Pinto Car Club of America

I have the vent gauge set in my '80 hatch that has oil, amp & temp. I was going to use them for now with my basically stock 5.0 since they work fine. Problem is the amp gauge never goes right of center but it does show slight discharge when running & drops farther with the lights on. The battery never goes dead & the lights arent dim. Is this common with the stock gauge or is it junk?

When did you notice the problem?  This sounds strange, almost as if the gauge is hooked up backwards (providing alt is working).   ???

Put a voltmeter across the battery to verify around 14V when running.
Common failure mode for ammeters is that the needle does not move or not enough. I never heard of them reversing direction.

I found that a Pinto will go a long way on a non-charging battery.  The only thing that signalled me that my alternator died was that the electric choke was not opening and the engine ran like crap. 

Do check the alt. output, both with and without lights on. 

Gauge hooked up backwards is the first thing I thought of....

Im going to have to check it with a meter. I know its not backwards since it reacts normally but only on the discharge side. Im wondering if my alternator is weak & only putting out 12 volts.

With the engine off, turn the lights on and see which direction the needle moves.
If in the same direction (to discharge), I suspect alt, regulator, or miswiring.  The latter can be eliminated if you did not modify wiring prior to this issue.
 
Yes, a weak ALT can cause this, too.  Let us know what the voltage checks at.