How to wire up an amp guage??

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How to wire up an amp guage??

Postby gm_fan » Sat Aug 20, 2011 9:36 pm

I installed a guage tri today in my Monza and cant figure out how to wire the amperage guage. It indicated it must be in series in a circuit , but the instructions left a lot to desire...I have installed the guage and ran a coulpe of wire through the firewall and over to the wiring block next to the brake booster where they come into the car. Anyone have any idea where i NEED to connect to make the amp guage work..I took out a pair of sun blue line guages that had 1978 on the back, an oil pressure and water temp . Guess I will use them in my 68 Camaro when I get around to restoring it...Thanks for any help,,gm_fan
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Re: How to wire up an amp guage??

Postby Rickracer » Sun Aug 21, 2011 1:47 am

You really do NOT want an ammeter. Back in the days of 30 and 50 amp generators and alternators, they were OK, but today's alternators have the capacity to literally FRY most ammeters. The problem with them is that in order for them to be accurate, all the current need to pass through them, coming and going, and the aren't up to "welding" size loads that many newer style alternators are capable of producing. I've rewired a number of 60s and 70s Mopars due to the factory ammeters burning up, shorting out, and taking a bunch of the harness with them. I'm pretty sure most aftermarket ones won't fare any better. A voltmeter will tell you just about anything you need to know, that's why most newer cars have them if they have a charging system gauge at all. :wink: 8)
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Re: How to wire up an amp guage??

Postby cosvega76 » Sun Aug 21, 2011 7:40 am

What Rick said.

A voltmeter is a better choice.

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Re: How to wire up an amp guage??

Postby Wizeguyrc » Sun Aug 21, 2011 9:57 am

On a side note....
If your Sun Blueline water temp gauge is the electric model, don't forget to pull the sending unit out of the engine and save it with the gauge. Those older gauges have specific senders for the model gauge being used. The senders are worth more than the gauge themselves.
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Re: How to wire up an amp guage??

Postby gm_fan » Sun Aug 21, 2011 11:29 am

Yes, I pulled the sender also..The car has a volt meter in the dash, just that the guage trio has the ammeter in it and wanted to hook it up,I actually connected it to my electric cooling fan relay supply voltage and its showed a forward on the guage when the car is running , but I have the trip leg on the relay hooked up to my ac wire to bring on the clutch for the air , so it only works when the ac is on..,,any ideas what circuit to use that doesnt pull a lot of amps?and is also on when the car is running ? thanks gm_fan
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Re: How to wire up an amp guage??

Postby spencerforhire » Sun Aug 21, 2011 5:56 pm

An ammeter is designed to measure the flow of amperage in and out of the battery; they were installed in the main power wire that runs from the battery to both the alternator AND the main power feeds to the ignition switch and fuse block. As was stated in some responses above, this was sort of OK when cars had 30-50 amp generators and (later on) alternators. The early GT Vegas had ammeters, but later ones(and all Monzas with full gauges) had voltmeters instead.

With modern (50-150 amp and even larger)alternators, in my opinion, an ammeter is really nothing more than a fire hazard; it brings all the current(except the starter) into the dash area.
My advice is to use a voltmeter instead.
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Re: How to wire up an amp guage??

Postby Rickracer » Mon Aug 22, 2011 3:11 pm

gm_fan wrote:Yes, I pulled the sender also..The car has a volt meter in the dash, just that the guage trio has the ammeter in it and wanted to hook it up,I actually connected it to my electric cooling fan relay supply voltage and its showed a forward on the guage when the car is running , but I have the trip leg on the relay hooked up to my ac wire to bring on the clutch for the air , so it only works when the ac is on..,,any ideas what circuit to use that doesnt pull a lot of amps?and is also on when the car is running ? thanks gm_fan

Not sure what you're trying to accomplish here. If you're talking about tripping the e-fan relay, you can use just about any "switched ignition" fed wire, it takes very little current to trip a relay. 8) Your best bet, with that ammeter, is to keep in the top of your toolbox as a testing tool to measure current draw of stuff you want to test, either that or just go ahead and toss it in the circular file right now and save yourself some trouble, :wink: 8)
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Re: How to wire up an amp guage??

Postby firebird77clone » Sat Apr 14, 2012 10:22 am

TRASH the amp meter.

The 78 LTD I gave to my mom went up in flames when the amp meter ,which I installed , failed catastrophically.
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Re: How to wire up an amp guage??

Postby bugdewde » Sat Apr 14, 2012 3:02 pm

A very well versed electronics dude sent me this about the ammeter in my '73 Vega.
They are only good for measuring voltage drop across a wire, to indicate whether the alternator is charging or not.

If you continue to use the ammeter, maybe this will help:

....... "The factory amp meter is really a very low voltage volt meter and it basically measures the voltage drop across the power wire from the alternator to the battery.
You can connect one side of the amp meter to the 12v post on the horn relay and the other terminal to the + battery post on the starter motor where all the fusible links are.
The amp meter will now measure the small voltage difference (due to the resistance in the wire) between the 2 points. If the meter reads backwards, just flip the 2 wires on the meter around.
Attached is a crude diagram for hooking it up.."



Cheers,
Tim


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Re: How to wire up an amp guage??

Postby Spode134 » Sun Feb 23, 2020 10:16 pm

Just read the post about how the ammeter is wired. Genius! I kind of had an idea it was like that and am glad I didn’t try feeding a big shot of current through that dinky little meter. The 16 ga wires feeding it made me rethink that. Thanks for the post.
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Re: How to wire up an amp guage??

Postby cjbiagi » Mon Feb 24, 2020 8:27 pm

Correct, these are not true ammeters in H bodies. They do not carry large amperage as they have small wires (16 gauge?) going to them. They measure voltage and "convert" the reading into a charge or discharge reading on the "amp" gauge.
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Re: How to wire up an amp guage??

Postby 74_Vega_GT » Wed Feb 26, 2020 11:11 am

I read this thread and it raised a question, Is the voltmeter a direct drop in replacement for the amp gauge?
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Re: How to wire up an amp guage??

Postby cjbiagi » Wed Feb 26, 2020 11:47 am

I believe you would need to use one wire and run a ground to the other side of a true voltage gauge. The ammeter does not have a ground wire, It is measuring the voltage difference from one end of a + wire to the other end.
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Re: How to wire up an amp guage??

Postby zeke » Tue Mar 03, 2020 5:47 am

74_Vega_GT wrote:I read this thread and it raised a question, Is the voltmeter a direct drop in replacement for the amp gauge?


No
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Re: How to wire up an amp guage??

Postby zeke » Tue Mar 03, 2020 6:16 am

The manus quit putting ammeters in cars 50 years ago as they put high current under the dash and can cause fires with all the extra connections and possible points of failure. A voltmeter is a much safer indication of charge circuit operation. Even a shunt style ammeter, as shown in a post above, is safer but the shunt meter gauge circuit should be fused.

Here's how pass thru ammeters were wired anyways.
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