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  • Does anyone have a functioning ammeter?

    Topic originally created by Zanny1 on May 8, 2006 9:22 PM and viewed 2839 times in the old forum.

  • #2
    Originally posted by Zanny1 on May 8, 2006 9:22 PM
    My ammeter has never moved since purchasing the car. Installing a temporary replacement still yielded no movement. The alternator is charging the system based on a shop test. I have heard that these ammeters never were very good, but believe the solution may involve more than a corrctly functioning instrument. Any ideas from you guys? Thanks Mike

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    • #3
      Originally posted by Mike Meehan on May 8, 2006 9:28 PM
      Mine has always worked,but now Ill be looking at it more regularly.I have heard that on many Ferraris the gauge will often fail. Mike

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      • #4
        Originally posted by Bryan on May 9, 2006 10:33 AM
        Mine works fine.

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        • #5
          Originally posted by shross on May 10, 2006 1:58 AM
          Mine failed when the alternator crapped out last year. Couldn't find a replacement so we put in a voltmeter from a TR (I think). Works fine, just gives volts instead of amps now.

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          • #6
            Originally posted by Aaron on May 10, 2006 9:33 AM
            My amp meter works very nicely and indicated clearly the low frequency oscillation due to the old, since replaced voltage regulator. (there lies a story about headlights that fluctuate.) Is should be noted that the amp meter in the C4 does not work like the typical amp meter found at Kragen auto supply. I actually think it is a very robust and much more professional setup. The amp meter in the car is actually a volt meter that measures the voltage across a low value resistor. This low value resistor, often referred as a shunt, typically is a metal bar with connections at both ends. Mine is mounted behind the “relay array panel” in the engine compartment. I have never had the shut out or measured it, but I would guess it is 0.1 ohms. It is about 2 inches long and ¾ wide. CAUTION - Be very careful about touching metal objects to the shunt, it is a direct, non-fused connection to the positive terminal of the battery. Metal contact between the shunt and the chassis will cause bad things to happen – melting, sparks, burnt wires, holes is things….. If you are messing around near the shunt – disconnect the battery. The risk of slipping with a wrench is too high. The way the meter functions – the shunt is in series with the positive battery connection. Electric current traveling to or from the battery passes through the shut. Because the shunt has resistance, there will be a voltage drop across it. (Voltage equals Current x Resistance. (ohms law V=IR)) There are two wires connected to each end of the shunt that also connect to the amp meter. The amp meter actually measures the voltage across the shunt. The shunt and amp meter are calibrated together such that the needle indicates the amps flowing through the shunt. CAUTION - be really careful of the wires that got to the amp meter. Both of them are also direct connections to the positive battery terminal. These wires are always hot and will cause bad things to happen if an end terminal is touched to metal chassis parts. I always disconnect the battery if I’m messing with any of the instruments in the consul cluster. It just is not worth the risk of shorting an amp meter wire. It should be noted that high current to the starter motor does not go through the shunt, and is not measured by the amp meter. If you meter is not working – My first guess – did someone disconnect a shunt wire out of ignorance. Most people do not understand what that “silly metal bar” is all about Second – the amp meter is really a very sensitive volt meter - If someone tried to connect it up like a “Kragen Unit” routing current directly through it. The meter will most likely have suffered damage – possibly burnt wires. Sorry I don't have more info on debugging the meter - I just have not needed to fix it yet. Aaron

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            • #7
              Originally posted by Zanny1 on May 13, 2006 9:46 PM
              Thanks Aaron. As always, your detailed explanation makes much sense. When I was testing the ammeter, I mearly installed one (from an old British motorcycle! in a temporary manner to look for an indication. I will test the shunt and have the ammeter checked by a specialist next time I have a chance. At some time during the cars history, the hot wire going to the ammeter touched the chassis adjacent to the instrument panel. I could see the contact point when dismanteling the instrument pod. Your advice about disconnecting the battery is right on the mark. Thanks Mike

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              • #8
                Originally posted by JeffLit on June 27, 2012 7:01 AM
                I'm debugging my recalcitrant amp gauge at the moment. It hasn't moved from zero in a number of years now. I pulled it today and determined that:
                a) both wires going to the gauge have 12v+
                b) the gauge moves when low volts are applied to it (I used a AAA battery and it moves in either direction depending upon polarity applied)
                So, that seems to point to the shunt? I tried to measure resistance across the shunt by using my ohms meter on the two wires at the gauge but my meter is old and adjustable and the resistance measured seems to the be same as if I just touch the two leads of the meter directly together. I haven't pulled the relay panel yet to visually inspect/test the shunt.
                Just checking with the electric brain trust to make sure my tests have some validity and that I'm barking up the right tree. Also wondering if anyone else has experienced a shunt issue. Seems like a simple piece so I am suspect of it failing in this way -- I'd be less surprised to see no voltage/current going across it than to see no resistance.

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                • #9
                  Originally posted by Aaron on June 29, 2012 10:13 AM
                  The amp meter works by measuring the typically small voltage that occurs accros the Shunt (A resistor with a very small value) when electrical current is traveling through the shunt.
                  The math >> Voltage = Current x Resistance. I do not know the resistance of the shunt, I'm betting it is around 0.01 Ohms. In this case, 20 amps of current would generate a voltage of 0.2 volts.
                  I imagine that a 1.5 volt batter will peg the meter in either direction quickly and I actually would not recommend doing this.
                  The shunt is placed between a battery terminal and all electrical devices such that any time an electrical device uses current or supplies current to or from the battery, that current must pass through the shunt. and generate that small voltage from one end of the shunt to the other, which is measured by the meter. (The exception is the Starter main windings which is direct to the battery. We never want any resistance between the starter main windings and the battery.) In our case one side of the shunt is connected to the positive side of the battery and the other side is connected to the electrical devices. When no current is running through the shunt, there will be no voltage accross the shunt and if we measure the voltage at the two terminals on the meter, they will both measure the positive battery voltage. In the case were we turn on a bunch of stuff and current is running "Out" of the battery, say 20 amps from the above example, and we measure the voltages on the back of amp meter, the terminal connected to the battery side of the shunt will measure battery voltage. The terminal connected to the electrical devices side of the shunt wil measure battery voltage minus 0.2 volts.
                  So to the problem - Because the voltage created accross the shunt is very small, it is important that the connections between the wires going from the shunt to the amp meter have REALY GOOD connections. Any corosion, loose solder joint, slopy terminal, deegraded wire .... due to the very small voltage the meter is trying to read, will cause the meter to just sit at 0. So this is where I would start, (My amp meter quit a while back as well and it was due to a dirty connection on the shunt)
                  Disconnect the battery first - this is important becuase the shunt is tied to the positive side of the battery and ground is everywhere else around you.
                  On the shunt, find the two wires that connect the shunt to the amp meter. unscrew the connections and clean them - I used 600 grit sandpaper, steel wool and a very fine wire brush. Doing this will likely solve the problem.
                  I have found the meter movements to be really top quality and tend not to be the problem.
                  Hope this helps

                  Aaron


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                  • #10
                    Originally posted by Ian Levy on June 30, 2012 7:11 PM
                    Hi Aaron
                    As usual thanks very much for your immaculate posting.
                    My ammeter has never moved much if at all but as the alternator is charging I never bothered.
                    I it missed out when the car was in bit & never seen the shunt as I never removed the relay panel.
                    It will be on my winter to do list & thanks again
                    Best regards
                    Ian L

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