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.
Thursday, March 2nd, 2017 AT 4:34 PM