Reverse Polarity Jump Start - Gen3 Prius wont Start

@ChapmanF, @rjparker, @Patrick Wong, @Elektroingenieur, are some electrical savvy members here, hopefully can help.

 

The 125 amp fuse is very difficult to access, but there are easy test points you can reach with a meter probe from the top. If voltage is there with the car off (or if there is continuity from there to the jump point) the fuse is ok. Stick a meter probe down into the square opening with the green arrow here, or (even easier), touch it to the green-indicated test points on any of those three fuses directly above it. (Isn't it handy these blade fuses have test points on top?)

 

You can just look at the 125a if you know where to look. Replacing it is another issue requiring removing the fuse box from the fender. Some have bypassed it but that is not recommended and requires almost as much skill. Typically a few smaller fuses go with the 125a.



Fuse box Bottom View

Two tabs shown on this bottom view release from the fender.

Other resources:

 

The relays are normally ok in a reverse polarity jump scenario.

These are blown fuses. One with stripped screws.

 

That square place, and all three fuse test points I also marked, are places you can test for voltage as a way to find out whether that fuse is blown or not.

If the battery is connected, and you see battery voltage at any of those points, the fuse is good. If you see no voltage, the fuse is blown.

Equivalently, if the battery is not connected, you can use an ohmmeter between any of those points and the provided jump terminal. If continuity is shown, the fuse is good. If no continuity, the fuse is blown. (Only if the battery is disconnected should an ohmmeter be used there.)

As rjparker points out, from certain angles you can also get a look at the fuse. Usually a blown fuse will look blown. My habit is still to use a meter, though, because I have been fooled by looking at fuses before.

 

Were you able to verify the 125a fuse in your fuse box was bad?

I would imagine the fuse in your salvage fuse box is fine. It is unlikely your existing fuse box is bad. Typically, once you determine your fuse is blown you replace the fuse. Yes, removing the fuse box from the fender is necessary but then the box comes apart. We just did not get to that point yet.

If you have already bought the salvage yard fuse box, you could take it back and just buy the fuse for much less.

If you do replace the fuse box, carefully verify that all fuses and relays in the new one match the existing. It looks like one of the mounting pegs of your salvage box is broken off but that won't keep it from functioning.

Here are some pics from both of us that are found elsewhere on this site. Unfortunately the moderators don't do much to organize or clean up the forum and members can't even update our own posts after 24 hours. These are not guaranteed accurate but you get the idea.

 

@chuktai you should let body shop know what they’ve dropped to you into.

Also, could Toyota be doing something better, say ensuring only one fuse need blow, and making it readily accessible??

 

First, Toyota has done something amazing with their module protection in that a high energy reverse polarity jump does not blow any of them. A fuse will not be fast enough to save subpar modules. Plus many are on the unprotected side of this fuse.

Second, the smart thing for anyone to own in 2022 is a lithium jump pack that does not allow reverse polarity jumps. Yes, a more accessible fusible link would be handy as would an easier to remove box.

A knowledgeable mechanic can bypass the 125a with an external fuse but the method requires a clear understanding of the circuit and proper corrosion protection. I don't recommend it because the downside of a bypass mistake could result in widespread burning of the wire harness. However if this happened to me I would be on my way the same day using a generic fuse and fuseholder.

 

I have never done it, but most people simply pry it off. It would appear that access from inside the fender well might be possible.

 

Everything seems fine now, apart from the check engine light? The car starts up and drives normally?

I think it's usual for a check engine light to stay on, even if everything is fine, for about 5 more start ups. If everything seems fine, you could just try driving for a few days, see if the light goes away.

 

Hopefully you have checked for codes, wrote them down and then cleared them. It is possible and likely that other normal fuses are bad but the engine still runs.

What did you do to change the 125a? We need to get a definitive procedure.