How to Find a Parasitic Draw

There is nothing worse than coming out to your vehicle in the morning and finding out that your battery is dead. There is nothing worse than calling your boss in this situation. 

But what if you just had your battery’s condition checked. Should you replace the battery just in case? 

If your vehicle has continuously been going dead and your local mechanic cannot find the issue. What do you do?

You can test your vehicle’s electrical system for a parasitic draw. 

Many people do not understand what a parasitic draw is or how to complete a parasitic draw test. We are going to get technical in this article and you will learn everything you need to know to complete a proper parasitic draw test, with an automotive multimeter and other tools that you may already own.

What is a parasitic draw or battery draw?

There are many different names for a battery that loses power over time.  You will most often hear it referred to as a battery drain. The technical term for a battery drain is a parasitic draw. 

A parasitic draw is when current flow is present in the electrical system of your vehicle even with the engine and key turned off. Over time a parasitic draw or battery drain causes the battery to lose all of its charge resulting in a dead battery.

Most people learn about a parasitic draw after they have their battery replaced and experience the same dead battery issue that they had before. This is when shops will then decide to properly test the electrical system for a draw. Draw testing needs to be done every time you run into a dead battery. 

To be honest, many shops overlook this step and do not complete the test.

What causes a parasitic draw?

Like we mentioned above a parasitic draw is when there is current flow in the electrical system of your vehicle when the key is turned off. A parasitic draw is caused by an electrical component that stays on and does not turn off. This is sometimes referred to as the component not going to “sleep”.

This can happen many different ways. The most common cause of a parasitic draw is an electronic control module that stays “awake”. Every electronic module on a vehicle is designed to turn off all of its internal functions when the key is turned off. Sometimes these modules will have an internal failure that results in it not turning off.

This is very hard to find as the module does not set a check engine light or any other indicator light to tell you what is going on. If you run into your battery going dead and the battery has been replaced or has tested good, you need to complete a parasitic drain test to find the cause of the problem.

Other times there may be a switch that is stuck that causes a component to stay on. One of the most interesting examples of this is a glove box light that I once found staying on when inspecting a vehicle.

 This one was a hard one to find as the light was on inside of the glovebox. The light was not visible when the glove box was closed. When the glove box was open the light would turn on as usual. This led me to think that it was working correctly, after a parasitic draw test I was able to pinpoint the problem however. 

This is a great example of how hard a battery drain can be to find sometimes.

How to complete a parasitic draw test

Before we start with this how-to guide we need to cover a few basics before beginning. Make sure your battery is fully charged before completing a battery draw test. A faulty or low charged battery can skew your test results making testing even harder. 

Test drive the vehicle and use as many of the electrical accessories that you can during the drive. If you have an electrical component that is causing the draw, this can help cause the issue to occur so it is present when you test. Also make sure the windows are open, all the doors are closed and the hood latch is manually closed to ensure the vehicle does not think the hood is open.

  1. Install a 10-amp fused jumper wire between the battery positive terminal and the battery positive cable lead. This will give us the ability to remove the battery cable without interrupting the power to the vehicle.

Note: it is important not to lose connection. If you do, the components on the vehicle will turn on when power is reconnected causing your readings to be incorrect. 

  1. Remove the positive battery cable without disturbing the 10-amp fused jumper wire. Observe the fuse in the jumper wire. If the fuse blows this means there is more than a 10-amp draw and will blow the fuse in your multimeter. If the fuse does not blow continue to the next step.
  1. Temporarily re-install the battery cable on the battery. Setup your multimeter by putting the leads in the correct ports for testing amperage. Ensure that you are on the amperage range so your meter has a 10 amp internal fuse protection.
Correct settings for multimeter to complete a parasitic battery draw test
  1. Remove the positive battery cable. The current will now flow through the meter and give us an indication of how much current is flowing through the battery.
Meter clamps hooked up to a battery to complete a parasitic draw test
  1. Monitor the multimeter. You will need to wait close to 15-20 minutes before getting an accurate reading. This is caused by many components of the vehicle that need to turn off. It can sometimes take up to 30 minutes after your vehicle is turned off before all the electrical components turn off completely. 
  1. Check the battery draw. If your battery draw is less than 300 milliamps I would suggest changing your setting on your multimeter to the milliamps range and repeat steps 3 and 4.
image of a multi meter being used to find a parasitic draw
  1. If your battery draw is 60 milliamps or below your battery draw is within specifications. If it is above you need to look for the cause of the draw. 
  1. Remove the lid of your fuse panel. Take an image where all of the fuses and relays are located in your fuse panel. This is very important for reference later when you re-install all of the components that you removed. 
  1. Remove 1 fuse at a time. Wait 2-3 minutes and observe the multimeter to see if the current draw has dropped within specs. Continue to remove 1 fuse at a time while monitoring for changes. Once you see a drop, mark which fuse was removed when you observed the drop. 

Note: By removing fuses one at a time you are removing the power from the component that is staying on. When the power is removed the current draw stops indicating you are closer to finding the issue.

Image of a fuse panel, showing a 15 amp fuse
  1. Reference wiring schematics for the vehicle that you are testing. If you need good wiring schematics, check out our service manual review to find out which manual will fit your needs best. 
  1. You will need to look at the wiring schematic and see which component is fed voltage by the fuse. Once this component is found, gain access to the component and remove the wiring connector. 
  1. Reinstall all of the fuses removed and re-start the test. If the draw is now gone you found the component that was causing the draw. If the draw is still present you will need to trace the wiring between the electrical component and the fuse block. 
  1. Once the part is replaced, complete another draw test to verify that your testing was correct. 
Image of a multimeter with an acceptable battery draw

Draw testing can be a time consuming test to complete correctly. 2 to 3 hours is recommended before starting your testing. The test is not something that you can stop half way through. Once you have started you will need to finish to make sure your testing was done correctly. 

Do not be intimidated by electrical repairs and electrical diagnosis. They are problems that can be solved if you take your time and have the correct tools. You will need to have a multimeter, correct wiring schematics and the ability to take your time. With all of these essential tools you will find the issue and have your vehicle back on the road as fast as possible.

John Morris
John Morrishttps://autoknowit.com
John Morris is the technical editor for AutoKnowIt.com. His years of experience in automotive repair as well as an automotive professor have prepared him to ensure that even the most technical information is accurate and concise at all times.

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