3rd Gen Reverse Polarity Disaster and 125A Fuse Access

20241023_174956 by oskr24 posted Oct 23, 2024 at 5:54 PM

Hi guys. I won the stupid lottery by reverse connecting the 12V battery when I was inspecting it. My fault but some batteries could do with better labels. It was only a split second but I guess that's enough. Prius won't start. Some functions work such as the headlights. Some don't. I've spent many hours on the problem since then.

I found the 140A fuse inside the red clamp on the battery. It's fine.

I then started pulling apart the fusebox looking for the 125A fuse. It's indeed as difficult to access as other posts indicate. Has anyone successfully figured out how to get there?

From looking through the crack I can't completely tell whether it's blown. It doesn't look as expected.

Does the 125A fuse simply connect the two white wires as another post implies? If so then it's definitely blown because infinite resistance. Should I just remove the two white wires from the box and connect them with a new fuse?

I tested some of the other fuses and I found at least one broken.

If anyone can direct me to where this problem has been properly solved it would be enormously helpful.

thanks

 

Ok. Solved. Car is driving again. So far this fuse seems to be the only damage. I'll write back if I subsequently discover further damage.

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I thought about making a video and even recorded some but I guess I'm more of a writing guy. I'll do my best to make it clear and easy to understand. I personally found some of the other threads on here confusing. None of them really go into enough detail about how actually the fuse was accessed. And as far as I can tell there are no instructions in the 7000+ page manual. It seems that Toyota installed a fuse that was never meant to be replaced. What follows may well be the closet thing to an instruction manual you're going to find(!).
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HOW THIS HAPPENED :

I was inspecting the 12volt battery. Car had been experiencing minor electrical issues. I wanted to look in the battery life indicator. To do that I had to actually remove the 12volt battery from the slot. In the process I turned it around (stupid!). It was a no-frills type battery. The entire lid was a single colour. The positive and negative terminals weren't labelled except for barely visible raised symbols. I left it disconnected for a while. When I came back I was running out of time and needed to lock the car. When I tried to reconnect the battery I got some bigger than usual sparks. Strange. I tried again. This time the car horn beeped. I realized the polarity was backwards, so I flipped the battery and connected it properly. Note that the reverse terminals were never fully connected. I only touched the reverse terminals for a millisecond, and then again for another millisecond.

SYMPTOMS:

Car doors wouldn't lock. Radio and most electricals were not functional. Car still registered keys in the car. Some symbols on the display. Just silence when pressing the start button.

DIAGNOSIS:

It took a lot of research online before I found this forum and learned about the 125A fuse. I knew there must have been some major fuse somewhere but of course never imagined it would be as difficult as this.

As other threads have indicated there is a tiny window in the fusebox where you can see the dreaded fuse. You'll need a torch, preferably a small one. In my case, I wasn't sure whether it was broken just by looking at it.

You can test across the fuse with a multimeter for resistance. See diagram below (which I found on this forum). Note that this diagram is roughly the reverse of what you're looking at - the lug is on the left, the plug is on the right.

The fusebox jump point is the red thing with a plus (this is how you jumpstart a Prius). The red plastic lifts up revealing metal. The fuses at the top right of this diagram are actually on the left of the fusebox, next to the longest white relay thing with the sticker and the number 2. The 50A and two 30A are grouped together. Use one of the "standard" car fuses directly above this. In my case they were a 7.5A, a 30A and a 10A (I can't see any 10A on the diagram). Put one of the mulitmeter probes in one of the tiny holes on the fuse and then touch the other to the jump point. If you have infinite resistance then you're likely to have blown the lottery fuse.

You can also diagnose the fuse by checking voltage, some of the other threads talk about doing that. In that case you want to just put the other probe on any unpainted piece of metal.


You're likely to have blown other fuses in this box too. You may as well test all those little fuses while you're here. In my case I had blown six(!) of them.

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REPAIRS:

After spending many hours on this !@#% fusebox, I've come to the opinion that the 125A fuse was never meant to be replaced. For whatever reason, maybe just pure sadism, Toyota put a fuse in a completely inaccessible location. Therefore it would make sense to ignore the fuse and just bridge over the top of the fusebox.

You can probably buy the components at a hardware/electronics store. You would be connecting something from the lug (on the left) to the white cable that plugs into the top of the box just next to it. You would want it to be as flat as possible so that you can still close the lid. You would need it to carry a high current.

