1977 280z Voltmeter only reads while engine is running.

[ckurtz2]

I have a new issue developing on my 1977 280z coupe.

I am running a 280zx internally regulated alternator that is about 7 months old, as has been functioning properly.

Anyways, the other week I got in the car and noticed my voltmeter on the dash was not working. I tracked it to a blown fuse on the passenger kickpanel, so I replaced it. I then started to notice the voltmeter would bobble slightly at idle which was odd.

Today I came out to the car and noticed the voltmeter was not working again. I immidietly checked the fuse and it was fine. So I turned the key to the on position and the voltmeter would bobble a little, but it still was showing 0V. I started the car and the voltmeter swung right to about 15V and stayed there. So the voltmeter only reads while the car is running. 

Any ideas what these symptoms might be leading to? 

[SteveJ]

Do your hazard lights work with the ignition off? They should be on the same circuit as the voltmeter.

[ckurtz2]

@SteveJ

Hazards do not work with the ignition on. They do work once the car is running though. Same with the rear defogger and radio. 

I don't really know where to start looking for how to start diagnosing this.

[SteveJ]

Check to see if you have voltage on both sides of the fuse for the circuits circled in red.

If you don't, check your fusible links. Also post clear photos of the links where they plug into the engine harness. Let's see the condition of your links.

[ckurtz2]

I went through the fuseable links. All were continuous, and also read 12.6V.

Went back to the fusebox. Started testing all those you circled. All read 12.6V with the ignition on. All fuses read continious. While doing this I heard a click and then the radio turned back on. Voltmeter worked again, and hazards/defogger worked as well. 

So far my theory is that one of the fuses is making a bad connection with the block. I think it read voltage when the car is on, because the alternator allowed it to overcome the resistance.  It may just be as simple as I need to clean the terminals on the fuse block.

However, this may just be a symptom of the issue, as I don't know why the first fuse blew in the first place. Only time will tell.

[ckurtz2]

Also now looking at the fuse cover, the radio, defogger, and hazards are on seperate fuses. So that makes this a little more intersting too...

[SteveJ]

Did you test the fuse box with the ignition off? Those circuits should have voltage when the car isn't running. 

[ckurtz2]

They read 0V when the ign was off,  when I tested originally. Now they read voltage. Hmmm. Hard to pinpoint the failure until the next time it acts up.

Edited November 20, 2022 by ckurtz2

[Zed Head]

1 hour ago, ckurtz2 said:

swung right to about 15V and stayed there

After you figure out the meter problem you might want to put a handheld meter on your charging system.  15 is too high.

[SteveJ]

I'm thinking you wiggled a connection somewhere, and there may be an intermittent connection. What did you do (touch) between the readings at the fusebox when you had no voltage and when you had voltage?

[ckurtz2]

@SteveJ I was putting pressure at both terminals at one of the fuses trying to read continuity and then I heard a click. Looked a little later at the dash and saw it was reading volts. So I didn't wiggle any connections, besides adding pressure at the fuse block.

Only other thing I could think of is cycling the ignition switch a few times. I also think it is an intermittent connection somewhere.