Volt Gauge

[n1265]

getting ready to make some changes to the 1977, Just wondering what you guys think of this volt gauge.

http://cgi.ebay.com/ebaymotors/ws/eB...tchlink:top:en

what gauge did you guys go with when you did your amp by pass ? are there any makers to stay away from ?

Many Thanks,

[n1265]

Or maybe this one ....

http://cgi.ebay.com/ebaymotors/VOLT-...ssoriesQ5fGear

[MoparFreak69]

getting ready to make some changes to the 1977, Just wondering what you guys think of this volt gauge.

http://cgi.ebay.com/ebaymotors/ws/eB...tchlink:top:en

what gauge did you guys go with when you did your amp by pass ? are there any makers to stay away from ?

Many Thanks,

My honest opinion on this one; screams China built/quality. While it is only a volt gauge, and there isnt much to one, going with a more respected manufacturer reduces risk of component failure, and even reduces fire hazard. The reason the Chinese can build this stuff so cheap is that they skimp on everything. The wiring is always only just barely large enough to handle the task, no wiggle room whatsoever.
Just my opinion but I would steer clear of this one, and probably most others in this very low price range. This one http://www.summitracing.com/parts/ATM-4391/ would be a more preferred option for anything I would put one in.

[DODGEBOYS]

what he said above

[n1265]

What size wire should I use to wire a gauge in ?

12,14,16,?

[MoparFreak69]

The Painless wiring harness I ordered for my '59 is all 14 gauge except a few of the higher load circuits that run 12 gauge. 14 guage should be plenty for gauge duty, just make sure you get your wire from a reputable supplier. The cheaper (read Chinese) producers like to make the insulation thicker to make the wire appear to be the correct size, it is always smaller than spec though. I have found NAPA to be a pretty good supplier or wire, Auto Zone (and Schucks/O-reilley) tends to be more of the lower buck, lower quality stuff.

[n1265]

Many Thanks....

[Paul in MN]

Wire gauge for voltmeter

The gauge needed for any wire is determined by the number of amps expected as maximum to flow through it (and total length of wire). Commonly a #12 gauge wire can adequately carry 20 amps continuously, #14 15 amps, #16 10 amps. In the case of wiring up a voltmeter, it is probably in the range of 1 to 10 milliamps, meaning probable maximum of 1/100 amp. Different gauge manufacturers will have slightly different numbers for current flow. But this range is adequate for this discussion.

A volt meter measures voltage from the + side of the truck's electrical system to the - side. It makes no difference where this measurement is made... at the battery, at the dashboard, at the cig lighter. The highest normal voltage is expected to be close to 15 volts as the battery is first charged, settling down to 13.2 volts. With this low voltage, and the extremely low amperage, the smallest of common wire gauges is adequate. And in your wiring supply, this might be 20 gauge. The advantage to using this thin gauge wire is the prevention of electrical fires. A very thin wire, if abraded by rubbing on a sharp sheet metal piece behind the dashboard, will quickly heat up and burn out acting as a fuse protecting the rest of the wiring from overloading and overheating. POOF!!!!, the thin wire is gone, there is some smoke and stink, the voltmeter no longer works, but that does not prevent the rest of the truck's electrical system from working normally, and it does not prevent the truck from running.

The wiring to an amp gauge is quite different, because it is built to carry temporary overloads of 60 amps or more as the alternator begins to charge a very dead battery. Thus the very heavy 8 ga coming off the alternator, but if its insulation is abraded (and resuls in grounding), the electrical harness is destroyed, and quite possibly the truck is burned. Lesson learned: do not use any heavier gauge wire than its particular circuit needs. I did learn this lesson in the college of hard knocks, a grad level course. The tuition was pricey.

Paul in MN

[74w300uteline]

fascinating Paul

I'm a big fan of overkill. however your post has me reevaluating some of my future wiring plans. I thought wires were like tires, the bigger the better.=)
I learn something every day

[74w300uteline]

makes sense now

at first i couldnt wrap my mind around the thin wire idea, then i realized that my multi meter has thin leads!!

