Reversed Jumper Cables and Car Won’t Start: What to Check

Reversed Jumper Cables: What Happens and How to Fix It

Everyone warned you to make sure you’re connecting the right jumper cable to the right terminal. But it was dark, you were cold, and well, it was a mistake. Here’s what happens when you reverse the jumper cables, the items that most likely got damaged, and what to check to get your car started again.

Reversed jumper cables always blow fuses

Well, at least in late-model vehicles, it does. They have high-current fuses that control the vehicle’s power. If you own a late-model car, start by checking for a blown high current fuse. But older cars don’t have those fuses. If you own a pre-2000 vehicle, scroll to the old car section for more advice.

Where are the high-current fuses?

Here a high current fuse box diagram for a 2013 Ford Focus. Here, you see two MEGA fuses; one is 150 amps, and the other is 80 amps. Using a multimeter set to the ohms scale, test each of these to see if it has continuity. If so, the fuse is good, if no, you’ve blown that fuse and it must be replaced. But don’t think you’re in the clean by replacing just one fuse. It’s common to blow multiple fuses when you reverse jumper cables. So it’s wise to test all the fuses in the high-current junction box before making the trip to the auto parts store for replacements.

After replacing the blown high-current fuses, try starting the vehicle. If it cranks but won’t fire up, you’ll have to check all the fuses that are related to fuel pumps, fuel injectors, anti-theft system and the ignition system.

If it won’t crank, there’s a whole series of fuses to check since energizing the starter motor involves the body control module (for the theft protection system and push button start switch, the transmission control module to confirm that the shifter is in the park/neutral position and the starter relay.

If all those fuses are good and it still won’t crank, you’ll have to get the diagnostic procedure from a pro-grade shop manual.

If all the fuses are good, there’s a good chance you’ve damaged at least one computer

At this point, it’s the job of a technician with a scan tool to track live data to see what the ECM is seeing.

What to check if you reversed polarity on an older vehicle

Older vehicles don’t have MEGA fuses, but they do have fusible

Fusible link

links. A fusible link is a wire that melts when the current exceeds the link’s rating. Once it melts, it creates an open circuit, just like a fuse. The only way to fix a blown fusible link is to locate it and splice in a replacement.

What else to check

Check the battery terminals and posts for burned or melted spots

Check the insulation on your jumper cables for melted insulation. Discard the cables if you find that.

Check the battery to ground wires. They’ll most likely be melted as well. Replace the wires that have any sign of melting.