I recently started having problems with the battery voltage on the truck. I have a 100 amp 1 wire alternator. When I start it up the voltage usually shows 14V, sometimes a little lower. I checked it from the post on the alternator to ground, I checked it between the battery posts and got the same voltage. When I turn on the AC and the radiator fan, the voltage drops to 13.4V. Again the voltage is the same both places. If I turn on the headlights with the fan and AC, the voltage drops to 12.4V.
I have checked the wiring coming from the alternator to the fuse panel and to the battery terminal on the starter. All appears OK. My theory is if the voltage was different at the alternator than the battery, then it would be a wiring problem, but since it is the same I think it is the alternator.
Any other ideas?

 

sounds like a bad ground.. maybe body to chassis..

Sam

 

Don, is this a new problem or has it always done this? I know that you have your battery relocated behind the seat and I think you told me that you are using a one wire alternator. At the risk of sounding like I know what I'm talking about, are you sensing voltage at the starter solenoid or at the battery? It could be that the voltage drop in the sensing wire is causing the problem. Google "Mad Electric" and go to their tech pages as they can explain it better than I. I tried to post a link but am using FireFox now and can't figure out how to do it. Let us know what you find out as I'm getting ready to install a 12si alternator on my truck. I will be using the 3 wire with remote sensing. Hope this helps.

Gil

 

A 1-wire senses voltage at the alternator. On a system with as much load as you have, you would likely do better with a 3-wire.

What size wire do you have at the alternator output? It should be a 10 ga. at minimum, better an 8 ga.

 

What does it do when you raise the rpm's? Some of the 1 wire alternators don't really kick in till off idle. On occasion a slight change in pully size will solve this.

 

Since you are getting 14 plus volts I would say the RPM sensor is on because you are reading above battery voltage of 12 VDC. Looking at your pictures,the battery cables may be undersized. Larger than normal size is needed, such as what is used when mounting the battery in the trunk of a car. Battery size in amps should be high as you live in a area that the AC and fan runs often. Since the battery is in the cab, all your system has a increase in resistance due to longer wiring. I would suggest at lease four relays placed on the inside of the fender for lights,AC, Starter, and fan. Run one heavy battery pos to a junction next to the relays. Use the fuse box to run the relay coils. Prior to all this I would have the alternator bench checked under load at a shop. Check the voltages with your meter in case your meter is reading low. Finally the rectifier diodes wear when you have a constant high load draw, high heat and will wear giving lower outputs. Just my opinions and guesses. chuck

 

I have been running the truck for 3 years with NO problems. This problem just started. I was checking the voltages with a meter, not relying on the gauge.

 

In that case you may have blown one leg of the diode pack. It would cut your output by 1/3.