Reverse Polarity Jump Start - Gen3 Prius wont Start

If you feel like taking a small risk, you could remove the fuse now and use a short piece of 14 gauge copper wire across the two screws as a temporary "fuse". If you do this, this is just for testing purposes - to see:
1) if the Prius will become READY,
2) whether the combination meter will show signs of life, and
3) whether the inverter can produce 13.8V.

If you can answer yes to all three, then the car will be good after you replace the 125A fuse. However I would not want you to depart on a long road trip with that temporary measure installed.

If your answer is no to one or more of those issues then you will have more work to be done after the fuse is replaced, and it would be good to learn that sooner vs. later.

 

Sergey K's pictures of the location of the 125A DC/DC fuse (not fusible link) really are the most useful info I have seen on here in quite some time. Thanks again for doing this and posting.

Even with the Repair Manual and Electrical Diagrams there was no good indication of where this was located. It was good that Patrick knew.

What was puzzling was that the Electrical Diagram showed the symbol of a high current fuse for the 125A DC/DC device yet the Repair Manual uses the term DC/DC Fusible Link.

 

Thanks for picture. I can see how Toyota would want to differentiate this from a typical fuse.

However a fusible link is typically an insulated wire where the wire is sized to melt at a specified current rating, similar to a fuse link. So I would be looking for a wire or either a metal link that melts, not a fuse mounted on a piece of copper bus.

But thanks to your help we all know what to look for now.

 

Patrick.

You could put a "Schottky" diode (lower forward voltage drop) in series with the jump terminal which only needs to supply a short burst of 20-30 amps to run some pumps during start up.

JeffD

 

Very interesting and informative thread and I'm glad you saw it through to the very end, including keeping us updated. I can't help wondering how long it would have taken the dealer to find/fix the problem and what the bill would have been.

 

Congratulations, and please excuse my earlier pessimism! It's at least somewhat less painful to blow a horribly inconvenient secret fuse by reverse polarity than to fry the inverter and a few computers, as we tend to assume might happen.

 

Ouch, out of warranty. Well hopefully you can get your hands on that fuse and get it back on the road soon.

 

Congratulations on your success and thanks for sharing your discovery about the DC/DC fuse location.

I believe that the 125A DC/DC fuse was deliberately mounted under the main relay/fuse box to make it difficult to replace - with the thought that most people will need to contact a Toyota dealer for assistance when that fuse blows, and then if the DC/DC converter within the inverter is found to have failed, the root cause of a botched jumpstart will be obvious.

Hi Jeff - I agree this would work - unless the 12V battery has a shorted cell or two which causes excessive current flow from the donor battery - which would necessitate a diode with sufficient current carrying capacity to account for that possibility. Also, if the 12V battery was directly accessed for the jumpstart then the diode would be out of the picture.

 

Congratulations on your repair and glad it was only a fuse !!

Thanks for the post and following up on your final results. This post needs to go under the Sticky posts so everyone can refer back to this in the future.

Thanks for this explanation as that does make some logical sense as to why they would bury the fuse that way.

 

First: a big congrats, and thanks for the informative posts.

The one time I had to jump start my Prius I decided to turn the jumping car off out of paranoia, which worked fine. It really does not take much current to start up a Prius since the engine is turned by the traction battery. I have thought for a while that it would be prudent from a convenience and car safety aspect to buy a small back-up battery of the type sold online and in home improvement stores for ~ 80 USD to use for charging (and NOT jump starting) if needed and never connect the Prius to another car. Certainly Toyota advises to never use the Prius to jump start another car.

Toyota wants us to use the terminals in the engine bay to jump start the Prius and not the 12v itself. I don't know if that is safer for the car electronics in the event of an errant cable swap.

 

I would imagine the jump start "port" is by design. Probably safely powers up only the systems to start and engage the ICE. Going direct to the 12v likely exposes the whole system to a "bad" jump.

 

There is no difference between using the dedicated positive jumpstart terminal vs. connecting directly to the battery, since the terminal is connected directly to the battery with the 140A MAIN fuse in line.

That fuse is large enough to allow the inverter to short out, if the jumpstart polarity is reversed. The 125A DC/DC fuse will blow, and perhaps the inverter output transistors as well, to protect the MAIN fuse...

 

That jump-start terminal looks kinda thin. Perhaps it's designed to act as a fusible link if hit by absurdly high current, whereas the battery terminals and its associated cable would not?

 

^ I do not know, it looks to be end of the line, can't see how it could break a circuit. Maybe Toyota's just keeping it cheap?

 

IF the under-hood jump-start terminal itself melts away from the jumper cable jaw, that would break the circuit between the donor battery and the Prius 12-v system, MAYBE before worse damage is done. However, I don't have enough confidence in that hypothesis to recommend anyone test it!

 

I bought an emergency battery kit from Amazon

Am I right in thinking that it will be a bit more foolproof(and therefore safer for the car electronics) if I instruct all concerned to never jump start a Prius with the kit, but it is OK to charge the 12V ? My thought was to include instructions in the kit that say:

1. If you think the 12v is dead, do the following:
2. Turn the car OFF
3. Attach the emergency battery to the terminals under the hood: red to red, black to black
4. Power up emergency battery
5. Wait 30 minutes
6. Disconnect emergency battery
7. Try to start Prius. If no go, repeat (1 through 6.)

Is the hook up to the terminals correct ?

 

Overkill? And impractical, ie: likely won't work?

A properly hooked up jump pack works fine. Before we signed on dotted line I watched the dealership do it. And the Owners Manual condones jump start, shows a second vehicle as the source.