Used a jump starter to get the car started (battery needed replacing). And I switched the positive and negative leads when I attached them. The power arced and threw a spark.but I still didn't realize what I had done until the car would not start. Realizing my mistake, I switched the leads to the appropriate poles, and the car started. The jumper started emitting a high pitched tone (which it continues to make after 2 weeks) and some things stopped working on the car. Now the rear backup sensors don't work, using the turn signal to the right makes the bright headlights come on until I jiggle the turn signal lever, the radio and CD changer will turn on for 2 seconds - display light comes on but no sound comes out - and then it turns off. Headlights are fine, and all other lights and devices work. SO MY QUESTION IS THIS: If I take it to my mechanic, will he be able to fix this (maybe by installing a few new fuses) or have I wrecked the electrical system? What do I do? I don't want to spend $$$ on diagnostics if it is obvious to one of you experts that this damage is permanent or exorbitantly expensive to fix, What would you do? I can replace a few fuses myself if that's all it is, but I don't want to replace the radio/CD player and the back up parking sensors. Feeling really stupid since I never made this mistake before. Please advise.

 

i would start by checking all the fuses myself, since its easy and doesn't cost anything to look. also check your relays by making sure theres power coming out of them with a test light or preferably a multimeter.

 

I would start by asking the dealer what gets fried when the terminals get reversed. Not all vehicles have safeguards in all systems for reversed current flow. I don't think that the fuses would blow since there was no short circuit, but a reverse of polarity.

 

I am skeptical that you caused much damage. I don't know how smart the charge box is, but if you left it hooked up, and it was actually working, it would have melted. Yet you say that you reversed it and then started the car. So based on that I think it turned itself off, and that it's designed not to melt itself. That probably mitigated some of the damage.

The things that aren't working on your car may all have stopped working just because the battery ran down. Those XC90's are pretty finicky.

If you're right, though, I know of a related example here. It blew out the headlight relay, and on any car it'll usually melt the charging wire. I'm not sure what else got fried, as they did not let me fix it. Anything that uses a diode would look like a short circuit with the voltage reversed, and the alternator has 3 diodes that are pretty darn big. if your car is charging, then the list of things you damaged is getting very short.