Ammeter not working - Thoughts??
Thread Starter
Racer
Joined: Apr 2005
Posts: 484
Likes: 5
From: Madison Wisconsin
Ammeter not working - Thoughts??
I am doing a frame off restoration on a '73 convertible. When I purchased the car the previous owner had recently replaced the alternator, but I noticed the ammeter was not working. During the restoration I chose to replace all the wiring harnesses with Lectric factory harness, (expensive, but very high quality). I also replaced the ammeter. When I started the engine the gauge never moved from -40 amps. I tested the wires at the gauge and I get battery voltage (about 12.47 volts) on both terminals when the engine is off. If I start the engine, I get about 14.77 volts on both terminals. Should I have the same voltage across both terminals? If not, any thoughts on why this might be the case? I tested the resistance across the gauge terminals and it reads zero. I did try putting a AAA battery cross the gauge terminals and I get no movement regardless of the polarity. Any thoughts on what might be wrong? Bad gauge?
I really appreciate any advice.
Thanks,
Jim
I really appreciate any advice.
Thanks,
Jim
Drifting
Joined: Oct 2005
Posts: 1,615
Likes: 150
From: Forth Worth TX
You say the gauge reads -40 amps all of the time. Wouldn't/shouldn't a properly working gauge (even if disconnected) be centered at zero (0) to start with?
Last edited by Revi; Oct 15, 2014 at 11:16 AM.
Melting Slicks
Joined: Sep 2004
Posts: 2,655
Likes: 19
The way the ammeter works is that it's wired into the harness at 2 points. It uses the harness as a shunt resistor. Very little current flows through the gauge, but it should always show 12 V at both posts.
Too much current through the ammeter (incorrect wiring) will fry it.
What does it read with 1) everything off, including key, 2) key in run position, engine off, 3) engine running
Too much current through the ammeter (incorrect wiring) will fry it.
What does it read with 1) everything off, including key, 2) key in run position, engine off, 3) engine running
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,116
From: Crossville TN
Garys 68 is correct. There will be approximately the same voltage on both terminals of the C3 ammeter. There is very little current that flows through the ammeter...only that required to move the needle of the meter. There is a small voltage drop through the harness wire between the two connection points. (That voltage drop is 'proportional' to the current flowing through that wire...hence, the ability to get a gauge reading in AMPS.)
If your gauge is properly installed/wired and the harness is wired properly, then there should be more than 40 amps being drawn through the harness wire. If the battery is somewhat low, that would not be a surprise. But, that level of current draw should reduce as the battery is being recharged. If you find that it maintains that level of current draw, there is likely some problem with the wiring. Since the meter is 'working', it is not likely to be a problem with the meter.
If your gauge is properly installed/wired and the harness is wired properly, then there should be more than 40 amps being drawn through the harness wire. If the battery is somewhat low, that would not be a surprise. But, that level of current draw should reduce as the battery is being recharged. If you find that it maintains that level of current draw, there is likely some problem with the wiring. Since the meter is 'working', it is not likely to be a problem with the meter.
Advanced
Joined: Oct 2010
Posts: 80
Likes: 2
From: near Nuremberg Bavaria
While I'm not absolutely sure about a '73, that's not quite correct for ammeters ca. around '70. Here, there actually flows a not insignificant amount of current through the device. Someone here in this forum has measured the current required through such ammeters in order to produce a full scale (40 Amps) deflection: It's 2.2 Amps.
That's true again. At least, almost. There is a voltage drop of approx. 1/10 of a Volt, which is fully compatible with the observation mentioned above, because the internal resistance of such ammeters is very low.
To your immediate problem: The first thing I'd do is disconnect the ammeter and measure across it's terminals whether it has continuity. Then go from there.
Melting Slicks
Joined: Feb 2001
Posts: 3,330
Likes: 13
From: Europe , Luxembourg
Hi
2.2 Amps can be considered as very minor in regards to it's reading of 40 Amps. Guess this is what was meant .
My gauge was internally fried. The amp-meter coil wiring has a like internal fuse that melts ( thinner cable ). Guess this is to avoid high current flow through this amp-meter wiring in case the main wire is disconnected on purpose or accidentally.
I soldered a wire in and the gauge works fine again.
If your gauge shows -40 all the time, it could be the pointer that is installed incorrectly. Pull it off and reinstall it 12 o'clock position.
Rgds. Günther
2.2 Amps can be considered as very minor in regards to it's reading of 40 Amps. Guess this is what was meant .
My gauge was internally fried. The amp-meter coil wiring has a like internal fuse that melts ( thinner cable ). Guess this is to avoid high current flow through this amp-meter wiring in case the main wire is disconnected on purpose or accidentally.
I soldered a wire in and the gauge works fine again.
If your gauge shows -40 all the time, it could be the pointer that is installed incorrectly. Pull it off and reinstall it 12 o'clock position.
Rgds. Günther
Thread Starter
Racer
Joined: Apr 2005
Posts: 484
Likes: 5
From: Madison Wisconsin
Guys thanks for all the great information. sorry I did not respond sooner. I was out of town for the last week. I suspect my gauge is either stuck or the pointer is off because it is always at -40 amps regardless of whether the key is off, on, or engine is running. The battery is very new and I keep it on a battery tender so it should be fully charged. I'll pull the gauge and check the movement.
Thanks again,
Jim
Thanks again,
Jim
