i put a voltmeter in my coin holder in my 04 f150 xlt i havent hooked it up yet. i was thinking about getting power from my headunit but then i thought that may not be the best source. where is the best source (color of wire) for power ground i was going to use my headunit ground and the best source for the light so it comes on with my headlights.

 

Best reading would be to run the line to the battery so you see the exact voltage the alternator is putting out. You can tag the brown wire at the headlight switch*for illumination. That wire however is not dimmable.

 

if i draw power right from the battery would this be a good setup? i want the meter to turn on as soon as i turn my truck on other wise i would just use a toggle switch.

 

yes that will work you can also try tagging an ignition wire at the key cylinder coming out of the steering column. there are two 12ish awg wires red/light green and white/yellow. hook your voltmeter to that and compare the voltage difference between those wires and the battery voltage. if it reads the same or very close, it can save you some time from running the wire to the battery and wiring up a relay. sorry i couldnt give you a better response the first time...i posted that from my phone.

 

Perfect set up right there! The battery will give you the most accurate reading consistently. And a relay there is a must if you want the meter to shut off when your ignition is off.

 

now i just have to wire it up. lol. what size relay should i use? also should i fuse it?

 

You could grab a 5 amp Switched live & Dimmer from the instrument cluster.

 

im just gonna run a wire from the battery to a relay then to the meter.

 

You should put an in-line low current fuse (500mA) as close to the battery as possible.

 

Any reason you need to have a voltmeter in the truck in addition to the gauge already in the cluster? (assuming you have the gauge in the cluster)

 

i like to have real numbers and not just l and h. and i already had the gauge in my other truck that im not driving right now.

 

I'd get a programmer like a Gryphon or similar so that you have many gauges in one, all with "real numbers".

 

give me 400 bucks and ill put a programer in there. i had this at my house already so it isnt costing me any thing.

 

Don't get testy, I was just making a suggestion. I just questioned redundant gauges, especially ones that don't really help you avoid much of a problem, especially problems that are pretty rare.

 

The OEM gauge will tell you if you have a problem, it will read lower than "normal". So, you don't need actual numbers. If you notice where the OEM gauge is reading with the ignition in the "run" position, without the engine running, this will be close to 12.6V, the battery voltage without the alternator charging. You can then use this reading to indicate a problem.