14 volts at the battery and alternator and 12 volts on the gage

There are 4 Mega Fuses on this truck and 3 have output wires. One is open. There are 3 batteries.

All Measurements taken with the engine running.

All Mega Fuse to Ground = 12 volts

All Fuse Panels to Ground = 12 Volts

(Mega fuse to Battery Positive = 2 volts)

I'm taking my readings from bat ground where I can but on the other side of the truck I'm taking it from the frame.

I'm still seeing 14 volts from Alternator to Frame and from Battery Positive to Battery Ground/Negative.

I don't have a schematic but I'm sure it points to one place since all the mega fuses and fuse panels have 12 volts.

It appears that the battery positive runs out in the truck and then comes back to the battery box where the mega fuses are mounted and feeds them there.

They get there power from some place out in the field as opposed to the battery running straight to them.

(There is one large Red wire that comes back into the battery box to feed the mega fuses but I'm not sure at this point where it comes from. No Schematic.)

Maybe the starter? Maybe a high resistance connection there?

 

One minor Mistake here. The Mega Fuses are being fed directly from the batteries.

I guess that would point directly at the connection between the fuses and the batteries?

I found it.

Thanks for the help guys.

I found an early print for a 387 and it points right at the problem. Bad connection between Mega Fuses and Battery.

That simple.

Gage shows correct 13.8 or so volts now as do all fuse boxes. Just a very low value resistance in the connection between the Mega Fuses and the Batteries and that's all it took to lower the voltage.

I feel stupid but I thought the wire (could not see it completely until I removed it and ohm ed it out) went out in the truck somewhere and came back to the fuses from some place else.

Consider this thread as complete. Thanks for the help. Sometimes just talking to others helps you to find the problem.

 

That's just it. I was seeing 14 volts at the alternator and the batteries but the connection between the batteries and the mega fuses (3) which feeds everything else in the truck was bad and caused a 2 volt drop for every fuse panel in the truck. Everything worked but the bad connection was taken the voltage down 2 volts for everything else. The guy that was driving the truck told me that every now and then it would come back to 13.8 and then go back to 12. That was the key that I missed acting on. I was expecting it to be a bad alternator but much to my surprise the alternator was fine. That's when the troubleshooting began. Was not expecting to find a bad connection on a heavy gauge wire. That's exactly what it was though.


Fixed now

 

I read your post but High resistance as taken from your original post is actually relative.

While the bad connection resistance was higher than it should have been, it's not a high resistance/value at all.

The resistance that I found in the circuit was only .1666 ohms and that caused a 2 volt drop at 12 amps of current. Granted, it should have been less resistance than that but as you can see it not really a high resistance number wise being way less than 1 ohm. I had an amp probe on the circuit so i knew the total current and I knew the voltage drop. I knew the source voltage. I had everything I needed to analyze the circuit from an electrical perspective but I couldn't find the bad connection/problem because I thought the wire feeding the Mega Fuses was running to something out in the truck and not the batteries. The way they had it routed fooled me. I couldn't tell it was running to the batteries and I didn't have a print to go by.

Thanks for the help Philip

 

46 years as Rocket Scientist............