However if you want to access the actual 125A fuse you realistically have to destroy parts of the box. Ideallly you'll do it as efficiently as possible - getting access to fuse as quickly as possible and doing the least damage.

The first step is to pull out the main cables entering the top of the box. The big white cable just clips out. There are also two multi-pin type cables that pull out of the sockets with a bit of jiggling. The lug cable is concealed under a very unusual black cover. This can be removed carefully by pressing in all the right places. Or less carefully by just prying it with a screwdriver. It's not that important. Underneath you unbolt the lug. Once you've done this you probably want to check the continuity again with a mulitimeter, this time directly from the lug to the socket of the white cable. By doing this you're testing exactly and only across the 125A fuse.

ABOUT THE BOX:

The box is roughly a rectangular prism. There are bundles of cables entering at the bottom NW and the bottom SE corner. These cables are so rigidly installed that there's no chance of lifting out the box entirely. The box is also constructed with several sections. At first glance it appears that these can be pulled apart. On further inspection it seems that doing so with all the wires attached is impossible.

There is a top and bottom section of the box. These are held together by multiple, very rigid clips on all sides. There are also two long prongs that extend into the fender on the right-hand side. Although they are right at the base of the box, amazingly these prongs are attached to the top-side of the box. This means there's no chance of lifting the top off the box. Additionally any work you do on the right hand side is severely restricted by the overhanging part of the hood.

It's probably possible to reach under the fender from the wheel side and cut or push the prongs through. I think it would involve removing the wheel though - too much work. I decided to destroy the prongs. Working from the hood I wedged a long screw-driver into the crevice above the prongs and whacked it with a hammer until the prongs were disconnected. This improved movement and leverage to the box.

The box is also bolted onto the chassis in two visible places. I used a small-size socket wrench with a long attachment. You may not have one of these things. I was glad I had it, I used it a lot. There's a picture of it below. Using the wrench I got the bolts off. This also improved movement to the box.

There are clips all around the exterior of the box. I found them to be extremely stubborn. Using a screwdriver and sometimes a hammer you can disengage them. But they are so inflexible I doubt anyone will be able to close them neatly again. There are also clips on the top face of the box. This makes it look like entire sections of the front face can be disengaged and possibly raised out of the box. After messing with the clips a long time I doubt this is really possible. However, to get access to the 125A fuse you probably want to at least partially separate the bigger middle section from the rest of the face. So use a screwdriver and hammer if required and get the middle section of the face loose enough to move a little.

The next thing I decided to do was cut a chunk out of the SW corner of the box. I used a small brush saw because it was the only saw I had that was narrow enough to fit in the narrow space there. This was pretty satisfying after hours of frustration. I also used a boxcutter to cut more precisely towards the fuse.

The box was by now loose enough that the top section of the box could be pulled upwards. Meanwhile I pushed the partially disconnected middle section downwards. The 125A is attached to the bottom of this middle section. I also used a screwdriver and wedged it between the two sections to force even further space. With a lot of pulling and wedging it was possible to get access to the 125A, via the section I had cut out obviously. The photo below was taken with my phone pushed down into the gap. It was still very hard to see the fuse while standing at the hood.



Using the attachment on the wrench I was just able to unbolt the fuse. I got a replacement at Jaycar. It was $12 for two of them. You can see it in the picture below. Also note the screwdriver which is still wedging the box apart. Also note the minor dent in the chassis caused by so much prying and wrenching (I bent it back later).


I put everything back together and taped the cut-out section back in place. The lid still fits on the box pretty decently. The box is obviously less water resistant than before.

PS: There are plenty of other pictures online in various places. Have a look on this forum obviously. I also found it useful to look at pictures of the fusebox on ebay. It's hard to get an idea of the actual shape of it just by looking down from the hood.
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RESULTS:

Car started as expected once I put the 125A fuse back, along with the 6 smaller fuses. Haven't yet taken it for a long drive but so far I can't detect any proper damage. Other reports on this forum and elsewhere suggest that the inverter (DC-DC converter?) can be destroyed by a reverse polarity. From reading posts on this forum however that mostly doesn't seem to happen.

CONCLUSION:
As I said earlier I think it's probably faster and smarter to simply install a new fuse above the box somehow, but that's for someone else to pioneer. I'm sure many other people are going to blow this fuse and struggle to comprehend the placement of it. Hope this has been useful to you. Good luck.