Connected Jumper Cables the Wrong Way

Car owners may accidentally connect the jumper cables in reverse order or install the battery backward, which causes the vehicle to no longer start.

The purpose of this troubleshooting guide is to help answer the following questions:

  • Hooked battery up backward now car won't start.
  • What happens when you connect a car battery the wrong way
  • The car battery is connected backward. 
  • The car battery was installed incorrectly.
  • What happens if you hook up battery cables wrong
  • Accidentally installed car battery in reverse
  • Hooking up battery cables backward
  • Hooked battery up backward on a four-wheeler

Connecting the jumper cables backward or installing a new battery the wrong way is never fun. In this article, you will find troubleshooting steps that will help you diagnose such problems. 

Installed new battery backward, the car won't start.

car battery short wrong connection

You decided to change the car battery but accidentally put the cables backward. Instead of connecting the positive (+) cable to the positive battery terminal and negative (-) cable to the negative battery terminal, you hooked them up backward.

 All of a sudden, you see a scary spark, and your car goes completely dead.  The car will no longer start. Dashboard lights are off, and everything is dead. The key may not even turn the ignition. Similar symptoms will be experienced when you try to jump-start a dead battery, but you accidentally connect the jumper cables backward. 

When a car battery is connected backward, a fuse designed to protect vehicle electronics should blow. If your vehicle doesn't have a fuse (almost all cars do) designed for this purpose, you will send electrical current backward through systems in your car, including ECU, transmission control unit, and more.

If current flows backward through lights, that's not a problem. The issue is when current flows backward through electronics with diodes such as the ECU / ECM (Engine Control Unit / Module).

It is not common to damage the Engine Control Unit / Module by disconnecting the battery cables.

Most of them are designed to withstand reverse polarity. In the worst case, the ECU/ECM can be removed and inspected if a diode has failed. 

Troubleshooting Car Won't Start Problem

Step 1

Car?battery backward?damage

Check the high ampere fuses in your car. Most vehicles have a large fuse that will blow and avoid damage to the ECU / ECM. This may be a 40, 60, 80-ampere fuse, and it is usually hard to find.

Here is a picture of this fuse on a Honda Civic. When the jumper cables were connected backward, this fuse blew. This is from a 2015 Honda Civic. 

Notice the fuse strip. More than one fuses can blow. Once you replace this fuse, start up the car.

In most cases, the car should start right up. If it doesn't, continue to Step 2. 

Step 2

What happens when you connect a car?battery backward

After you replace the blown a fuse, you may still have a problem starting the engine. The engine may turn over and crank, but it will not start.

Now it is time to check all the smaller fuses. Ensure that the fuse for the ignition system, ECU, fuel pump, Immobilizer is still good. 

Step 3

If all the fuses are still good and your car still refuses to start, here are a few things you need to check. 

The car turns over but won't start. 

  • Check if there is a spark at the plugs.
  • Check fuel pressure
  • Check if direct injectors are getting power.

The car won't turn over or crank. 

  • Immobilizer system
  • Engine Control Unit is getting power
  • Check to see if the starter is getting power.

Step 4

Car?battery?installed incorrectly

Hopefully, you were able to diagnose the problem yourself. If you are still having problems with starting the car, you may want to consider having a mechanic take a look at your car. Avoid letting water or rainfall on your fuse panel. 

Do not allow water or rain on your fuse panel. 

Comments

dummydun, March 23, 2021

i put it in backwards, i’ve checked the ecu and i can not figure out which wire or which fuse or anything it could be do i need to replace my ECU?

Almost every car has a fuse to protect for this case. The problem is that the fuse that blow is usually not listed in the fuse list. You may have to do some research. Since I don't know which car you have I can't really help you there. The only thing that I would say is that the fuse you are looking for it will most likely be over 75A and be large. Not like the small 7.5A, 15A, 30A that you have for lights, radio etc. 

Travis, March 31, 2021

There isn't many ways to find out this problem anywhere on the internet!

Mazda6racer, March 31, 2021

I have 2011 Hyundai sonata with 127K miles and i see metal shavings during the oil change, but the motor runs fine.

Check with Hyundai and let them know. It may be covered by their extended warranty. There are tests that the dealer can perform. Don't ignore it though. There should not be any noticeable shavings in the oil. At least not easily noticeable by the naked eye.