Headlights won't turn off, even with the car off.

Welp, if you're reading 11.5 at the battery, there's nowhere for the 13.7 to be coming from unless the converter's working, so I doubt there's a problem there.

What piques my interest is a whole 2.2 volt drop between the harness wiring where the scangauge measures, and where you measured at the battery. Were you measuring right at the posts?

I would also try a measurement between the clamps (just move the probes off the battery posts and onto the clamps sitting on the posts), and then maybe a measurement between the positive clamp and a bright unpainted body screw, etc., and just keep moving the probes around trying to see where you might be losing most of the 2.2 volts.

(BTW, the inverter pics aren't mine, but hobbit's.)

-Chap

 

I suggest measuring voltage from the negative battery post to the negative terminal clamp, and then from the negative terminal clamp to an unpainted part of the body, near where the negative cable is bolted. In all cases the voltage drop should be near zero.

 

I understand that when the Prius is READY, you are measuring 12.2V across the battery. Try measuring 12V at some other convenient point, such as the auxiliary 12V socket.

If you get a similar measurement there then that is proof the DC/DC converter in the inverter is not operational. Check the DC/DC fuse and if that is good then either wiring is bad or the DC/DC converter has failed.

 

Yep, it is pretty clear that your DC charging circuit is not working.

 

Does anything (including your ScanGauge) still read 13 or higher when the car is on and READY?

Is the ScanGauge the only thing reading that high, or can you also get a reading above 13 on your voltmeter anywhere in the car (say, at the cigarette lighter 12 V power socket), when the car is on and READY?

-Chap

 

So now you know the inverter is operational.

The 2G Prius has a fusible link assembly that the 100A DC/DC fuse is part of, located in the main relay/fuse box near the inverter.

Since you have a Classic, check your owner's manual to see if the DC/DC fuse (or fusible link) is depicted. If not, and if no one else responds, obtain repair manual and electrical wiring diagram info at techinfo.toyota.com which is a subscription website. My guess is that the DC/DC will be located within the relay/fuse box near the inverter.

 

Here's a quick easy check...

Make the car on/ready, such that you're seeing the 13.6+ up front at the scangauge and lighter power socket. While it's still ready:

Back at the aux battery, squeeze and unplug the two cable connectors (a fat one and a thin one) from the red positive clamp assembly.

Your granddad's voice is probably yelling "never unhook the battery with a running engine, you'll toast your alternator/voltage regulator!" That's ok, granddad's car had an alternator and a vibrating reed regulator, your regulator's all electronic and will be just fine.

Put one voltmeter probe in the (now disconnected) end of the fat cable, the other probe at a clean metal ground point.

What reading do you get?

Now plug the cables back in.

-Chap

 

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As you're learning, pulling on fuses isn't going to be the fastest or most efficient way to track this problem down. As Patrick points out, we already have good reason to look elsewhere than the DC bus fuses at the moment. The test where you make the car ready, then pop out the two cables at the positive battery clamp module (super easy, they just squeeze and pull out), then measure the voltage at the free end of the fat cable (to good body ground) I suspect will tell us the same thing (but we'll see what your measurement says). Using a voltmeter, a little patience, and Kirchhoff's second law, I don't think it will be hard to get a pretty good idea where to look without having to disassemble a lot of stuff.

Since you're wondering about the larger fuses, the high ratings in that style have mounting screws into the blades at the bottom, so you won't be pulling them out as if from a socket. It takes access from underneath, which means getting the engine room J/B loose for access to the bottom. Everything I've read by other people tackling that job kind of makes it sound like that J/B is the first part of the car and the rest was assembled around it.

You may end up doing that later anyway, if you broke the top off the fuse, as you probably don't want to leave it that way, but it can probably wait until we've tracked down whatever the real problem is for now. I'm also not sure what the price on that particular fuse is.

I'll stay tuned for what you report on the battery-disconnected bus voltage measurement.

-Chap

 

I suggest you disconnect the 12V battery and when IG-OFF, measure resistance from the positive battery cable, to the 12V aux power socket. You'll need to run a jumper wire to the positive battery cable, and account for the resistance of the jumper wire and the digital multimeter's test leads.

If you find the resistance is greater than 0.1 ohm you'll know there is something wrong with the cable running back to the battery. A resistance of 0.1 ohm and current flow of 10A would result in a 1V drop.

 

My apologies. I had somehow totally missed that earlier, or I had seen it at first but was distracted by your 13+ volt readings at the scangauge and cig socket.

It's pretty confusing to be seeing 13+ anywhere at all, which proves the converter is putting out power, and at the same time have the car unable to continue running with the battery unhooked. Confusing enough to make me dig out my wiring diagram.

I either had never noticed, or hadn't remembered, that some of the 12V power distribution is fed from terminals F10 and F11 (on the converter side of the 100A DC/DC fuse), and some is fed from F12 (with F13, on the battery side of that fuse; F13 is for the cable to the battery itself). These are terminal numbers, not fuse numbers; all four of these terminals go on (the two ends of) one fuse, the one you were trying to pull.

The OBD-II circuit (where the ScanGauge reads) and the cig circuit both get fed from F11, so it's no surprise they show 13+ when the converter is active. Several critical engine circuits (EFI, THRO, AM2, ...) are fed from F12. I don't know why they designed it that way, but if that fuse is open (as you seem rightly to have been thinking all along) it would totally explain why the engine dies if you unhook the battery. There could still be other explanations, but none as simple and tidy as that.

I'm sorry to have sent the thread in other directions temporarily. I'm now right with you in thinking of that 100A fuse as the prime suspect. I've heard it's tedious work to get that junction box loose enough to access the terminal connections underneath, but it seems like that's the right place to go next.

-Chap

 

Absolutely. I think people fairly commonly subscribe for a short period and, in that time, download stuff for reference later. At the price of the short subscription (plus some tedium doing the downloading) it's not a bad way to go.

My faith in humanity has been boosted by seeing that most people here do not seem to go putting their downloaded materials up in torrents or whatnot. It seems there's been a pretty widespread tacit feeling that it's fair to send $15 Toyota's way in exchange for such complete information access, and download the stuff directly for personal reference, and encourage others to do the same.

I'm old-fashioned enough I bought my manuals on paper, but if I were starting now, probably downloading's the way I'd go too.

-Chap