12V system not charging after reconnecting inverter.

I recently had to do a bit of work on my sons 2007 Gen 2, including replacing the brake control unit (and yes, I have Techstream and bled it correctly). As part of replacing the brake controller, I had to disconnect the main battery and remove the inverter.

Since reconnecting everything, the 12-volt battery is NOT charging. I can bench charge the battery and it holds fine, and I had it tested at the local parts place just to be safe, so the battery doesn't seem to be a problem. With the power on and the car in Ready, the battery starts at 12.4v and slowly drains as the car runs, until the VSC/Brake lights come on at 10.8 volts, the warning squeal comes on at 10.5v, and the red triangle of death appears at 10.4v. The computer will usually throw a C1241 error (Low battery voltage) followed soon after by a collection of other random errors (changes every time...everything from HV failure to 02 sensor failure to door lock circuit malfunction....I presume it's just the computer misreading the sensors as the voltage drops).

So. The HV battery appears to be charging correctly, but it looks like the 12 volt system isn't charging at all. The connectors and wires all appear to be seated and tightened properly. Other than the low battery warning, no other errors are consistently appearing in the computer. Because this was working properly BEFORE I disconnected the inverter, I'm assuming that I broke something .

Anybody have any troubleshooting workflows or suggestions on where to start diagnosing? Are there any relays or sensors to test to determine whether the DC/DC inverter is working at all? Or am I looking at simply replacing the whole inverter unit? Anyone have any good wiring diagrams of the system? My son is quite peeved at me right now, and this is one of the parts of the Prius I'm least familiar with, so I'm looking for some suggestions on where to start testing.

BTW, I have a CSEE degree, so feel free to get technical. I'll probably work this out eventually, but I'm hoping that I don't have to spend the next week trying to diagnose it on my own. More than anything, I'm looking for suggested starting points or the most common sources of failure.

 

I've checked the fusible link and it's not the error. Besides, checking power at the jump start terminal shows the same low and declining voltage when the car is in Ready.

It looks like the car is relying 100% on the battery for 12v power and isn't charging the 12v circuit, which suggests that the DC/DC inverter isn't working at all. Correct me if I'm wrong, but it's my understanding that the car should be pulling 12v power from the DC DC inverter when its running, and should only be pulling power from the battery when the HV battery is too low or when starting the car. That doesn't appear to be happening, and it looks like the car is simply relying on the 12v battery for all of its 12v power.

And, frustratingly, it's not throwing a code when this is happening. The car is completely code-free until the battery voltage drops below the threshold for the C1241 warning.

I'm looking at opening the inverter again to double-check its internal connections. Before I move to that step, I'd like to rule out any possibilities outside of the inverter.

 

I use pinch pliers on the coolant hoses before disconnecting them. Other than a little top-up to account for the small amount lost from the inverter itself, I didn't have to drain and refill the system

 

Thats interesting, and would certainly change how I approach it.

I presume theres a control circuit for the DC DC converter that dictates its output. I should find a wiring and circuit diagram to find out whether its testable. I need to determine wherther the inverter is broken or is simply not being activated properly.

 

Thanks! I found and watched through that video a few minutes ago, and it does a great job at explaining how the Prius HV/LV systems work and connect together. The two big things I learned from it are 1) If the A/C works, I probably hooked up the inverter correctly because both tap the same power input, and the A/C doesn't run off the accessory battery (the A/C works). 2) The video identified the 12v power output location from the inverter. I should be able to connect a VOM to this and see 13.6v if the inverter is working, even if there's a wiring/connection problem elsewhere or the battery is shot. If there's no 13.6v, I'm either looking at a computer problem or a toasty inverter.

 

How to turn on the gen 2 Prius 200V-12V DC-DC converter – nkirkby

This one is also quite interesting, and goes into detail about how the converters actually work (and how to test them!)

Fixing this may be a pain, but a "good day" is any day you learn something new...and I'm learning a lot today.