How to Test My APU Alternator?

I watched some videos on how to test car alternators, but my APU alternator is connected to 4 truck batteries, or to be more accurate 2 truck batteries. Should I use a multimeter on just the APU negative and positive cables and not the batteries? A voltage under 13 means bad alternator? My APU is throwing an ALT code.

 

Test the voltage at the terminals with it off. Then test it after its started. You should see an increase in voltage with it running. Maybe 13ish volts

 

If it's an older Tripac, make sure the 50 amp Maxi-fuse by the starter isn't corroded or blown. Then check the voltage on the output post of the alternator. If it's less than 13.6 after running for a few minutes, it's bad.

You may want to swap over to a newer and higher voltage alternator anyway. Most of the newer voltage regulators top out at 14.3 volts. They charge better.

Do a search on here for swapping to a chevy alternator.

 

The other problem is that the APU won’t run for more than 3 minutes. Hopefully that’s enough to get an accurate reading of the alternator.

 

Does it have the inverter that comes with a Tripac? It should have a voltage display.

 

So I just finished checking the voltage with the inverter, and the alternator appears to be charging. The volts move up, but then stops at 13.7. There is nothing else running. No A/C, heater, microwave, etc. Maybe there is something wrong. I would think the voltage should show 13.9-14.1 volts without any accessories running, right? When the volts stop at 13.7, it takes 2 minutes and 20 seconds for the APU to then turns off. The APU manual says the ALT code will appear if there is no output after 2 minutes of operation.

Ok, so now I tightened the alternator belt with a crow bar, and the volts raised to 13.9, but the APU still turned off. I am starting to think there is some bad wire or connection.

 

I had a simular problem and it was the wire in the rear of apu box had corroded and broke that kept track of battery voltage. I ended up pulling the big fuse and wrapped the wire around 1 of the prongs so i didnt have to run the wire again from rear passenger side to under my drivers door. That was about 2 yrs ago and havent ran into that problem again. Hope this will help

Ps the wire is so the computer to knowns when to fire up the apu to recharge the batteries. And also if you have the usb cord hooked up to your ecu you can plug up your laptop to it and get all fault codes and parameters. The software is built into the apu brain itself.

 

Take the cover off the control box and check the voltage at the D+ terminal. It should read the same as the inverter. If not, you have a wiring problem.

 

I do have one semi-thick white wire behind the APU box that has some corrosion, but the whole wire is still intact. I’m going to have to open the wire up to see what else is in there. Maybe multiple small wires and one broke off.

How do I get the APU software? I have a Tripac, and I think there is a USB plug near the fuse box.

 

I had the same problem as the op. In my case the problem was the sense wire is broken somewhere. What I did was make a jumper to go from D+ to sen. on the diagram you posted.