Problems with ThermoKing Legacy 650 APU

Bought a refurbished APU a couple years ago and apparently got a lemon. Trying to track down some strange electrical issues revolving around charging the batteries, and maybe it has some other problems as well but I just haven't gotten a good diagnosis yet. So here's the story.

About a year ago it started tripping a relay in the glove box of my '13 Cascadia. The relay would first shut off, then repeatedly and rhythmically flip on and back off. It would do this at exact intervals for hours on end sometimes, but it would stop doing it just as randomly as it would start. Sometimes every night it would do it, sometimes it could go days without doing it. Sometimes it would only do it for ten minutes, other times all night. But it only does it when the APU is running.

Now here's another quirkto keep in mind. Apparently this thing is supposed to just sit quietly and monitor battery level when it's on, and start itself up and charge the batteries when they get down to 12 volts. But mine won't do that. In order to get mine to charge the batteries, I have to set it to run the fan.

Anyway, I've had it to a TK shop a couple times now and they say it should only need to charge the batteries once or twice per night. But mine needs to do so much more often than that. Unless the batteries are brand new, they'll only take enough charge to last maybe half an hour, and then it'll need to run for close to that to charge them back up, so basically it's running like 50% of the time to keep the batteries from dying.

When it first started, I tried replacing all the relays but that didn't help. Recently I found out which relay it is. It controls the dome lights by the visors. If I turn one of them on while the relay is flipping, the light will flip on and off in unison with the relay.

There's apparently some sort of short or draw or something that's killing the batteries prematurely. I replaced all four batteries on November 21, and had to replace them again yesterday.

Maybe related, maybe not: in the past few months it would crank slower and slower when trying to start. A few weeks ago it started having a real tough time starting. It would crank real slow for like 20 seconds and give the low battery alert. But the truck would still start fine. Sometimes if I tried to start the APU over and over, after about 3 tries it would finally cough to life and then be fine for the rest of the night. But then a couple weeks ago I passed out with it running and then woke in the morning to discover it showing the low battery alert. It had failed to start overnight.

That''s when I brought it to a TK shop. They said the starter tests OK but the truck's batteries are shot. I replaced the batteries yesterday and they lasted thru the night with no charging, then today they finally got down to 12v so I tried running the APU but it doesn't hardly do anything any more. The first time it just rebooted the control panel which shut off the fan, so it was on but not running anything. I tried it again, it cranked once reeeal slow and then rebooted the control panel again. The third time it cranked twice real slow and then gave the low battery alert. Again, the truck started just fine, so I'm running the truck to charge the batteries.

Every shop I've brought it to can't find any problem. It rarely reproduces the problem at a shop so I have to explain it, and the most common answer I get is it shouldn't be possible for these things to be happening. I'm about ready to just have someone pull this thing off and install something new, but the only thing that stops me is the fact that it seems perfectly fine mechanically. It's just this odd electrical issue(s) apparently causing all the troubles.

Any insight is appreciated. Thank you.

 

Thanks for the info. Can you please expand on this passage a little bit? Are you talking my AC inverter? And would the high voltage be a setting or a defect. Ive never understood electricity real well. Anyway, thanks again.

 

It has a readout, says between 12 and 12.7 most of the time. Not sure if that's just input and there's a different measure for output.

 

I guess I don't understand how that would make the APU unable to start, unless the wire with the problem is one of the ones that was installed with the APU. But my understanding is that the only connection between the APU wiring and the rest of the truck's wiring is via the APU's connection to the truck batteries. It's tough for me to imagine a truck problem that only manifests when the APU is on (or trying to be), but like I said, I never understood electric very well.