How to perform a parasitic draw 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 a parasitic draw?

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

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 car 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 a 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, the proper selection of tools and other needed equipment is of vast importance.

Tools of the trade

• Battery Charger,
• Battery Tester,
• Digital Multimeter (Capable Of Checking In Amp Scale),
• Quality Multimeter Leads (Clamp Style Leads Highly Recommended),
• Wrenches (Of Specific Size To Remove Battery Cables).

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 and preventing damage to your meter or blown meter fuses.

Step#1 – Charge and test battery

Begin the testing process by checking your battery’s condition, which you can achieve best by charging your battery using 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 essential 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 the negative battery cable

Once you have verified that your battery is in satisfactory operating condition, you will disconnect your negative battery cable from the vehicle’s battery. While you can conduct testing at the positive cable, we do not advise this, 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 to allow all power accessories and modules to “time out,” allowing for accurate testing to occur in the same conditions as would be present as a vehicle sits for an extended period. The exact length of time 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 car.

Step#4 – Set up the meter

Before conducting actual testing, you must first adequately set your digital multimeter. Make sure that your black negative lead is in the ground port on the meter. Arrange your red positive lead so that it connects 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 too 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, which essentially makes the meter an active part of your vehicle’s circuitry to assess draw.

Step#6 – Change amperage ports as needed

Suppose your digital multimeter shows no reading at this point in the testing. In that case, you will need to move your red lead from the amps port to the milliamps port on your meter, which allows you to detect lower thresholds of a 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 vehicles 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 you have verified the presence of a parasitic draw, you must conduct further testing to diagnose the draw’s source, which you can accomplish in three easy steps.

Step#1 – Remove fuses to find affected circuit  

You will now need to locate the circuit from which your current draw is originating. You can accomplish this by removing fuses in a one-by-one fashion, replacing each fuse after being 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 found the affected circuit.

Many vehicles have multiple fuse boxes, with some located in the interior of the car. 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, which prevents spiking amperage from damaging your meter as dome lights and accessories activate upon opening the door. Depress the plunger that facilitates dome light activation with 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 the wiring diagram

You now must assess which components operate on the affected circuit, which you can achieve using a wiring diagram. Diagrams can be located on the Internet by various means, including All Data and Mitchell On-Demand. Once you have found a wiring diagram, you will need to study all switches, accessories, and other components tied to the circuit in question carefully.

Step#3 – Disconnect circuit components

First, ensure that you’ve moved all switches on the affected circuit to the off position. If there’s no change 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 part, carefully study your meter for any fluctuation in amperage. This process will continue until there’s an amperage drop in a particular component’s disconnection. Once you have made this observation, replacement of the part in question is warranted to rectify your vehicle’s parasitic draw condition.

Common sources of parasitic draw

Although thorough testing using the above-outlined procedure is advisable, specific components display a historical tendency as conventional sources of the parasitic draw, thus warranting careful consideration if you’ve noted such a draw. 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 car back to cranking with reliability.