Ammeter to voltmeter conversion
I am getting ready to order a wiring harness kit for my 68 barracuda. I am finding out I need to change from an ammeter to a voltmeter. How do I do that while keeping the stock gauge and have it read correctly
That is a very wise thing to do. I have replaced 5 a-body wiring harnesses, and each one showed some heat damage on the firewall "alternator in" and "battery out" contacts. On 2 of them, the plastic was melted to the point of allowing a short to the adjacent contacts. The problem can only get worse over time. Also, the new alternators and the newer remanufactured alternators have higher maximum current output, so, these days there can be even higher charging current through the firewall connector contacts, and those contacts are already over 50 years old. It could get even worse if high current lighting or audio were added with their power still routed through the original firewall connectors (not the right way to do it, but it happens).
As you know, the need to convert from an ammeter to a voltmeter is driven by removing charging current from the firewall connector contacts. As many others have posted, there are several ways to convert the dash ammeter to a dash voltmeter. I recently called Mr. Heater Box, and asked them if they offer a ammeter to voltmeter conversion. He replied that they don't currently, but they do have one under development, and it should be available within a few months. I have had good experience with Mr. Heater Box services and products.
Not your question, but I can't resist adding some thoughts you might find helpful, or annoying, or someone else might find annoying:
There are several ways (and even more opinions on) how to bypass the firewall connector to get charging current from the alternator to the battery. In any of those options, a new, properly sized charging wire should be run from the alternator to the battery, and the fusible link (or equivalent fuse or breaker) still needs to be there near the battery. To avoid problems, You should disconnect the original alternator wire from both the alternator and the firewall connector. Same for the battery charging wire, disconnect it from both the starter relay terminal and the firewall connector. Do not reuse those original wires/connections.
Another option, if you don't mind the appearance, it is easy to add an aftermarket voltmeter mounted under the dash, powered from an "accessory" power source. The voltmeter will read over 13 volts when the the battery is charging. Ammeter just sits there, doing nothing.
To get power to to the dash, I run a new, separate wire from the alternator (separate from the new battery charging wire), and route it through a bulkhead feedthrough that I install in the firewall. It is very convenient to connect that dash power to the "alternator" stud of the ammeter (where the original black dash harness wire connects to the ammeter stud), so that's where I make the connection. Leave the original black dash harness terminal connected to that stud, because that's how power gets routed everywhere. Some people add a fuse or circuit breaker in the dash input power line from the alternator. The new wire must be sized properly.