Best place to tap 12 V that's only active when the engine is running?

[Discoverado]

Ecoboost

I'm trying to tap into a 12V source (for a signal, not power) that is only hot when the engine is running (and not hot when the key is just in the "on" position). The 12 V signal will be used for an aux battery charger to know when the alternator is running. I don't want the charger to drain my starter battery so it's important the 12 V signal is active only when the alternator is turning.

Anyone know of an easy to access location I can tap?

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[Exranger]

Fuse box. Use a test light to find and tap per your parameters

 

[fossil]

I you're planning on a second battery consider buying a battery isolator, that's what their made for no charger needed.

 

[Discoverado]

Fuse box. Use a test light to find and tap per your parameters

Fair enough. I poked around in the fuse box under the glovebox using the link below as a guide, but I couldn't find a fuse that is only hot when the engine is running. It seems like when the key is turned to on the entire fuse box gets power.
https://fuse-box.info/ford/ford-maverick-p758-2022-fuses

I you're planning on a second battery consider buying a battery isolator, that's what their made for no charger needed.

Thanks for the suggestion. I have a Renogy 50A DC-DC MPPT charger installed. It's supposed to function by charging the aux battery from the starter battery when it detects a high enough voltage on the starter battery (presumably the alternator is providing that voltage). However, for vehicles with "smart alternators", AKA variable voltage alternators, the charger has a signal line that you can connect to let the charger know when the engine is running because the alternator may not be applying a high enough voltage to the starter battery to activate the charger.

My initial thought was to tie the signal wire directly to the alternator, but the alternator is mounted pretty low on the engine and isn't super convenient to access. Maybe it's easier to access from underneath? I'll have to inspect it again.

 

[Exranger]

Did you try under the hood

 

[Discoverado]

So I looked at the fuses under the hood, but they're installed upside down and the connectors all lock closed on top. I'm not sure how to probe them with a multi-meter to see which ones are only powered with the engine running. Even if I did find one that was only hot with the engine running, I'm not sure how I'd actually tap into it.

 

[BlueSpec1]

Passenger Compartment Fuses - has some ignition run fuses listed.
Is anyone else conFUSED by the Ford Maverick under-dash fuse diagram?

 

[GPSMan]

So I looked at the fuses under the hood, but they're installed upside down and the connectors all lock closed on top. I'm not sure how to probe them with a multi-meter to see which ones are only powered with the engine running. Even if I did find one that was only hot with the engine running, I'm not sure how I'd actually tap into it.

So ironic. I went through this myself yesterday while wiring up fog lamps.

They for sure did not make it easy to tap the fuse box under the hood.

On my 2005 escape I did something similar. I think I tapped the fuel pump or fuel injector wire. I forget which.

 

[RogueCheddar]

search the forum for dashcam installs, you should be able to piggyback off the same fuses for you needs

 

[Discoverado]

I'll do some more probing in the fuse box under the glovebox, but I think they were all active when the key was on but the engine wasn't running. I think this is fine for a dashcam since it's low power draw. My case is a bit different because I only want power when the engine is running. I'll double check tonight - maybe I missed a fuse that would work for me.

Otherwise I've got two other ideas:
1) Get a fuse tap and figure out how to install it in the under hood fuse box. I can then move this tap one fuse at a time and probe the extension with a multimeter with the engine off and on to identify/confirm operation.
2) Simply tap a fuse under the glovebox and then add an inline switch. I can leave the switch in the closed position 99% of the time and it'll work as I want. But the 1% of the time when I want to turn the key on and listen to music is when I would open that switch so as not to drain the starter battery charging the house battery. This isn't preferred...I'd rather get it wired to a "hot when engine running" fuse and not have to think about it again.

I appreciate the help so far!

 

[MakinDoForNow]

I'll do some more probing in the fuse box under the glovebox, but I think they were all active when the key was on but the engine wasn't running. I think this is fine for a dashcam since it's low power draw. My case is a bit different because I only want power when the engine is running. I'll double check tonight - maybe I missed a fuse that would work for me.

Otherwise I've got two other ideas:
1) Get a fuse tap and figure out how to install it in the under hood fuse box. I can then move this tap one fuse at a time and probe the extension with a multimeter with the engine off and on to identify/confirm operation.
2) Simply tap a fuse under the glovebox and then add an inline switch. I can leave the switch in the closed position 99% of the time and it'll work as I want. But the 1% of the time when I want to turn the key on and listen to music is when I would open that switch so as not to drain the starter battery charging the house battery. This isn't preferred...I'd rather get it wired to a "hot when engine running" fuse and not have to think about it again.

I appreciate the help so far!

I don't know about EB dash fuse panel but read in a post that the green and red wires shown in picture one is switched other is hot. Do not know if they are on fuses or not or are just there to provide power. Need to check modifiers guide to maybe find out what those wires are for. The large black wire withe the red band appears to be main supply to fuze panel. Might check these and see. They might have position next to each for a fuse? Picture is of my Hybrid panel!

 

[RCollins]

I don't know if this engine has an oil pressure sender, but if it does that would be a source for a signal when the engine is running, but maybe not the alternator.

 

[Darnon]

Just use something like a 13V zener diode triggering a relay?

 

[TheWizziard]

I have been looking for the same start signal for a different reason. I want to energize a relay to switch on my power inverter so that it will only run when the Maverick is running. I have a hybrid and need the vehicle in run mode to be able to use the ICE to keep the 12v battery charged through the DC to DC converter.

The Cuban Rednick picked up this signal at the steering column wiring for his Automatic Eco-Mod mod. At 12:17 on his video
If this is a momentary on signal I will wire it to a latching relay and use key off to unlatch it.
This signal may not be available at the inside fuse box.

I would love to have a wiring diagram but not for $170

 

[Discoverado]

Just use something like a 13V zener diode triggering a relay?

Can you expand on this? I guess I don't understand how I'd use this.

I want to energize a relay to switch on my power inverter so that it will only run when the Maverick is running.

Yeah sounds like we're looking for the same thing. You bring up an interesting point, though. I have the factory 400W inverter, and I think it's only powered when the engine is running. Maybe there's an existing remote line for that we could take advantage of?

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