Crossed connection on jump start

CAMRONSIS300

MEMBER
2002 LEXUS IS 300
3.0L
6 CYL
RWD
AUTOMATIC
150,000 MILES

My battery was dead so I jump-started it. I accidentally cross-connected the battery. I went to start the car and I noticed on the panel, it showed that my high beams were on and my emergency flashers were on but not flashing, it showed them being solid. So I disconnected the jumpers and sure enough, yep I am a messed up and crossed them. I placed them back correctly and tried to start the car, it started! But none of the instrument panel was working, or radio, clock, power windows, pedal, sunroof, etc. The only thing that worked was my headlights and dome light. I tried revving the engine and I noticed that the gas pedal had to be depressed close to half way down before the engine would increase in revs. So the throttle is not responding correctly but the car was still running, a plus I guess. Well I got out and disconnected the jumper cables and right when I removed them the car stopped running? I have noticed that the etcs fuse is blown along with the radio number one. All other fuses under the hood at the battery are good. I am currently checking the ones at the kick panel inside the car.
I am not sure where any other fuses are located and I do not know how to check the relays if that even pertains to this issue. If someone could give me some ideas on this matter, and directions on where other areas to look, that would be awesome! Thank you for your time!

1 Reply

CARADIODOC

MECHANIC
33,916 POSTS

Computers cannot tolerate reversed polarity so they have diodes added between the twelve volt supply and the ground wires. Diodes are one-way valves for electrical current flow. They are in the circuit backward so they do not do anything, until the battery cables or jumper cables are reversed. That places the diodes in the circuit "forward biased". They act like a dead short, causing the fuses to blow, thereby protecting the rest of the circuitry. The fuses often do not blow fast enough to avoid damage occurring to a computer.

The place to start is by replacing all the blown fuses. While it is possible for a relay to be damaged, it is not common. They also have diodes across the relays' coils, but for a different purpose. In this application they are also backward, to dampen the voltage spikes all relays develop when they are turned off. If the polarity across the coil is reversed, the relay itself would still work just fine, but it is those spike suppression diodes that would appear as a dead short. First of all, the voltage would have to be applied to the coil, and it would have to be reversed, to do possible damage. Second, almost all of the relays are turned on by computers, and those will not be operating with reversed voltage. Do not even think about a bad relay until further testing suggests otherwise.

See what works and what does not work once the fuses are handled.