I have a bunch of gen 1 prius on the road. The 12 volt system usually runs about 13.4 volts when the car is in the "ready" state. One of my 2002 Prius only puts out 11.4 to 11.9 volt. It is constantly discharging my 12 volt. What do I do?
The 12 volt battery is new. I'm only reading 12.2 volts on all my fuses. And it decreses as the 12 volt battery discharges from not being charged. I checked all my small fuses they are all good. Does MG1 or MG2 charge the 12 volt battery? Is there a dc/dc invertor relay?
The battery is charged by the output of the DC/DC converter, which is part of the inverter/converter assembly and I don't think was available as a subassembly replaceable part in Gen 1. I'm used to seeing 13.8ish as the output of the converter in my old Gen 1. If you can't see that voltage at any point in the front fuse boxes while the car is READY, that would sound like a converter problem. If you do see 13.8ish in some places but not at the battery, again with the car in READY, something else may be going on. It would be a good idea to use Techstream and see if any trouble codes about the converter are present, or look in the live data list for clues about its operating status.
I just replaced the mg2 and the invertor with ones I don't know for sure they were good. The old mg2 was burned and the t-iv fluid smelled burned. The car now drives but discharges the 12 volt battery. By the way, it was discharging the 12 volt battery before and would not go into gear.
After car is starts, will it still run if you disconnect the negative terminal on the 12V battery? If so, what's the voltage on the battery cables after the 12V battery is disconnected? If no voltage check the big 100A fuse in the engine compartment fuse box. Sounds like an issue with the DC/DC converter, but can also be something else loading down the 12V circuit when the car is in Ready mode.
With the inverter/converter assembly just recently having been replaced, it wouldn't be a bad idea to carefully recheck all its connections.
I just changed my battery to OEM Toyota, when I use the MFD check detailed here on PC for the 12V battery it shows it at just above 12V (12.1-12.2), so not especially high at rest with a brand new battery. It says to turn on to accessory, when I do it just stays above 12V. I think I'll check it with a multimeter to gauge the accuracy of the MFD reading, but I'd expect a new battery to be higher, maybe 12.4 at rest.
In every Prius where I've checked it, the MFD "signal check" voltage reading has been a solid fraction of a volt lower than readings elsewhere in the car at the same time. (I don't think that means the MFD uses an inaccurate voltmeter, but just that it's measuring its own supply voltage, at some internal point, and the voltage is just a bit dropped there.)
I figured it out. I had a donor invertor and replaced all the upper components. MG1 and MG2s components. Not knowing the the dc convertor is on the bottom of the invertor. I couldn't use the donor invertor because all the cover bolts broke off and wouldn't seal if I used the case. The connectors and gold casing by the yellow arrow is the 12 volt (13.8) supply. I'm glad it's over.
I took the bottom of the bad invertor apart. Bad DC convertor disassembled. Found burned component. See pictures. Sure enough... it's bad.
Possibly you can fix it by just replace this transistor but it wouldn't hurt if you check all power components, there can be a fuse or 2 that need to be checked too.
