12 volt key on power source

[brokemechanic3000]

I am sure this has been asked before but where is a good 12 volt key on power source to tap into for a switch panel on a 1999 Mazda B3000? It is for a light bar on the front that I want to turn off when key is turned off. Also can I make it so that I can have my fog lights and high beams on at the same time?

 

[RonD]

Best method is to use relays mounted in engine bay and then run a smaller ground wire into the cab switch(es) to activate each relay, they can be switches that light up when device is on

Easiest place to get key on power in engine bay is the RED wire on MAF sensor, this is NOT to power lights, JUST RELAYS

You power lights or added devices directly from the battery thru the relay using a FUSED wire

Relay on/off
MAF sensor RED wire---(5amp fuse)---------1 to 4 relays-------------------cab switch---------cab ground

The 1 MAF wire can feed 12v(key on) to 1 to 4 relays, you would need 1 ground wire per relay into the cab to activate each relay individually, these ground wires only need to carry .5amps so 18-22gauge wire is fine to use


Device power
Battery positive-------(fuse)--------1 to 4 relays------------Devices(lights)------------ground

The wire and fuse used would be based on the AMPS each device might need
i.e. if lights are rated as 6 amps then you need to used wire size rated for 15 amps and a 10amp fuse
Wire size must always be rated higher that the fuse used or the WIRE becomes the fuse


The "trick" in all automotive wiring is to keep the 12volt wires in one place and as short as possible to prevent possible shorts to Ground
And since most of the metal parts in a vehicle are already grounded it save you time and money to use those grounds

If a ground wire used to control a device's relay shorts to ground the WORST that happens is that the device comes on
If a 12volt control wire shorts to ground the WORST that happens is a vehicle fire

 

[brokemechanic3000]

Best method is to use relays mounted in engine bay and then run a smaller ground wire into the cab switch(es) to activate each relay, they can be switches that light up when device is on

Easiest place to get key on power in engine bay is the RED wire on MAF sensor, this is NOT to power lights, JUST RELAYS

You power lights or added devices directly from the battery thru the relay using a FUSED wire

Relay on/off
MAF sensor RED wire---(5amp fuse)---------1 to 4 relays-------------------cab switch---------cab ground

The 1 MAF wire can feed 12v(key on) to 1 to 4 relays, you would need 1 ground wire per relay into the cab to activate each relay individually, these ground wires only need to carry .5amps so 18-22gauge wire is fine to use


Device power
Battery positive-------(fuse)--------1 to 4 relays------------Devices(lights)------------ground

The wire and fuse used would be based on the AMPS each device might need
i.e. if lights are rated as 6 amps then you need to used wire size rated for 15 amps and a 10amp fuse
Wire size must always be rated higher that the fuse used or the WIRE becomes the fuse


The "trick" in all automotive wiring is to keep the 12volt wires in one place and as short as possible to prevent possible shorts to Ground
And since most of the metal parts in a vehicle are already grounded it save you time and money to use those grounds

If a ground wire used to control a device's relay shorts to ground the WORST that happens is that the device comes on
If a 12volt control wire shorts to ground the WORST that happens is a vehicle fire

Already have relays installed. I am looking for keyed 12 volt power inside the cab just for the switch panel.

 

[RonD]

Fuse 20 in cab fuse box is key on power for the radio, yellow/black stripe wire on radio

 

[brokemechanic3000]

Fuse 20 in cab fuse box is key on power for the radio, yellow/black stripe wire on radio

Will tapping into this cause radio interference?

 

[RonD]

no, not at all

 

[brokemechanic3000]

What if I grab power right from the ignition switch wire

 

[RonD]

Sure, but fuse your added wire, 5amp fuse if you are just powering relays

Page 2 has the ignition switch(bottom) so you can see the wire colors that have 12v with key in RUN, they all run to the cab fuse panel