Charging problem

What's a meter saying at the batteries and at the alternator? If you're going to try and diagnose it yourself, you need a meter. They are less than $10 at harbor freight for a cheap multimeter. Put it on DC voltage and check. Could just be a bad gauge.

 

If it's showing 9v at the alternator, then it is bad or not hooked up correctly.

 

You have a bad cable between the alternator and batteries. You replaced the alternator already and still have the same problem, I highly doubt you need a third alternator. I have seen battery cables totally corroded from the inside out and show little to no signs externally. Also Freightliners have those fuse link type of grid plates near the starter that like to corroded and burn themselves open. Freightliners also like use the frame rail as the ground connection between starter/engine to the batteries. I have seen where the starter to frame connection is actually tight, but arched between the eyelet and frame and making a poor electrical connection. So you may need to go connection by connection and remove them and clean them and retest.

 

I'm going to agree with Heavyd. I'd start by following the wires from the alternator to the next connection, usually the starter, but some go to a mega fuse block. Check voltage there and see if the connections are loose, dirty, corroded, etc. When you find the point where it shows battery voltage, you've isolated the problem.

When checking voltage use a good battery ground, not the ground on the alternator. Then you check the alternator ground the same way using good battery positive. If you use the alternator positive and alternator ground to check voltage, you don't know if the problem is in the ground or positive cable, so check them separately.

 

I had a similar issue but not low as 9 volts. It was a relay or junction box that is mounted from inside the frame rail above the starter. The connections were loose and corroded. I took them apart, cleaned everything and mounted everything back. Problem solved. This is/was on my 379 Pete.

 

That is still low, but way better than 9 volts.