Pardon my ignorance as I don't have much experience in electrical work, but hoping someone can at least guide me on this. Vehicle is a 93 F350 5.8

Recently my voltmeter on the cluster has started to act pretty erratic, jumping back and forth from 8 to 18 - almost looked like the needle was loose or something and it was just jumping around as the car moved. It will occasionally stop jumping, and will settle in random spots - usually close to 18 or back down around 10. It's pretty inconsistent, although when it does settle, it seems to settle more often near 18.

I have a cigarette lighter voltmeter that I plugged in and with the truck running it sits around 11.8 - 12v, which usually leads me to believe the battery is going. When the car is not running it sits around the same voltage. I put a tester on the battery itself and cold the battery reads about the same as it does on the cigarette voltmeter - about 12v. When I crank the truck the voltmeter jumps around violently, staying mostly under 10v, but occasionally it will jump to 11 or 12 before dropping back to 2 or something. This does seem to correlate to what's happening with the voltmeter on the cluster.

I'll cover my bases and have the battery and the alternator checked at the parts store when I get off work, but is there something else that could be amiss here? Perhaps faulty wiring or some sort of voltage regulator that has malfunctioned, thus causing the volts to jump all over the place? I really have no idea and welcome anyone's thoughts here.

 

It could be bad connection, but the first thing to check is the alternator. I believe your truck has the regulator built into the alternator, so getting it diagnosed and possibly changed out will change both those components at one time. You can be looking at the connections and the wiring as you are pulling the alternator off. MAKE SURE YOU PULL ONE OF THE BATTERY CABLES OFF before taking the alternator out. It's very easy to let the large wire touch a bracket or something else metal and that will blow out the fusible link in that wire, and then the new alternator will not charge at all.

 

I would not be concerned with the voltmeter's behavior while cranking as long as everything else seems to be working. It's going to be wonky while that much current is being drawn from the battery in a non-linear manner.

Monitor the voltmeter during normal operation and while the cluster's meter is acting up. That should tell you if it's the cluster voltmeter or if there's an issue with the rest of the electrical system.

 

Battery voltage with engine running should be upward of 13.5 volts. Below that, it is not charging properly.

 

Seems like the alternator could be the issue. I have a 2G anyway so this weekend am swapping in a 3G unit and will see if that takes care of it.