This may be a long shot and is more likely on positive ground cars.....Many years back I purchased a positive ground Ford where the ammeter read backwards. The owner said not to worry, that was just the way the gauge worked - stupidly I believed him. After some time I noticed that the battery was installed backwards! I switched the battery to the proper positive ground and the ammeter then read correctly. Oddly I never noticed any difference in the performance either way. If I hadn't experienced it I would have thought the car would not run with a backwards battery but it did.