jim perrotti

I had the the amp gauge replaced on my 75 corvette and its still not working.Does anyone have an idea where I should look next to fix the problem. thanks, Jim

DUB

You have to verify that the two wires that go to it are not broken. You may need to get a wiring diagram to check to make sure that they are getting 12 volts.

DUB

jim perrotti

Thanks for the info DUB.I droppped at a corvette repair shop and they replaced the guage and obviously never did that.

noonie

Both wires are protected by fusible links.

jim perrotti

Thankyou for the info Noonie, much appreciated
Jim

rberman999

Hi Jim,

Let me start by stating I fixed my own amp meter. Having a simple understanding of the amp meter and its circuit are required in order to fix either the circuit or the meter. The first step is to understand that the amp meter really is not an amp meter but a voltage meter. Now before you or anyone else rips my head off for this statement let me try to explain. The amp meter measures a voltage differance between two different points in the charging circuit. The way it reads "positive current" is if the voltage polarity diffenence between the two points of measurement within the circuit is positive the meter reads on the positive side of the meter (a positive polarity between two different points in a circuit indicates the flow of current, hence the amp meter). An example of this would be if the alternator is putting out ~13.5 volts and the battery is at 13.0 volts then the meter reads a positive current. But in reality the meter sees a + 0.5 voltage differance and moves the meter over to the positve side of the meter. Now say your alternator was only putting out at a voltage of 12.9 volts and your battery was at 13.5 volts then the meter would show a discharge of current but in reality the meter would be reading a -0.6 volts and would read as a negitive current on the meter. If both points in the circuit are at the same voltage then the meter reads a differance of 0 volts and shows no needle deflection. Now what does this simplified explaination do for you? It allows you to be able to simply test your amp meter. If you take a discharged 1.5 volt AA battery (the more discharged the better (one that reads say 0.9 - 1.1 volts) you can connect leads from the battery to the amp meter and see the meter deflect in one direction, reverse the leads and the meter deflects in the other direction (the needle deflection would be for a good meter). If you get no meter movement then the meter is no good. Don't leave the meter connected to the battery, just use the battery to tell you if the meter is working by touching the leads for a moment. As far as how I fixed my meter, I carefully opened up my meter, found the broken wire inside it (the meter is a winding of fine coil wire used to induce a magnetic field which causes a deflection of the meter) and soldered the wire. If your meter is working then move onto the circuit. For that you need a circuit diagram, a meter (Volt/Ohm) and the ability to trouble shoot a circuit. I hope this simplified description of the amp meter helps you or someone else. Lots of luck, Russ

Hammerhead Fred

Try this diagram.

noonie

There is a small gauge ammeter wire at the starter solenoid with a fusible link and the other at the outside firewall fuse panel junction with another fusible link.
As per diagram linked.

rberman999

Hello again,

In my post above where I stated "I fixed my own amp meter" I should have included the fact that before fixing my old gauge I bought a NOS gauge and replace the old gauge. Well when I tried the NOS gauge no go, it did not work. Never thinking the NOS amp gauge could be no good I then moved onto checking the circuit. Yes there are two fusible links but both mine were good (easy to see when they are bad, the wire burns up and becomes carbonized). I fully checked the circuit and could find no issue, fully tracing the wires. That is when I started to dig further into how to verify the gauge as being good, hence the post above. It turned out that the NOS gauge was defective as well as my old gauge. Once I got a gauge to work (my old gauge) my amp meter worked. So, what I'm saying, just because the "shop" put a new gauge in, don't assume it was good, you need to know you have a good gauge, and that is where testing the gauge with a discharged AA battery comes in handy. Russ.

corvetero

Hi Russ,

Sorry to bring up this topic again.

