Jump-Starting Weirdness, 100 Amp Fuse

Are you referring to the 100A DC/DC fuse? If the DC/DC fuse popped, that is evidence that the battery polarity was reversed, as the DC/DC converter output circuits will present a short to a reversed battery. In that case there's a good chance that the inverter will need to be replaced (which will probably cost around $3K at the Toyota dealer.)

The lights are controlled by the light switch. However the body ECU is also in the circuit. It could be that the application of reverse polarity voltage caused that ECU to turn the lights on.

The only other way to blow the 100A DC/DC fuse would be if the car was READY and then a sizable short developed from the 12V bus to ground, sufficient to conduct 100A+ so that the fuse would blow.

 

It's great to hear that the inverter was not damaged. You (and your friend) are extremely lucky!

I'm not sure how to interpret why the lights "slowly fade out". If the polarity of the jump battery was reversed, that would force substantial current flow through the Prius 12V battery. Maybe that had the effect of slightly charging the Prius battery so that it would power the lights for a brief moment after you disconnected the jump cables.

The body ECU controls miscellaneous systems. For example it keeps track of whether the doors, trunk and hood are open or closed. It controls the interior lighting and allows the gradual dimming of the lights when you enter the car and then close the door. Minor things like that.

The headlight switch provides ground to the taillight and headlight relay coils, thus energizing the relays so that the lights will come on.

However the body ECU is also connected in parallel with the switch, so if you had the switch off, then I assume the body ECU was effectively providing ground to the relay coils so that the lights would turn on.

It definitely is not normal for the headlights to come on when jumping the car, assuming that the headlight switch is off. That would almost be like a Stephen King novel.

 

Yes, if you see considerable sparking, that is evidence that the current flow is abnormally high. That would be due to incorrect battery polarity or an electrical short.

Incorrect polarity would not affect your Mazda donor vehicle because 1) the jump cables are attached directly to the Mazda battery so no fuses are in the loop and 2) the Prius battery was dead or close to dead, so it would not be able to reverse the battery voltage polarity as seen by the Mazda's electrical systems.

Now, if the Mazda was the recipient vehicle with a dead battery and you were jumping it from another vehicle with a good 12V battery and reversed battery polarity, then you would probably destroy the Mazda's alternator, as the diodes within would be blown. However it's a lot easier and cheaper to replace an alternator than it is to replace a Prius inverter.

 

You were very lucky not to have destroyed anything more expensive than that fuse.

General rule: when you do something, and weird and/or alarming sh!t happens, immediately undo whatever you did.

 

One cannot take somebodys word that the cable in your right hand is POSITIVE, you must walk up to the donor vehicle and verify that that cable is connected to the plus or the larger terminal, then connect, THEN, deal with the negative cable in exactly the same manner. It is sometimes somewhat difficult to discern the positive from the negative terminals. To pre-verify the positive cable under adverse conditions ( a storm ) strike the assumed pos to ground, IF, you get a myriad of sparks .........that's POSITIVE!

 

Patrick, I did the same thing, hooked up a booster pack, and it appears the 100A fuse is blown, from the reverse polarity. However, when I left the car for a time, after the car door was opened and closed, ignition turned on and off, battery cables were reconnected, and I opened the car door, the car horn set itself off and would not stop, until I disconnected the battery. Now I have nothing? What do you make of this? Is it simply the 100 amp, or is likely I also fried the inverter?

 

Hi Sandy,

I don't know why the horn would be blowing. Does the car have an aftermarket alarm system? I suggest you disconnect the horn for now, while troubleshooting.

See if you can find any other blown fuses. Remove each fuse, one at a time, and measure continuity with an ohmmeter. Do not rely on visual inspection. Replace all damaged fuses and make a list of their names for future troubleshooting reference.

While you are checking the fuses, put a charger on the 12V battery. Make sure the battery is fully-charged or replaced with a new battery if the existing battery cannot be charged. Your problem started with a botched jumpstart so apparently the 12V battery was not in good condition at the time you decided a jumpstart was required.

The goal is to get the Prius to the point where you can make it READY again. Once it becomes READY, measure voltage across the 12V battery. If much less than ~13.8V, then the DC/DC converter in the inverter has failed.

 

Hi Patrick, thanks for the info.

The alarm is the manufactured installed alarm.

I don't have tools to test the fuses, or diagnostic tools for reading codes.

I've been researching a bit online. Trying to locate someone who can possibly do install of the 100 amp and a salvage inverter if it is fried. Any suggestions for contacting someone in Los Angeles/ Orange County area? I'm located in the Long Beach/airport area. I don't want to give up this car, I'm original owner with 93,000 miles and the care is in running great shape.

Many thanks.