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How To Perform A Parasitic Draw Test With A Multimeter

Here's where you can find out how to perform a parasitic test with a multimeter
In this article, you’ll learn how to perform a parasitic test with a multimeter

After a weekend of enjoying leisure time at home, Monday morning has arrived yet again. You slide behind the steering wheel of your vehicle and prepare to crank the engine to head to work like any other day. However, today is different. Absent is the sound of your engine firing and settling into its usual smooth idle. 

This morning you turn your key only to be met with no response. But how could this be? Your car has been a reliable companion day in and day out, only to leave you hanging after sitting for the duration of the weekend. If this is the case, you could very possibly be dealing with a case of a parasitic draw.

What Is Parasitic Draw?

A parasitic draw is characterized as a slow draw of current from your vehicle’s battery on the behalf of an accessory or another component even when the vehicle is shut off and the particular component is not intentionally in use. 

This consumption of electricity robs your battery of its standard level of charge. This constant draw becomes evident when your vehicle is not in use, thus rendering your vehicle’s alternator unable to generate a level of charge that would overcome such a draw.

Symptoms Of Parasitic Draw

A vehicle owner often does not become aware of the presence of a parasitic draw until their vehicle does not start as expected on a given morning. The vehicle will usually crank as expected when the duration of time between uses is relatively minimal. However, during prolonged sitting, such as overnight or throughout a weekend, the vehicle in question exhibits cranking issues. The most common symptoms of parasitic draw are a no crank, or slow crank condition.

How To Test For Parasitic Draw

If you suspect that your vehicle is exhibiting signs of a parasitic draw condition, you must verify that your assumption is correct utilizing accurate testing. However, like any other diagnosis that is to be made, the proper selection of tools and other needed equipment is of vast importance.

Tools Of The Trade

A multimeter can be used to test for parasitic draw
Use a multimeter to test for parasitic draw

Verifying Parasitic Draw

The steps to testing for the presence of a parasitic draw are not complicated. However, there is a general order to the process that assists in achieving accurate measurements, as well as preventing damage to your meter or blown meter fuses.

Step#1 – Charge And Test Battery

Begin the testing process by checking the condition of your battery. This is best achieved by charging your battery with the use of a battery charger. After your battery has been allowed adequate time to charge, use an in-vehicle battery charger to test the battery’s status. It is important to validate that your battery has a resting, key off voltage of at least 12.6 volts and that no other forms of internal battery damage such as bad cells exist. Alternatively, if you do not have access to a battery tester, most auto parts stores will conduct a courtesy check for you upon request.

Step#2 – Disconnect Negative Battery Cable

Once you have verified that your battery is of satisfactory operating condition, you will disconnect your negative battery cable at the vehicle’s battery. While testing can be conducted at the positive cable, it is not advised, as any contact with a vehicle ground can lead to a short condition.

Step#3 – Let Accessories Time Out

You will now let the vehicle sit undisturbed for a duration of time. This is to allow all power accessories and modules to “time out.” This allows for accurate testing to take place in the same conditions as would be present as a vehicle sits for an extended period. The exact length of time that is required for all electrical components to time out varies from one vehicle to the next. Consult vehicle-specific service literature to obtain details regarding the necessary wait times for your vehicle.

Step#4 – Set Up Meter

Before actual testing can be conducted, you must first properly set your digital multimeter. Make sure that your black negative lead is placed in the ground port on the meter. Arrange your red positive lead so that it is connected to the amps port. You will always want to start with your red lead in the amp port, as opposed to the milliamp port, because this will prevent meter damage or blown fuses, should the draw be extremely excessive. Next, turn your meter’s dial so that it is set to the amps setting.

Step#5 – Connect Meter In Series

You will now connect the leads of your meter in series between the negative post on the battery and the end of the negative battery cable. This essentially makes the meter an active part of your vehicle’s circuitry to assess draw. 

Step#6 – Change Amperage Ports As Needed

If your digital multimeter shows no reading at this point in the testing, you will need to move your red lead from the amps port to the milliamps port on your meter. This allows you to detect lower thresholds of current draw that might not have registered on the previously used standard amp setting.

Step#7 – Assess Reading

You will now assess your findings. Your reading is to be used to compare against acceptable current draw ratings for your vehicle. Although most models of a vehicle will specify their own particular acceptable current draw thresholds, anything above 50 milliamps of the current draw is generally considered excessive and will drain a battery during prolonged sits.

Diagnosing The Source Of A Parasitic Draw

Once the presence of a parasitic draw has been verified, further testing must be conducted in order to diagnose the source of the draw. This can be accomplished in three easy steps.

Step#1 – Remove Fuses To Find Affected Circuit  

You will now need to locate the circuit that your current draw is originating from. This is best accomplished by removing fuses in a one-by-one fashion, replacing each fuse after it has been pulled. As you pull each fuse, you will be studying your meter to see if the excessive amperage reading drops out. Once you locate the fuse, that when pulled, drops the draw to an acceptable level, you will have located the affected circuit.

Many vehicles have multiple fuse boxes, with some being located in the interior of the vehicle. Before proceeding to pull fuses from an interior fuse box, you will first want to disconnect your multimeter test leads from their point of contact. This prevents spiking amperage from damaging your meter as dome lights and accessories are activated upon opening the door. Depress the plunger that facilitates dome light activation with the use of tape or a block of wood lightly clamped into the door jam. Once a suitable time out period has been allowed, you can resume testing.

Step#2 – Locate and Consult Wiring Diagram

You now must assess which components operate on the affected circuit. This is best done with the use of a wiring diagram. Diagrams can be located on the Internet by various means, including All Data and Mitchell On-Demand. Once a wiring diagram has been located, you will need to carefully study all switches, accessories, and other components that are tied to the circuit in question.

Step#3 – Disconnect Circuit Components

First, ensure that all switches on the affected circuit are moved to the off position. If no change is noted in amperage readings on your digital multimeter, you will proceed to unplug components related to the circuit in a one-by-one fashion. As you unplug each component, carefully study your meter for any fluctuation in amperage. This process will continue until an amperage drop is noted in relation to the disconnection of a particular component. Once this observation has been made, replacement of the component in question is warranted to rectify your vehicle’s parasitic draw condition.

How to verify a parasitic draw and locate the source

Common Sources Of Parasitic Draw

Although thorough testing with the use of the above-outlined procedure is advisable, certain components display a historical tendency as common sources of the parasitic draw, thus warranting careful consideration if such draw is noted. The following are some examples of these common sources of the parasitic draw.

  • Alternators (Bad Diode)
  • Radios
  • Hood Lights
  • Trunk Lights
  • Glove Box Lights
  • Alarms
  • Aftermarket Stereo Systems

Parasitic Draw Solved

If you seem to have repeated issues with your vehicle refusing to crank after sitting overnight, bypass anxiety that comes with such an inconvenience, and diagnose your issue step by step. Although it can be frustrating when your vehicle exhibits the symptoms of a parasitic draw, with the use of quality testing tools and an afternoon under the hood, you can have your vehicle back to cranking with reliability.

Read More: Multimeters User Manuals for Free Download

Josh Boyd

Josh Boyd is an automotive technician with 10 years of experience. He has worked on a wide array of projects in his career. From compact cars to dump trucks, Josh has diagnosed and repaired a little bit of everything in his career.In recent years, he has shared his passion for automotive technology by assisting others through his writing and speaking with high school age students enrolled in trade programs about a future in the automotive field.

Web: https://obdadvisor.comEmail: [email protected]
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