I’m working on a 1975 914. I just replaced a non working volt meter with a working period correct vdo volt meter. When the car is shut off the volt meter is reading the voltage.
Is this correct? Is the volt meter supposed to be switched on with the ignition or does it always show the voltage? I’m concerned that it will slowly drain the battery. The car has the stock wiring harness and originally came with a center gauge package.

Not wired correctly, showld not be on when ignition turned off.

Yeah, definitely not on. It's best to rewire it anyway. Unless you are interested in watching it bounce around when you hit the brakes or turn signals. It really is totally worthless otherwise.

I'm wondering if you have it plugged into the clock wire by mistake. That one would be powered all the time.

That’s a very valid point! I’m going to check. That could definetly be the problem since the clock isn’t working!

Bingo. That fixed one problem. The clock wires and voltage gauge wires were incorrectly plugged in and reversed.
Now the volt meter only works when the ignition is on.

New question:
When I turn on the headlights the volt meter drops to zero. I measured the voltage at the battery and it’s still roughly 13.7 volts so I know the charging system is working.
What would cause the volt meter to drop to zero when the lights are on?

The volt meter could just be inaccurate- not uncommon. Mine got less and less accurate right before it stopped registering anything.

As I said, with the original wiring it is worthless. I put a relay in the car that connects it to straight battery voltage and now it works great. I used the original wires to trigger the relay.

It sounds like you've hit on the wrong wire. Find a fuse maybe fuse 9 which also powers the GEN light. I believe this one turns off.

The positioning of the voltmeter in the 914 wire harness is not optimal. When lights, ventilation fan are on, the voltmeter likely will read a bit less than the battery. It's only use is as a rough estimation of battery voltage. As some say, it's just for vanity. For me, it gives a good indication of battery voltage when I first turn the car on and running no lights or fans.

Crappy voltmeter is one of the reasons (space being the main driver) that I removed my console entirely. I got tired of seeing bad (and fluctuating) readings while driving around.

The final straw was when my big feet popped a fuse out without my knowing it and I thought I was stranded out in the middle of nowhere at night, trying to decide if I should chance the long drive through nowhere to get where I needed to be (clued in with no GEN light, I finally figured it out).

For me, the idiot lights and the fuel/temp gauge combo work perfectly fine to tell me when it's time to park the car.

I have actually found the volt meter (functioning as I have wired it) actually is a little helpful. Before I updated my regulator to solid state (and an optima battery that can take the voltage) I noticed that it dipped dangerously low with lights and wipers and heater all going. So low that I bet the battery discharge would have eventually caused the car to stop. If I were still running an old analog voltage regulator, I would want to know the details. Maybe the gen light would tell you about running a deficit but I suspect by the time it turns on from low battery charge, things might be too far. It's more of a "your alt belt just broke" kind of thing.

Every once in a while I try to drive blind, but eventually I just want to know what's going on and put the 'super console' back in (on both of my fun cars). I think I approach this more like a pilot. They have no problem with tons of gauges letting them know what is up, and what may likely cause them to come down.

Then I suggest you will absolutely hate my aviaton treatise from circa 2000...

The "Minimalist's Panel" Theory of Equipment and Operation
https://gatm.org/flying/minimalist.html

(As an aside, to each his own of course, but the one alternator failure I've had in the 914 did not show up on the voltmeter, but it did show up on the GEN light. I can explain why if you're interested.)

"Perfection is finally attained not when there is no longer anything to add, but when there is no longer anything to take away." - Antoine de Saint Exupéry

"More gauges?? I don't want the monkey distracted by gauges, I want the monkey to be driving the damned race car! Tell you what, I'll give the monkey two big lights: an orange one telling him to shift and a red one telling him to park the damned car! Otherwise, shut up and drive." - tGA

Yeah, simplicity has it's attraction, but so many noncatastrophic failures ignored lead to catastrophic engine failure. I would rather be able to get in front of them.

I like the voltmeter and even its faults. Mine actually matches pretty close to what I read at the battery but does dip when load is placed on the system. I kind of like that because it gives me a rough idea of the load. Sure, an ammeter would be better for that but I have the voltmeter.

A couple weeks ago my alternator light came on when I was 250 miles from home and the sun going down. Knowing the "normal" behavior of my voltmeter and monitoring closely helped me decide whether to soldier on or find a hotel. Revving the engine showed the needle moved up slightly so I knew I was getting at least a little juice out of the battery. The voltage held steady at ~11.8 for nearly a hundred miles. Then I had to turn the headlights on. I've converted them and all bulbs to LED so the draw wasn't bad and the voltage dropped about another 2/10ths. That held for another hundred miles so I decided to continue the last 50 to home. Voltage held the rest of the way and I rolled into the garage at 11:30 pm.

A shorted wire found later between the armature and commutator ring leads me to believe I developed a high resistance leak to ground on my trip home which later became a full short and fried the wire. Having the voltmeter made the difference between being stranded 250 miles from home and getting home and making repairs in the convenience of my own garage.

That actually demonstrates why the original wiring was not optimal. Your load you were seeing wasn't the same load the battery/alternator were seeing. It was some weird load world within the dashboard and light systems.if you had seen the actual voltage at the battery you would have had a better sense of how your charging system was keeping up. Before I fixed mine I would get predictable (and scary) drops in the volt meter from the headlights and fans etc but stuff like the brake lights and turn signals would make it fall as much as the headlights. Pretty soon I would be at 10 or 11 sitting at a stop light.

Now my voltage drops as I add load, but not when I hit my brake pedal or turn signals. Those don't pull enough power to make a meaningful load. Now with everything on I go from a 13.8 to maybe a 13.2 on the volt meter.

I always found it very weird that the voltmeter dropped when using the turn signal and even more when braking simultaneously. When I put on the the ventilator it drops to 10 volts. So I think it's a good idea to connect it directly to the battery using a relay. After almost 7 years of ownership I still learn a lot. Thanks for the tip.

Connect the voltmeter to the point you want to measure. If that is the battery, run a wire thru the tunnel and up to the positive battery post. Make sure it has a good chassis ground, or run a second wire back to the negative post. 26 or 28 gauge stranded will work fine, there is virtually no current load. For that same reason, you can leave the voltmeter connected all the time. Check it against a good DVM for accuracy.

Then if you are troubleshooting a problem, connect the DVM to the load point, measure voltage and compare to the battery meter.