How to Test My APU Alternator?

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.

 

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

 

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 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.