Ammeter
Thread Starter
Racer
Joined: Mar 2011
Posts: 359
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From: Elma NY
Pro
Joined: Nov 2016
Posts: 729
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From: Kelowna British Columbia
Why not? Use a battery and a known load. Preferably adjustable resistor. If it is a 30 amp guage then you could get a 1 ohm resistor and two batteries in series = 24 volt that should read 24 amps on gauge. Then 12 volt battery with 1 ohm load - 12 amps etc. Then 12 ohm load and 12 volt battery= 1 amp.
If not sure go into an electronic shop and they should be able to do it for you.
If not sure go into an electronic shop and they should be able to do it for you.
Drifting
Joined: Mar 2011
Posts: 1,347
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From: Kernersville NC
Can you explain more? How long before it goes back to "0"? If it is a few minutes that is pretty much what it is supposed to do. If it goes to a + charge and immediately falls back to "0" then something might be amiss. What happens if you turn on your headlights?
Melting Slicks
Joined: Mar 2005
Posts: 2,093
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From: Midlothian VA
When you first start the car the meter should read + for a bit until the drain on the battery from starting the car is replenished.
Once the battery is topped off the needle should sit right at/or just a bit to the right of center = 0.
If you turn on the head lights with the engine off you'll see the gauge deflect well into the negative - indicating that the battery is being discharged.
Last edited by Hammerhead Fred; Nov 25, 2017 at 09:32 PM.
Le Mans Master
Joined: Aug 2006
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From: mount holly NC
2025 c3 ('74-'82) of the Year Finalist - Unmodified
2019 C3 of Year Finalist (appearance mods)
To bench test your battery gauge:
Connect a ground to the top terminal and momemterally apply 12 volts to the bottom terminal,
Meter should deflect to minus.
Reverse the wires and the needle will deflect to positive.
Connect a ground to the top terminal and momemterally apply 12 volts to the bottom terminal,
Meter should deflect to minus.
Reverse the wires and the needle will deflect to positive.
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,114
From: Crossville TN
The C3 Corvette ammeter is not a 'direct reading' gauge. In other words, it doesn't measure AMPS directly; it measures the voltage drop over a fixed length of the main power wire in the engine harness. If you send significant current into that gauge, you have a good chance of wasting it.
To test it, just use an ohmmeter to check that it is not "open circuit" or get a AAA battery and just make a very quick [touch] contact with the ammeter terminals to see that the meter has movement. Reverse the leads if you want to see it move in the other direction (but that isn't really necessary).
The ammeter is one of those gauges that either works...or it doesn't. It does have AMPS as the scale measure, but it actually 'reads' millivolts.
P.S. A wire of known type and size has a specific "resistance per foot" value. Knowing that value, one can calculate how far apart the ammeter leads need to be (wire then becomes a 'shunt' for the meter) when attached to that wire. Cheap, easy, and reasonably accurate for its intended purpose....
To test it, just use an ohmmeter to check that it is not "open circuit" or get a AAA battery and just make a very quick [touch] contact with the ammeter terminals to see that the meter has movement. Reverse the leads if you want to see it move in the other direction (but that isn't really necessary).
The ammeter is one of those gauges that either works...or it doesn't. It does have AMPS as the scale measure, but it actually 'reads' millivolts.
P.S. A wire of known type and size has a specific "resistance per foot" value. Knowing that value, one can calculate how far apart the ammeter leads need to be (wire then becomes a 'shunt' for the meter) when attached to that wire. Cheap, easy, and reasonably accurate for its intended purpose....
Last edited by 7T1vette; Nov 26, 2017 at 05:34 AM.
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Thread Starter
Racer
Joined: Mar 2011
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From: Elma NY
Melting Slicks
Joined: Mar 2005
Posts: 2,093
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From: Midlothian VA
-40 with the engine running means your battery is discharging.
Might want to verify that the alternator is in good working order (voltage regulator too depending on year).
NOTE: Would be helpful if we knew year model etc. when you post
Last edited by Hammerhead Fred; Nov 26, 2017 at 04:10 PM.
Thread Starter
Racer
Joined: Mar 2011
Posts: 359
Likes: 13
From: Elma NY
Amp
If the gauge moves when you put the AAA battery on it then there's nothing wrong with the gauge.
