Have you confirmed the voltage when the alarm goes off? The sense wire is what provides voltage to the board. If it was broken the apu would run nonstop.
The issue may not be the APU. My truck has a master battery shut off switch. That switch went bad. The voltage for the truck goes through it, but not the voltage to the TriPac. So my voltage alarm would go off in the cab, because of high resistance from the bad switch, but the APU would not come on. I figured out it was that switch by comparing voltage in the cab with the voltage showing on the inverter. The inverter also does not go through the shut off switch, so it was showing the actual battery voltage.
Thank you for the input the low voltage shuts the lights and everything off and I have to turn key back on. The issue is that darn cpap machine goes out and I feel like I am suffocating. Will look into it more when I get home from vacation. Thank for your help
Don't know what issue was but now the apustarted working like it did before. I checked plugs and messed with a few wires and made sure everything was fine. Went down to truck this evening and it was running so I am bumfuzzled
If you have a master shut off switch and it is going bad, like mine did, the problem can be intermittent. If you have a voltage readout you can watch while the main truck engine is running, if you suddenly see very high, above 14.5 readings, the voltage from the alternator is not going through the switch to the batteries. (And that high voltage caused me other sensor issues.) And low voltage through the switch could cause what you were seeing.
Don't have a voltage readout but I am getting a power inverter and will check to see that it has a readout on it. Thank you for the input.
Updated on my apu not kicking in. It started the same issue again of not starting when batteries got low. Took to thermo king and they discovered the settings where lost. Reset parameters and now all is good.
+1 I want to mention one more thing. original tripac alternator's voltage regulator is set for 14.5v Bosch AL928N has 13.5v setup. Little resistance due to loose or corroded wire will set AL3 code