I'm looking to put a voltmeter in next weekend and I wanted to do it relay style. Instead of tapping direct into an ignition wire, I want to use an ignition wire under the hood to power a relay, and have the relay wired direct to the battery and send it on through. When I wired my tach it was a pain in the *** to find the ignition hot and an illumination wire, I ended up having to do a hackjob behind the radio. So basically I'm curious if you guys have a suggestion for a keyed hot wire thats under the hood I could safely use to power a small relay. Thanks friends.

 

A 12v relay only needs 0.4-0.5 amps max, so can safely be added to any circuit

The MAF sensor heater wire(red)
The EVAP system solenoid(red)
EGR solenoid(red)
IAC Valve(red)
Any fuel injector(red)

Any of these would be fine to use, they all get 12volts with key on, from 30amp EEC fuse and relay

 

Thanks. So I can splice into the MAF, have a wire running over to terminal 30. Take the battery HOT and put it on terminal 86. Take the battery ground and put it on terminal 85. Run the power wire to the gauge on terminal 87. Sound good? Would you think this would give any better improvement over tapping into the OEM wire harness directly?

 

NO.......................

85 and 86 are a relays activation tabs
Red wire from MAF to either 85 or 86, the other one to a ground there is no polarity, just need 12v and ground

30 and 87 are the LOAD tabs, they are connected when relay is activated
Either can be FUSED battery 12volts, the other to the LOAD, the device you want to be powered on with the key
Have to have an in-line fuse between battery and relay, and it needs to be rated below relays amp rating but above added devices amp rating

Relays are rated by voltage and AMPs
12v 40amp relay means:
it can be activate by 12volts(85 and 86)
And its Load tabs(30 and 87) can handle up to 40 amps

If your device needs 10amps max then use a 15amp in-line fuse and at least 12ga wire, 10ga if it more than 15ft of wire to the device

 

alright that makes sense. Shouldn't be too hard to do. Have you ever heard of a gauge kit requiring an inverter? The OEM says they ship with an inverter for the backlighting that converts the dc battery voltage to 200V AC. Never seen this **** before.

 

Not sure what you are doing but I guess a maker could use AC bulbs for back lights, not sure why they would???

As long as the watts are low shouldn't be an issue

 

IDK why I need it. It must be some sort of special backlighting. The brand is Speedhut. They seem to have some good stuff.

 

Yes, it does show Speedhut uses an inverter for back lights, not a concern just an extra thing to hook up

But there seems to be a bit of confusion on dimming these gauges, using vehicles existing dimmer wheel, they are older posts so maybe it has been worked out

 

well @RonD its a good thing my dimmer is always cranked to the max

 

I'm posting up here to confirm that a wire tapped into the red wire from the IACV works as a keyed 12V source. I have the relay set up and am awaiting the package from UPS.

One thing I did find weird, the IACV only has two wires, a red hot and a white. I would've thought it had 3 wires. A 12V hot, a ground, and a 5V signal wire

 

IAC Valve is not a sensor, its a Control, a solenoid, so uses 12volts

The Computer controls the ground(white wire) using PWM(pulse width modulation)
So it has 12v in(key on), but the ground side is pulsed by the computer to reduce the voltage inside the IAC valve's coil, so if you were to put a volt meter on the terminals you might see 11v engine off, but only 7v engine running because computer is reducing the voltage to close the IAC valve a bit to set idle
If you were to test the red wire to engine ground it would always be 12v because its the reference voltage, the ground(white) wire is the control

Pretty much all solenoids on vehicles are done this way, EVAP, and EGR solenoids get 12v key on, then computer controls the ground to each to apply a little or alot of vacuum
Fuel injectors are the same
Reason for this is its safer to run a majority of ground wires as control wires since they can't blow fuses, or start a fire, if they short to ground
So shorter 12volt wires around engine bay and longer control wires to computer and switches


If you were installing a switch on the dash to control something like lights for example
You would install the relay with lights 12v power, and relays 12v coil power
Then run just 1 small wire into the cab and to the switch on the dash, ground switch to cab ground
So when you turned ON the switch it grounds the relay to activate it
No 12volt wires running back and forth just waiting to short out, lol

 

Thanks @RonD . This makes sense. And I also never thought of using the switch on the ground side. See in the HVAC world we always switch the hot side, NEVER SWITCH NEUTRAL (ground in 12V DC terms). So it's unnatural for me to switch a ground. But what you said makes perfect sense.

Just FYI I used an old post from you years ago trying to tap into an illumination wire to power my illumination on the gauges. This time I tapped into the passenger parking lamp for a switched hot wire. This powers my illumination relay, the only thing is is it doesn't allow brightness adjustment on the gauges, just ON/off. Oh well.

IDK how you know all this **** about rangers but they need to start paying you

 

Yes, AC is a different animal, alternates + and - , a very confused electrical system doesn't know what it wants to be when it grows up, lol, but nothing wrong with that