[MoparFreak69]

The gauge needed for any wire is determined by the number of amps expected as maximum to flow through it (and total length of wire). Commonly a #12 gauge wire can adequately carry 20 amps continuously, #14 15 amps, #16 10 amps. In the case of wiring up a voltmeter, it is probably in the range of 1 to 10 milliamps, meaning probable maximum of 1/100 amp. Different gauge manufacturers will have slightly different numbers for current flow. But this range is adequate for this discussion.

Great information Paul.
I do however have to disagree with the second part of your statement, referencing chafe damage control. Any properly installed and routed wiring harness will not have this risk at all. Using bulkhead connectors to run a wire through a piece of metal is the only real solution to preventing wire damage. Proper wrap up and retention of wiring is also as essential as the quality of the wiring itself.
Another point I would like to make in reference to your burn through thought. Generally one wire is not floating out in space by itself. They are usually bundled into a harness. When one wire burns, it almost always takes more along with it. A small wire can burn through just as much insulation as a larger wire can when directly shorted to ground. All circuits in a wiring harness should also be protected by a fuse, which will only be rated right at the maximum normal circuit draw.

I like to play the devil's advocate.

Safety is always a concern, no matter what you are working on/with.

[MoparFreak69]

at first i couldnt wrap my mind around the thin wire idea, then i realized that my multi meter has thin leads!!

My DMM actually has 12 gauge wires for the leads.

[KRB64]

Thanks Paul, thats good to know. And splained so I can understand!

[Paul in MN]

DMM lead wire gauge

My DMM actually has 12 gauge wires for the leads.

The gauge of wire used in the leads of a meter depends upon the functions the meter is designed to perform. Commonly, a Digital Multi Meter has a function for determining the amps flowing through a circuit. Often times this is limited by an internal fuse rated at 10 amps. So the wire of the leads is carrying a possible full 10 amps, and a good meter manufacturer does not want any resistance within the leads of the meter. To have Zero resistance is impossible, so they compromise to a wire size that is still oversized to minimize the resistance and consequent line loss, and yet still be flexible enough to be convenient to use. And so it is with my Fluke 23.

But if the meter is just used for measuring voltage, i.e. DVM, then the lead wire can be a much thinner gauge. A good DVM should only pass a few milliamps (1/1000 amp) per volt it is measuring. And the less milliamps per volt is the sign of a better quality meter. So even though the meter leads may look thin and "cheap", they do the expected job quite adequately.

I know that the trucks of the 60's and maybe through the 70's had the heavy gauge #8 or #10 going from the alternator through the firewall to the ammeter, and then back through the firewall to a junction bolt on the starter solenoid where the current flow would continue through the heavy battery cable to the battery. Heavy gauge wire is expensive and puts the wire harness at risk if there is some short circuit (maybe a fault within the alternator). I don't know what year this design was changed, but I think my 86 W250 does not bring the heavy gauge alternator-battery wire through the fire wall. I believe it is designed with a SHUNT in that heavy gauge circuit. A shunt is a short piece of conductor with a known resistance. Essentially a voltmeter is connected (light wire gauge) across this shunt and measures the voltage drop across the shunt. The greater the voltage drop, the more amps that are flowing through the shunt. And now the voltmeter (dashboard instrument panel) is recalibrated to show "amps". But really there only a few milliamps or maybe a few 1/100s amps flowing through the thin wire to the "ammeter". The theory is good, but the real world application (connector corrosion, etc) leads to an ammeter that never even wiggles off its center "0" mark. And this is in a truck built with the snofighter package with the 100+ amp alternator.

I hope this helps to explain some of the electrical "magic" we deal with as we work to keep these old beasts in useful condition.

Paul in MN

[n1265]

All good reading.

Where is the best place to pickup the power going to the volt meter ?

I know it should be a keyed power source, but should I tap into a wire, plug a spade into the firewall connector, or run a wire all the way to the fuse box ?