From your explanation I understand that after unplugging the 2 lines I should have continuity on the amp, otherwise would mean the internal wire is blown.

rberman999

Hi Corvetero,

What your saying is in theory correct BUT don't use your VOM to check continuity since most VOM's use a nine volt battery and that much voltage could fry your gauge. The reality of a C3 amp gauge is it is a voltage meter, but one that reads a very slight voltage relative a reference point. So if it sees a slight positive voltage you are charging, a negative voltage you're discharging. That's why using a VERY discharged AAA battery can tell you if the gauge is good or not. With a battery at say 0.4 volts ( you can use your VOM to measure the battery voltage) you can test the gauge. Place the battery across the two gauge terminals just for as short as you can (less than a second) to see if the needle deflects (to either the positive or negative). If you reverse the battery from your initial test the needle should deflect the opposite direction. Again just touch for as short as you can ( less than a second), If you get no deflection of the meter meddle then the amp meter is no good. Again, just a small voltage for a short amount of time will test the amp meter. If you determine the meter to be the fault then the winding inside the meter is likely broken. If you are careful opening the meter and don't destroy the meter face or the needle you should be able to soldered the winding and fix your meter. Then put it back together and make sure it now works. That's what I did back in 2014 and it's still working fine. Lots of luck, Russ

Peterbuilt

Both leads to the ammeter should have 12 volts to them, so disconnect the battery before you start.

To test the ammeter:
Connect a ground to the top post and 12 volts to the bottom post.
The meter should deflect downward.
Connect 12 volts to the top post and a ground to the bottom post.
The needle should deflect upwards.

Your meter will handle 12 volts with no problem.

See this post:

https://www.corvetteforum.com/forums...n-ammeter.html

rberman999

Corvetero,

I have only described what I personally used to troubleshoot my amp meter and fix it. Therefore, I know it to have worked in my situation. I'm sure there's more than one way to troubleshoot and fix a non-working amp meter. Good luck, Russ

Peterbuilt

Hi Russ,
You gave good and correct advise.
Testing with a almost dead AAA battery will work.
It's better to err on the side of caution rather than applying battery voltage and destroying your meter.
Your 100% correct that there are many ways to do a task.
Regards, Pete.

Vogast

EDIT: Problem solved, most likely bad contacts on horn relay

derekderek

Everybody running in ammeter in their car should have one of these. https://www.walmart.com/ip/Digital-L...l12=125501206& people confuse short and open circuits. Every time you turn off a switch, you have created an open circuit. A short? Put jumper cables in the battery. Put them together. Arcs and sparks. Open circuits make something not work. Short circuits make insurance adjusters come see whether your car is too burnt up to repair. So generally, when you have a short, a fuse will blow or wire will melt and you have an open...

wadenelson

As an Electrical Engineer who's worked on Auto Electrical a long time I feel compelled to re-phrase some of what Russ wrote here. Hopefully make it a little easier to understand.

>...understand that the amp meter really is not an amp meter but a voltage meter.

In olden days big thick cables went to a "real" ammeter mounted in the dash of most cars. Every electrical draw, sometimes even the starter, passed through the meter. 100% of alternator output went through that gauge. But big fat cables are clumsy under the dash, expensive, and if they chafe, you can end up with a serious electrical fire. Great way to burn up a classic car.

57 Fuelie -- dashboard fire:

https://imgur.com/a/L9P1blM

So manufacturers looked for a better and cheaper way. One is simply using a shunt. A second electrical path. Run 9/10 of the current through the primary path, 1/10 through an ammeter, and calibrate the gauge accordingly. (You adjust the % of current by adjusting the relative resistance of the primary path vs the shunt). If this is confusing, ignore it and go on.

The other way to measure current is to DEDUCE how much current is flowing using Ohms law. V=IR. If 10 amps flowing between the alternator and the battery, and the resistance of the wire between the two is .01 ohms . ( a fixed length wire of a known gauge) a voltage difference V = 10 * .01 = 1 Volt must exist between the two points.

So you can simply stick a voltmeter across ANY piece of wire WITH KNOWN RESISTANCE between the alternator and the battery and DEDUCE how much current is flowing. 1 V = charging at 10 A, -1V = discharging at 10A, -2V = discharging at 20Amps.

