I also did the ammeter to voltmeter conversion. I used the guts out of a sun voltmeter gage, and the ammeter gage face and pointer. I cut the volt gage pointer off and glued on the original ammeter pointer. I was able to adjust the potentiometer on the new volt gage with its lighter ammeter pointer needle.
If you go this route, you will have to grind off a spot on your rallye gage housing so the bottom half of the gage doesnt hit and allows full swing of the gage. This is shown shaded in red in pic#3. I also added in a 12V led discharge light because i am running a denso mini alternator, and there is a spot in the back to wire in a discharge/fault light.
How i wired the gage in, is where the positive leg of the gage comes through the back of the housing , i enlarged that hole for a plastic grommet, and some plastic washers to insulate it. On the back side i ran a wire from the 12V+ stud of the gage , and jumpered it to the 12V+ feed lug coming into the cluster at the 12V+ feed going into the fuel gage.
The negative stud off the volt gage i cleaned the gage housing to bare metal so it would ground to the housing.
(Remember the gage housing is negative ground to the dash frame )
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