Reverse polarity

Haven't seen as many posts about this for gen 1, maybe just because there weren't many gen 1s made. These are common posts in gen 2 and gen 3 forums.

Often, one of the casualties will be the DC/DC main fuse in the underhood fuse box, so checking that wouldn't be a bad place to start. (Of the multiple gen 1 underhood fuseboxes, this is the one on the left inner fender, 'left' meaning, as it always does in a car, on your left when you face the same way the car's pointed.)

It's a 100-amp fuse in gen 1, and I believe it has legs with bolt holes; it doesn't just pull out of slots. It's mounted in its own little modular island in the top inboard corner of that fusebox, and I think you unclip and lift up that island and you can get to the bolts holding the fuse in. (Don't do this with the battery connected.)

Sometimes other fuses join in the fun, so there might be more to check, but that's where I'd check first.

 

If you do a search for "reverse polarity" you'll come up with a whole bunch of hits, some of them relevant to your '02: https://priuschat.com/search/974367477/?q=reverse+polarity&o=relevance&c%5Bnode%5D=87

If it is the 100 amp fuse, there are directions for getting to it (not an easy task, apparently). Since you do have signs of life (horn) you may not have blown the fuse.

 

Location of the 100A fuse. The window on top lets you see if it looks blown or not.

My '01 had another 200A (I think) fuse on the positive battery cable, but it didn't blow when the jumper cables were reversed.

 

This is my post about replacing the fuse. https://priuschat.com/threads/reversed-jumper-cables-blew-100a-fuse-in-engine-compartment.215888/#post-3041152

 

Oh, the island comes out through the bottom of the fusebox. Yuck.

With every Prius generation whose wiring diagram I've looked at, I have struggled to guess what rhyme or reason is behind Toyota's choices to put things on one or the other side of that fuse. The gen 1 horn is on the battery side, so still will work even if the fuse is blown.

A person who wanted to could probably even squint at the wiring diagram and come up with a way to deduce whether that fuse is blown or not, all by checking what other things still do and don't work, without ever checking the fuse itself.

But I think it has a clear cover and can probably be checked at a glance.

 

Which battery? Must be the HV, since OP was trying to jump start the 12V (presumed dead?).

 

No fuse in the car's 12-volt system has the HV battery on any side of it.

The horn is on the same side of the 100-amp fuse with the 12-volt battery.

The exact condition of that 12-volt battery isn't known to me, other than that the OP mentions trying to jump it at some recent time.

 

I've done this one several times. It's a PITA just from the number of clips, bolts, connectors, nuts to take off. The hardest is the 10mm or 12mm, I forget, holding a harness against the firewall. Another pain is accessing the actual nut holding the wire to the fuse, you gotta bend some ears on the plastic cover to slide it out. And finally, for me and my fat hands, just getting all the connectors back into the fuse box by touch. Most are easy, but it seems like there's one or two that hose me up every time.

To the OP, there's almost a 100% chance that fuse is blown. Don't get it at the dealer, they charged me almost $50 for one. Then I found out the parts houses can get one for a few dollars.

But you should check all your fuses. I've had others blow too.