I have a 69 corvette. The problem is with the amp gauge , it reads at +20 even with the engine off and with the battery disconnected. The gauge does move when charging or discharging. I’m going to replace the gauge but could I be missing something.

 

I agree with your diagnosis, no battery connected, should read zero.

 

Most likely that gauge has been subjected to an instantaneous, very high current spike (discharge). As a result, the needle hit the low-end stop on the gauge...but the gauge mechanism kept on going. When all was done, the needle has shifted to the positive side and the gauge reads +20 amps offset from the actual current being passed thru the circuit.

You can experiment with readings from the gauge by 1) turning on headlamps (should be approx 12 amp discharge...your gauge might read +8 amps); 2) turn on headlamps and go to hi-beams (appros 20 amp discharge... 0 amps on your gauge); 3) start the engine after running the lamps without engine running [not charging battery]....your gauge should read somewhere more positive than the +20 reading when 'off' [+30 amps?].

If that checks out, you could pull the gauge and just re-set the needle to 0 [zero] instead of the +20 it presently reads. {You must hold the spindle in position when you remove the needle so that it doesn't rotate prior to the needle being reset to zero. There is no resistance to moving the needle when there is no signal being read.}

If that does not check out, replace the gauge.

P.S. You should be aware that the ammeter is really just a galvanometer that measures millivolts from the current shunt imbedded in the engine wiring harness. The gauge never actually 'sees' the true current flow in the harness.

 

IF you are not going for NCRS points I would get a voltage meter. It will be more helpful if your alternator dies and you dont realize it until its too late. I resisted for a couple of years until I blew it up when my wiring harness fried. I forget how i screwed that up but Im definitely glad I switched to the volt meter. Check Willcox first then on Ebay, some of the venders have online stores on there and you might save on shipping

 

Those who don't know the difference between the ammeter and a voltmeter...and how much more info you can glean from the ammeter in your car...just throw it out with the bathwater.

 

Theoretically they can provide some information, if they worked, and you had one for every seperate system in your car. BUt they dont, and they arent and they were replaced in the 70s by voltmeters

 

Basically because MOST folks just didn't get it. When they just "stay at zero most of the time", I guess they just lose interest. How many owners actually watch the ammeter (if they have one) immediately following engine start and for a few minutes thereafter to assess the integrity of battery capacity? How many watch it when at idle to assess the charging system?

The list goes on and on. Once you know what an ammeter can do for you, it's almost automatic to check things out as they occur during a drive.

Oh....a voltmeter. Yes, it lets you know how many volts are in the main system......