kmead
Old enough to know better
The issue I have noted is the meter is showing the voltage in the switched wiring loom off the ignition switch as opposed to the voltage at the battery. This is evidenced by the changes you will often see when you: use the turn signal (the gauge will go up and down in time to the flashes), turn on the HVAC fan, press the brake pedal, the wipers and a few other actions.Dr.Jeff you are exactly right, I worked on Fiats for 27 years, I have a 79 X1/9 and I tried everything I could think of to get the electrical gauge to read properly, I ran a ground wire directly to the gauge, I wired a positive directly to the gauge but it still behaves the same, at this point I say, don't worry about it, check your charge rate at the battery and if it's good carry on !
Ultimately to get a good indication on the gauge, a dedicated wire should run from the positive of the battery to the gauge (this would likely need to be on a relay controlled by the ignition switch to not run the battery down).
This is one of the reasons I am changing most of the loads to large relays to switch the ‘switched’ circuits and off load some of the other loads to relays for the wipers, headlights, starter and a few others. I am hoping removing load from the ignition switched part of the loom will offer enough power to have the gauge indicate more correctly. I did say ‘more’