So you're using a Voltmeter pretending its an ammeter, all you really gotta do is paint the #'s on the dial in the right places, and you've eliminated the need to run high currents and fat wires up under the dash. (In a lot of the postings here, that calibrated wire is also referred to as a "shunt" - I might argue it technically isn't, but that's no mind.)

This all makes the General happy. Low cost, small wires, fewer dashboard fires.

Mind you, if somewhere along the way some repair dude replaced that calibrated wire with one of a different length & AWG, your ammeter is going to read correspondingly higher or lower. No big deal; most of us are just glancing at an ammeter now and then to see that it's generally centered during most driving events, and that the alternator isn't failing to keep up with the load --- or failed completely, and the battery discharging continually. The gauges themselves aren't all that accurate unless you just got lucky.

As for blowing up an ammeter with a VOM; unlikely. If you have your meter set on OHMS it's going to apply about .7 volts across the leads. If you put it on DIODE/BEEP it might apply a little more. I'm not aware of any that apply full battery voltage to the leads on any setting.

If you don't believe me, take two VOMs, set one on volts, the other on ohms, touch the leads together, and you'll typically see 10M ohms (on a quality meter) and .7V (or thereabouts) on the other. Light bulb just go on for you? One meter is measuring the internal resistance of the other, the other measuring the voltage output of the first. Now set it on DIODE test and see how much it increases the voltage -- most diodes require a little more than .7V in order to pass current)

Whether using your VOM, a AA battery, or a discharged AAA battery, a slight touch to the terminals is plenty to see if the needle deflects. On another forum posting another poster runs 2 headlights, a fan, etc. through a Vette ammeter apparently without damage suggesting it's pretty bulletproof. (Although I failed to see where he tests the calibration before and after to SHOW that he didn't damage it in some way --- short of smokin' it)

>Now before you or anyone else rips my head off...

Was I gentle enough, Russ? I try not to sound condescending online but sometimes come off that way. If so my apologies.

Peterbuilt

Hi, wadenelson,
Thanks for the follow-up and I have a few questions.
Is the ammeter in my '74 a galvanometer?
Is the area in the rectangle the shunt?


Also interesting about Rogers test, was the meter still accurate after the test and why wouldn't it be?

wadenelson

[QUOTE=Peterbuilt;1598228318]Hi, wadenelson,
Thanks for the follow-up and I have a few questions.
Is the ammeter in my '74 a galvanometer?
Is the area in the rectangle the shunt?

Apology for not giving you quick SIMPLE answers but:

>Difference Between Galvanometer & Ammeter. ... The galvanometer is converted into the ammeter by connecting the resistance in parallel with the circuit. And if the resistance is connected in series with a galvanometer, then it is used as a voltmeter. The ammeter is also known as the ampere meter.

I suspect what you are really asking is do you have a voltmeter parading as an ammeter in your 1974, and from what I have read so far about early Vettes, the answer is YES. I don't think the General used "real" ammeters past the early 60's, just from my reading here and elsewhere.

Is the area in the rectangle your "shunt."
I'm going to assume that the black and black / white wires going off-screen in your schematic go directly to your ammeter. In that case, YES, what is blocked is indeed the shunt.

BUT... remember this is a schematic - a simplified drawing showing raw connectivity. Not a wiring diagram, harness picture, whatever. Something shown as an (apparent) 2" piece of "wire" running between two other pieces of wire might be 10 feet long in reality. It might in reality tie to the ENDS of one of those other two wires, splice in the middle of the second, whatever. A dozen different ways that shunt could actually be "wired" under the hood. The CONNECTIVITY is identical but the length & AWG of the actual wire will produce a certain amount of resistance-- that the ammeter is calibrated to see.

Whatever wire, wires, wiring connect BETWEEN the leads coming of the two terminals of your ammeter constitutes "the shunt." Period.

I apologize my answers are so theoretical. Hopefully someone with a similar '74 can give you a real world answer, tell you where that shunt is and what it looks like, where the ends actually connect, etc. I'm a new C3 owner and can only tell you what Electrical Engineering principles demand, not how stuff on these great cars is ACTUALLY wired. Not yet, anyway!

Cheers!