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Summary
  • Accidentally inserting a battery backward can have a lot of consequences for your vehicle. If you’re lucky, you might just get away with a blown fuse and nothing else.
  • After connecting a battery backward, you’ll need to check components like the powertrain control module (PCM), and alternator, plus your vehicle’s fuses and relays for damage.
  • You’ll need to find all the damaged components and either fix or replace these before continuing to drive your vehicle. This can be a daunting challenge for any DIY mechanic, so we recommend leaving this task to a trained professional.

If you connect your vehicle’s battery cables to the wrong terminals, it can cause a wide range of issues. Reverse polarity in a car battery occurs when the positive and negative terminals are  incorrectly connected, often leading to electrical system malfunction. This can happen when you jump-start your vehicle or if you install a new battery on your vehicle. This can also happen when you get a new battery with reversed battery terminals.

, What Happens When You Connect a Car Battery Backwards?

Pro Tips are nuggets of information direct from ASE-certified automobile technicians working with CarParts.com, which may include unique, personal insights based on their years of experience working in the automotive industry. These can help you make more informed decisions about your car.

Pro Tip: Also, if a battery was drained all the way to being totally dead and somebody connected the charger backwards and charged it back up that way, even connecting that battery the right way can cause the same kind of damage as connecting a battery backwards.

To better understand the potential risks and consequences of reversing battery polarity, let’s go through the components that are directly affected.

When a battery is connected backward, a positive charge is applied to the other end of a device’s circuit, which typically connects to the battery’s negative terminal. This is typically used as a ground source for various vehicle electronics and devices.

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You’ll typically see a welding machine-grade spark when you make your final connection if you’re connecting the battery backwards. If this happens, you need to stop making that connection and hope you didn’t destroy anything.

The diodes in the alternator will feel the surge before anything else does, but today’s vehicles are loaded with dozens of modules that are easily destroyed this way, so pay close attention to what you’re doing and don’t connect the battery backwards.

Caution: Never work around a car battery without eye protection under any circumstances.

The Consequences of Connecting Car Batteries Backwards

So you’ve hooked up the battery backward and now the car won’t start. As previously mentioned, connecting a car battery backward can damage a range of components. The devices that can be immediately damaged include the powertrain control module (PCM), several fuses and relays, and wiring that isn’t rated to handle high-voltage circuits. The alternator and some sensors can also get damaged. The battery itself can even fizz out and leak toxic acid.

Don’t consider reconnecting your battery the right way hoping nothing was damaged. If any damage has occurred, you could just potentially worsen it by reconnecting your battery.

From Richard McCuistian, ASE-certified Master Automobile Technician: One 1998 Taurus that had the battery connected backwards came in with these complaints: “Wipers, fuel gauge, and power windows inoperative. Power steering feels stiff.” The car had to have several modules replaced to repair the damage. The car would still start, but there were several other problems.

What Should You Do If You Connect a Car Battery Backwards?

If you’re wondering what happens if you hook up a car battery backward, the answer is a lot of bad things. If you think you just made a fatal mistake and your battery is connected backward, you should do the following steps:

Note: If the car won’t start, have it towed to a shop. You probably won’t be able to fix it on your own, but if you want to try, you can do the steps below.

Remove Your Vehicle’s Keys From the Ignition

You should also ensure that the engine is turned off. The first course of action is to remove the battery. First, carefully disconnect the cable connected to the battery’s negative terminal to prevent any further damage to your vehicle’s electrical system. Then, you can proceed to remove the positive terminal. It’s recommended that you do this immediately, as leaving the battery connected can continue to damage your vehicle’s electronics.

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Inspect Wiring, Fuses, and Relays

Connecting a battery backward is a fire hazard, since this can cause certain components to catch fire and create smoke. This is why you should immediately disconnect your battery as soon as you’ve realized that the terminals are incorrectly connected. Otherwise, you’ll be applying high-voltage currents to sensitive components.

You should also inspect the visible wiring harnesses in your engine compartment for signs of damage like burnt wires or melted insulation. When wires melt or burn, there’s also smoke and a burning smell.

In addition, check your vehicle’s electronics such as the fuse or relay box. There are usually two fuse boxes in a vehicle. One box is underneath the dashboard, while the other is beneath the hood. You should refer to your vehicle’s owner’s manual to verify each box’s precise location. From there, you’ll need to open them and check the integrity of the fuses and relays.

Vehicle wiring harnesses also have a fusible link, which is a wire that acts like a fuse and protects circuits during a short circuit. When a battery is inserted incorrectly, these fusible links will typically be blown. The vehicle’s alternator will also typically short circuit.

If this happens, some vehicle manufacturers have a fuse that’s designed to protect all other devices. If your vehicle is equipped with this fuse, there’s a chance that only this fuse was blown and other components weren’t damaged. Depending on your vehicle this fuse can be replaced by simply unscrewing the old fuse and screwing in a new one with the same amperage.

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Test the Powertrain Control Module

In rare cases, the PCM can get damaged. If this is the case, you can connect a scan tool to find trouble codes that can help you diagnose the issue. However, sometimes a scan tool might not work when the PCM is damaged. You might need to send it out to be tested and repaired since you can’t test this computer without the right equipment. Alternatively, it might need to be replaced, which is going to be expensive.

If any of the previously mentioned components have been damaged, you’ll need to replace or repair them. If not, faulty relays and fuses will prevent certain vehicle electronics from functioning, which include components like the fuel pump and starter. Computers like the PCM are an integral part of a vehicle. Without it, the engine simply won’t function.

A shorted alternator will also prevent your vehicle battery from charging properly, so you’ll need to diagnose and repair this as well.

Overall, accidentally inserting a battery backward can have a lot of consequences for your vehicle. If you’re lucky, you might just get away with a blown fuse and nothing else. In a worst-case scenario, you might need to replace expensive components like the alternator or PCM.

Nevertheless, you’ll need to find all the damaged components and either fix or replace these before continuing to drive your vehicle. This can be a daunting challenge for any DIY mechanic, so we recommend leaving this task to a trained professional.

About The Authors
Written By Automotive and Tech Writers

The CarParts.com Research Team is composed of experienced automotive and tech writers working with (ASE)-certified automobile technicians and automotive journalists to bring up-to-date, helpful information to car owners in the US. Guided by CarParts.com's thorough editorial process, our team strives to produce guides and resources DIYers and casual car owners can trust.

Reviewed By Technical Reviewer at CarParts.com

Richard McCuistian has worked for nearly 50 years in the automotive field as a professional technician, an instructor, and a freelance automotive writer for Motor Age, ACtion magazine, Power Stroke Registry, and others. Richard is ASE certified for more than 30 years in 10 categories, including L1 Advanced Engine Performance and Light Vehicle Diesel.

Any information provided on this Website is for informational purposes only and is not intended to replace consultation with a professional mechanic. The accuracy and timeliness of the information may change from the time of publication.

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