72 ammeter needle doesn't move, always on 0. I have removed the map pocket panel and have access to the ammeter.With the battery disconnected and the plug removed from the ammeter, I connected an ohm meter across the posts and it shows open. Is this a correct way to see if the gauge is bad? Shouldn't it show a circuit? With the plug disconnected is there a way to test if the right voltage is at the plug? Any other ways to test if the gauge and or the wiring is bad?

Thanks,
Dan

 

It is most likely corroded terminal at the bulkhead connector on the firewall ( where the harness plugs into the back of the fuse panel )

 

Is there any way to test that theory? Like is there a known value that I'm looking for on the black and black/white wire at the meter?

 

You are correct; If you put an ohm meter across the two posts and it shows open the amp meter is bad.
The meter is a very simple wire wound circuit and the wire is less than hair thin.

 

At the ammeter, one lead connects to the starter at the battery terminal.
The other lead connects to the horn relay and then on to the alternator.
Both the black and black/white have 12 volts at all times.

The needle going + or - indicates if the current is flowing to or from the battery.

To Bench test the gauge:
Connect a ground to the top post and momentarily connect 12 volts to the bottom post.
The needle will deflect downwards (-)
Connect a ground to the bottom post and momentarily apply 12 volts to the top post.
The needle should deflect upwards (+).



http://www.keystonestatecorvetteclub...tte%201972.pdf

 

FYI....
I repaired my gauge tiny gauge wire which was burnt thru near the terminal inside the gage housing by unwrapping one turn of the tiny wire and resoldering it to the terminal.....it worked perfectly with the original 60(?) amp alternator...

Then I got the brilliant idea to upgrade to a new high amp alternator......it toasted the Amp gauge I had just repaired a few days earlier.....(as I discovered the hard way )

 

Peterbuilt's theory of ammeter operation is correct, IF that meter were actually an AMMETER. The C3 AMP meter is actually a millivoltmeter with a current shunt that is calibrated to read amperes. It is quite different in circuit design...for many reasons. In essence, the C3 meter measures the voltage drop across a measured length of the main current wiring; that voltage drop being proportional to the current flowing thru that line. So, the two "ammeter" leads actually are imbedded in the main wiring harness and attach to the same power wire, only at different positions about 16-18" apart. The resistance of the wire in that power line becomes the SHUNT.

This allowed for using a simple and inexpensive galvanometer for the meter; it also eliminated the need for high current to flow into or thru that meter.

Finally, that meter will read near "0" amps most of the time, if your charging system and battery are in good condition. But, if you turn the ignition key ON without starting the engine, then turn on your headlamps, the current being drawn by the lamps should show as a discharge on the meter. If it does not, there is a problem in the wiring or with the meter. Since you find that measuring resistance across the ammeter leads shows an OPEN circuit, I suspect a damaged wire 'somewhere'. I have repaired a similar ammeter which had one of the [two] coil connections broken [for whatever reason]. That wire is very thin and probably just failed from long term vibration. It can be re-soldered, once you strip the varnish insulation off that tiny wire. You can also 'test' one of the dash meters using a small 1.5v battery. Just briefly (a 'touch') connect the battery across the meter terminals to see if you get any movement. Disconnect as soon as you see movement so that you do not over-drive or overload the meter. If the meter moves at all, it is likely not your problem.

 

Hi 7t1vette,
Thank you for so eloquently explaining that it's a 'galvanometer' and not an actual ammeter.
Maybe that's why it says 'battery gauge' on the dash.

The OP asked for the time of day and you told him how to build a watch!
My intent was to determine if the gauge was good and then troubleshoot further if necessary.

!2 volts will not kill the meter but yes, a battery will work.

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

 

I want to thank all of you that have given me some greater insight into the workings of this ammeter problem. It appears that the gauge is at least one of the problems. Following the comments and the wiring diagram, tomorrow I'm going to test if I have 12 volts at both the black and the black/white ends of the plug to make sure that part of the operation is functioning properly.

Thanks again
Dan

 

Knowing how things really work can help folks make better decisions when trying to solve 'problems'. If my posts are too long and have too much content, you can easily bypass them.

 

easiest option...

 

So, I have 12 volts on both wires going to the gauge. Time to order a new gauge.

 

Could the connector on the back of the ammeter be pulled off the contacts or have a 'dirty' connection? Since you have to remove the old one to replace it, why not open it up to see if the problem has a simpler fix.

 

I did the same thing....except the brilliant idea part ...works fine