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Old 06-23-2012, 11:25 AM
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amp gage hookup

I have the starter,light's,horn working but im stumped on one thing I have a 1969 chevy pickup with that greenish printed wire circit for my dash for my gages. the amp gage has two post that [ I assume hold the gage ,and act as positive and negitive terminals]. If I am correct,witch post do I hook up amp wire to from the altinator,the pos. or neg.on the gage? any help out there?

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Old 06-23-2012, 01:36 PM
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AMP gauges at the dash are troublesome.
They should be by-passed, and then install a VOLT gauge.
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Old 06-23-2012, 03:09 PM
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ampgauge

thanks but have to use what I have for now,was told to hook to gauge but need to know hot post or neg. post? That is the wire from altinator
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Old 06-23-2012, 03:41 PM
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amp meters have gone the way of the dodo bird, as all of your alternator (or generators current goes thru the ammeter straight from the hot alternator wire, at least as my memory serves. (an AMMETER is in SERIES only. a volt meter is in parallel. ) Malc's got a link there to a diagram with all you need to know.

then you go from the other post to feed power to the rest of your ve hicular DO NOT HOOK UP ANY POSTS TO GROUND unless, of course, you like fireworks.

you could live just fine w/o it, too, btw.

Last edited by bullheimer; 06-23-2012 at 03:46 PM.
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Old 06-25-2012, 06:48 AM
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I don't know what year GM made the change, but my '75 GMC has an in-dash ammeter that uses a shunt, and it does not run the full flow of amperage through the dash. The dash gauge is connected to two points in the wiring and it is comparing the voltage between these two points and calculating an amperage. The gauge says amps, but its basically measuring a very low voltage difference to determine the amp flow.

However, the gauge reading with a shunt is based on comparing the voltage on two sides of the known resistance in the shunt. As wiring gets old it does not stay consistent resistance and this throws off the gauge. I don't think I've ever seen one of these shunt-based amp gauges that still works right.

Just switch to a volt-meter.

Bruce
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Old 06-25-2012, 08:04 AM
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amp. gage or volt. in 69 chevy pickup?

The wires going to the gage is a thin plastic printed circut that dosn't look like it could hold any power without burning, so I might have a volt gage?
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Old 06-25-2012, 10:47 AM
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amp gage

can anyone tell me if there is a volt meter that will take the place of my amp meter in my interment panel to look stock?
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Old 06-25-2012, 11:52 AM
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If your "amps" gauge works like the gauge in my '64 Corvette (and other old Corvettes), it is not really an amps gauge at all but in fact a sensitive galvanometer that reads a differential voltage.

In the case of the Corvettes one wire of the gauge goes to the positive post on the horn relay, which receives its power directly from the 12 gauge wire alternator hot lead.

The other leg of the gauge goes to the positive post on the starter solonoid, which then goes to the battery positive.

The galvanometer is actually reading the slight voltage difference between the positive terminal on the horn relay and the positive terminal on the starter. The voltage differential is then created from the length of wire and other applications that occur between the horn relay and starter.

When the battery is charging, there is a bit more voltage at the horn relay hot lead. When the battery is discharging, there is a bit more voltage at the starter and this is what is being measured by the gauge.

This amount difference under 1.5 volts and you should be able to test the gauge (if it's like the Corvette gauge) using a 1.5 volt battery. Flipping the battery around should flip the gauge polarity. But use caution because even a 1.5 volt battery can burn out the gauge if left on too long.

So at least in the case of the midyear Corvettes, you do not pull current through the gauge. However, on pre-'63 Corvettes, the gauge did carry the current of the system through a shunt (which essentually measures the voltage on either side of a set of windings doing the same sort of thing), so that the actual gauge only receives a small portion of that current.

BTW, if your gauge is like the midyear Corvette gauge, if you attach either leg to ground and the other to any 12 volt source, you will instantly fry the gauge.

Last edited by toddalin; 06-25-2012 at 11:58 AM.
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Old 06-25-2012, 12:56 PM
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If in fact I have this gage can I hook up a wire straight from my alternator? Im assuming all wires are hooked up except that one,Ihave every thing working but some one cut wires to altinator. On driver side fender the wires run up to radiator and were cut. I put cont. check meter on and big red was grounded as well as brown and a nother ,, there was one 10 gage wire hot so I hooked it to alternator. was wanting to run a new wire to gage from alternator.
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Old 06-25-2012, 03:21 PM
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Quote:
Originally Posted by dirtyfingers
If in fact I have this gage can I hook up a wire straight from my alternator? Im assuming all wires are hooked up except that one,Ihave every thing working but some one cut wires to altinator. On driver side fender the wires run up to radiator and were cut. I put cont. check meter on and big red was grounded as well as brown and a nother ,, there was one 10 gage wire hot so I hooked it to alternator. was wanting to run a new wire to gage from alternator.

You need to first determine how your gauge works and if it works at all.

Disconnect its plug and try putting a 1.5 volt AA battery across the leads for a moment so see if it swings. Then you can best determine how to proceed from there. There are places that can convert "amp" gauges to "volts" gauges were you can simply connect it to any switched power source and ground. They can even change the face from "amps" to read "volts."

On a GM (again, '64 Corvette) the red wire is a hot lead and the brown wire is a switched hot lead that feeds the accessories. Sounds like PO probably had a short and just cut out the circuit, more or less.

Last edited by toddalin; 06-25-2012 at 03:27 PM.
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Old 06-26-2012, 05:00 PM
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external shunt

The GM method in that era used the main accessory wire as a meter shunt.
This means that the ammeter measured the IR drop (voltage drop) across this section of wire.
The attached schematic shows this setup.
You may have noticed (2) in line fuse holders in the harness on that truck. These are for the ammeter.

vicrod
Attached Files
File Type: pdf 002 ammeter external shunt.pdf (17.7 KB, 38 views)
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