a multimeter today and gave up. I tapped into the above mentioned fuses and then just combined the dimmer wire and the power wire (red and orange iirc). The gauge doesn’t dim with the headlights but it works and after two hours in the rain that’s what’s important to me.

Thanks for checking. Had the same issue with the Mustang, ended up tapping into a wire for the parking light and running it into the cabin to the gauge.

 

[XanRules]

Yeah, we went to the headlight switch and the fusebox and couldn't find a single wire that only got power with the headlights. We got a few that shifted voltages when the headlights were on, but after two hours in the rain we gave up. So it goes.