Voltmeter reading low, alternator and batts good

The volt gauge is defective.

Check the voltage going to the gauge in the dash, if possible. If it doesn't match a known accurate voltmeter, its the gauge.

The voltage is the same throughout the system. Voltage doesn't change, amperage does. That's why you can install a voltmeter anywhere in a system and it will be accurate. This is why it leads me to believe it's the gauge itself.

 

Not exactly true. While in a normal system it is somewhat true the reading will be basically the same but when you get into the dash you may end up with a daisy chain of devices (ie multiple gauges, illumination lights, gps... ect.)being powered by the same circuit. In an AC system there would be little if any voltage drop. On the DC system a small amount of voltage is lost at each device in the chain also DC voltage does not travel well the further from the source the lower the reading will be. (this is the reason we us AC power in our houses it can be transmitted across the country while if we used DC there would be a power plant every few miles). A defective device in the chain or a weak short (not enough to blow fuse such as weak insulation on a wire) will cause issues getting the proper voltage to the devices at the end of the system branches.

 

I've got a 93 and 97 KW T-600 and both voltmeter gauges will read low for some reason. For example when lights are on truck and trailer both will read 12 volts and when I turn them off it will go back to 13.5-14.

I have checked with a voltmeter and I have good volts with lights on or not. I run a scan gauge in the trucks and it will show more volts then the gauge on the truck, especially with lights on.

I haven't bothered to change gauges to see if it helps, but I can understand your concern.

You could try a new gauge and see if it helps. Good luck.

 

I had the same issues, get under your truck and start chasing cables. I found sections with up to 10 inches of the cable jacket missing. Start at the battery box and work out and forward. All of my cable had to be replaced, over 100 feet of the stuff, all green. This came from the factory like this. I replaced with good copper welding cable, now truck has few if any electrical issues.

 

I had one that would run fine with no lights on , then the voltage would lower as you put more of a load on it.
It was just the alternator, there are several diodes in there, if some of them die, then the output of the alternator is deminished.

 

I'm sorry but you're wrong. Having multiple devices on the same circuit does not effect the voltage on that circuit. With a voltmeter you will always be checking the voltage in parallel, not in series. If multiple devices pull down the voltage, then the voltage of the entire vehicle is pulled down.

Take a voltmeter, with the engine off, and check the voltage at the cig socket on the dash. Now, put on all lights and the fan, heck put on the engine ignition. But don't start the engine. You'll see the voltage drop considerably. Now, go check the voltage at the tail lights. It's the same as the cig socket. It's all one big circuit on a vehicle, so voltage is the same throughout. They might have fuses for each little mini-circuit, but they're all tied to one voltage source, a battery or bank of batteries, so the voltage will remain the same throughout.

Why does voltage remains same over Parallel Circuit

I'm a licensed airplane mechanic.

 

Yep. Voltage only drops if you hook the devices in series. In parallel (98% of vehicle circuits are parallel) voltage remains the same. Building off the above principals, corrosion or poor connections act like a load in series with the device and cause less voltage to make it to the device.

 

I'd start here @Gumper. Get the alternator output tested. Then check for voltage drops between the alternator and batteries while the alternator is producing high output. Make sure to check the ground side too.