Changing The Ammeter Gauge To Newer Voltmeter

After redesigning and building new wiring harnesses for my Corvette, the old ammeter gauge wasn't really working. I did not really know how it was supposed to work in any case. If I understood correctly, it just showed which way the current was flowing: to or from the battery. With the new harnesses it always stayed dead center, so it wasn't really that useful. A voltmeter on the other hand would be much more informative as I could see how the alternator was working.

1976 Corvette is the last year that uses the ammeter and the 1977 model already has the voltmeter. I bought an used gauge a few months ago and decided to switch them. I heard that the sizes and shapes should be exactly the same but I would need to drill one hole to the back of the gauge panel. This information proved correct and one new hole later, the new voltmeter gauge was installed.

Then the troubles started. I reassembled the console and tested the gauge. A few hours of testing, fiddling and Googling later I learned that the voltmeter must be connected exactly correct or it won't display right voltages. I found an informative page from "European Corvette", that explains what's needed in the gauges. And this is needed for the voltmeter:

• Lowest connector must have the positive wire.
• Lower and upper connector must have the correct plastic resistor (with light blue sticker) between them. This resistor should have 125 ohms.
• The third connector is ground. No wire is needed as the holding nut should touch the body of the console.

With these points in mind, I went through the wiring diagrams. Most likely the wiring junction in the engine compartment would be the best place where to add the voltmeter positive wire. Then the gauge would show the true alternator voltage. Unfortunately then there would be voltage in the wire all the time. I needed this to happen only when ignition was on. Luckily this was easy as all the pink power wires for the other gauges work just like that. So I just added a 10 centimeter wire from the oil pressure gauge to the voltmeter. After all the work, this is how it looked:

I turned the ignition on and the gauge seemed to work. I checked the battery with a multimeter and got a bit over 12 volts. Unfortunately the fuel gauge didn't seem to work even after I changed it too. It is still showing too much. I'll have to investigate it further.