-40 with the engine running means your battery is discharging.
Might want to verify that the alternator is in good working order (voltage regulator too depending on year).
NOTE: Would be helpful if we knew year model etc. when you post
-40 with the engine running means your battery is discharging.
Might want to verify that the alternator is in good working order (voltage regulator too depending on year).
NOTE: Would be helpful if we knew year model etc. when you post
Thanks
Thread Starter
Racer
Joined: Mar 2011
Posts: 359
Likes: 13
From: Elma NY
Melting Slicks
Joined: Mar 2005
Posts: 2,093
Likes: 297
From: Midlothian VA
BTW 15v at the alt is a bit high.
You might check that the sensing wire on the double plug at the back of the alt isn't broken and/or disconnected.
Burning Brakes
Joined: Mar 2013
Posts: 750
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From: Springfield Missouri
If your ammeter is not showing a discharge with the engine running it is working properly. As with many gauges, except fuel and volts, center deflection is desirable. An ammeter, for all practical purposes, is monitoring system load while a voltmeter, in this case, is monitoring the alternator's charging ability.
In other words if you turn on the headlights, assuming the vehicle is running, a voltmeter might only flicker and the voltage will remain the same. The needle on an ammeter will increase slightly and remain there because of the added load.
In other words if you turn on the headlights, assuming the vehicle is running, a voltmeter might only flicker and the voltage will remain the same. The needle on an ammeter will increase slightly and remain there because of the added load.
Thread Starter
Racer
Joined: Mar 2011
Posts: 359
Likes: 13
From: Elma NY
Amp
If your ammeter is not showing a discharge with the engine running it is working properly. As with many gauges, except fuel and volts, center deflection is desirable. An ammeter, for all practical purposes, is monitoring system load while a voltmeter, in this case, is monitoring the alternator's charging ability.
In other words if you turn on the headlights, assuming the vehicle is running, a voltmeter might only flicker and the voltage will remain the same. The needle on an ammeter will increase slightly and remain there because of the added load.
In other words if you turn on the headlights, assuming the vehicle is running, a voltmeter might only flicker and the voltage will remain the same. The needle on an ammeter will increase slightly and remain there because of the added load.
Burning Brakes
Joined: Mar 2013
Posts: 750
Likes: 180
From: Springfield Missouri
Although uncommon, I have seen a solenoid short to ground internally, usually they show an open circuit when faulty.
Thread Starter
Racer
Joined: Mar 2011
Posts: 359
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From: Elma NY
Amp
That indicates to me that you have a direct short to ground. I would start by removing the negative battery cable and the disconnect that alternator wiring and reconnect the battery cable. If that solves the problem you have a bad alternator. If not pull the wiring off the starter solenoid and continue from there. Always pull the negative battery cable to keep from harming the battery and starting a fire.
Although uncommon, I have seen a solenoid short to ground internally, usually they show an open circuit when faulty.
Although uncommon, I have seen a solenoid short to ground internally, usually they show an open circuit when faulty.
Burning Brakes
Joined: Mar 2013
Posts: 750
Likes: 180
From: Springfield Missouri
Le Mans Master
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From your description I would suspect the leads running from the alternator via the horn relay to the starter, and the parallel path used for the ammeter. It's a rather simple circuit so it shouldn't be terribly difficult to ascertain where the fault is. You can try wiggling the wires (gently) while the engine is running to see if you get an ammeter response, or some deflection. You can search for the fusible links to get an idea where to wiggle(gently). Just be ready to secure the engine and disconnect the battery in case you get some sort of direct short.
Here is a link for where the location of fusible links for my 68 are--in case it might help you:
https://www.corvetteforum.com/forums...locations.html
Here is a link for where the location of fusible links for my 68 are--in case it might help you:
https://www.corvetteforum.com/forums...locations.html
Last edited by carriljc; Nov 28, 2017 at 12:44 PM.
Burning Brakes
Joined: Mar 2013
Posts: 750
Likes: 180
From: Springfield Missouri
I missed this earlier. A bad ground at the gauge cluster has ne bearing here since neither side of the ammeter is at ground. Do you get a decent spark when you remove the negative battery cable? If so that is an indication of something at ground that shouldn't be.