Molly Ayala
Testing for an electric draw, also known as a parasitic draw, can be done using a multimeter. A parasitic draw occurs when a vehicle is drawing power from the battery, but all of the lights and other electrical components are turned off. This can be caused by anything from a faulty radio or alarm wiring to more advanced electronics systems such as GPS or proximity key systems. To test for a parasitic draw, it is recommended to use a digital multimeter that can measure amperage and has an auto-ranging mode. The multimeter should be connected to the negative battery terminal of the vehicle, and then fuses should be removed one at a time while observing the multimeter for any changes in reading. A small parasitic draw under 1 amp will take a few days to drain a battery, while anything over 1 amp could drain it overnight.
| Characteristics | Values |
|---|---|
| Tools | Digital multimeter, voltmeter, Innova 3320, Innova 3340 |
| Vehicle preparation | Park in a safe, well-ventilated area, turn off all electrical systems, wait 20 minutes for vehicle modules to enter sleep mode, lock the vehicle, close the hood latch, engage the emergency brake, remove the key from the ignition |
| Multimeter setup | Switch to DC ampere (A) mode, set the range to a suitable value higher than the expected current draw (usually in milliamperes or mA), start with a range of 10A |
| Battery cable | Disconnect the negative battery cable, loosen and remove with a wrench or socket |
| Multimeter connection | Connect the red lead to the battery terminal and the black lead to the battery cable, plug the black probe into the common (COM) jack input of the multimeter and the red probe into the amps (A) jack input |
| Parasitic draw values | A certain amount of draw is normal (around 25mA), but a draw of 100mA or more indicates an issue |
| Circuit diagnosis | Check all fuses, relays, and fuse boxes, test the alternator, measure the voltage drop across fuses to identify live circuits, check each device or component on the circuit |
| Repair | Replace or repair the faulty component, disconnect the multimeter, reconnect the battery |
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What You'll Learn
Prepare the vehicle
To prepare a vehicle for an electric draw test, you should first park the vehicle in a safe, well-ventilated area. Ensure that all lights, accessories, and electrical systems are turned off, and engage the emergency brake. Turn off the radio, heat or A/C, lights, windshield wipers, and close your glovebox and lighted mirror covers.
Next, you need to open the vehicle's hood and locate the battery. Before proceeding, ensure that the battery is fully charged. If it isn't, use a vehicle battery charger to charge it to 100%. Many car batteries are 12.6 volts, so you can use a multimeter to check the power and ensure the battery is fully charged. If your battery is old or damaged, or isn't reading 12.6 volts when fully charged, you may want to replace it.
Now, you need to disconnect the negative battery cable from the negative battery terminal. Find the negative cable, which will be marked with a minus sign (-) and may have a black cover over it. Remove the cover, if applicable, and use a wrench to unbolt the negative cable from the terminal. Be sure to disconnect the negative, not the positive, cable!
Finally, attach a digital multimeter to the negative battery cable and terminal. Switch the multimeter to DC ampere (A) mode and set the range to a suitable value higher than the expected current draw (usually in milliamperes or mA). Start with a range of 10A if available.
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Set up the multimeter
To set up a multimeter to test for an electric draw, you'll first need to ensure you have the right type of multimeter. A basic digital multimeter is best for this test, as opposed to an analog one, which isn't as accurate. You can purchase a basic multimeter at a home improvement store, hardware store, or online. Brands like Innova are renowned for their accuracy and dependability, so you might want to consider one of their products.
Once you have your multimeter, you'll need to set it up correctly. First, make sure all parts of your multimeter are in good condition. Check for cracks on the device and any fraying or nicks in the wires. Do not use the multimeter if it is damaged. If you want to feel safer when testing, wear rubber gloves and shoes with rubber soles.
Next, you'll need to set the multimeter to the correct unit. In this case, you'll want to set it to the DC ampere (A) mode. You'll also need to set the range to a suitable value higher than the expected current draw, usually in milliamperes or mA. Start with a range of 10A if available.
Now you're ready to connect the multimeter to the circuit you're testing. The red probe should be connected to the positive terminal, and the black probe to the negative terminal. With the multimeter connected, you can observe the reading. It's normal to see some initial spikes, but within a few minutes, the current draw should stabilize.
If you're testing a vehicle for a parasitic draw, you'll need to disconnect the negative battery cable before connecting the multimeter. You'll connect the red probe to the negative battery cable and the black probe to the negative battery terminal.
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Disconnect the negative battery cable
Disconnecting the negative battery cable is a crucial step in testing for an electrical draw, but it should be approached with caution. Before proceeding, ensure that your vehicle is prepared and that you have the necessary tools, such as a wrench or socket, and a multimeter.
To begin, open the hood of your vehicle and locate the battery. The negative battery cable is usually identified by a "-" symbol or a black colour. Using your wrench or socket, proceed to loosen and remove this cable from the battery terminal. This step is essential for performing a parasitic draw test, as it allows you to measure the electrical draw from the negative side, eliminating the risk of accidentally shorting if testing from the positive side.
It is important to note that disconnecting the negative battery cable should be done with caution, especially in modern vehicles. Older vehicles without computer systems could handle this process without damage. However, in contemporary cars with multiple microprocessors, disconnecting the negative cable can potentially fry every electrical device. This occurs because, without the battery in the circuit to act as a buffer, the voltage regulator forces the alternator to output maximum power, which can reach up to 150 volts. Therefore, it is recommended to use a voltmeter or a multimeter to test for electrical draws, rather than directly disconnecting the negative battery cable.
Additionally, when preparing to disconnect the negative battery cable, ensure that your vehicle is locked, and if it has a hood latch, make sure it is securely closed. For keyless start vehicles, move the key fob away from the car. These steps help ensure safety and prevent any unintended activation of the vehicle's systems.
By following these steps and taking the necessary precautions, you can safely disconnect the negative battery cable to perform a parasitic draw test and identify any electrical issues in your vehicle.
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Connect the multimeter
To connect the multimeter, start by switching the multimeter to DC ampere (A) mode. Set the range to a suitable value higher than the expected current draw, usually in milliamperes or mA. It is recommended to start with a range of 10A if available.
Next, connect the multimeter leads in series between the negative battery terminal and the negative battery cable. Ensure that you connect the red lead to the battery terminal and the black lead to the battery cable. Alternatively, you can plug the black probe into the common (COM) jack input of the multimeter and the red probe into the amps (usually A) jack input. Then, connect the red probe to the previously disconnected negative cable and the black probe to the battery terminal.
It is important to note that the multimeter must be connected in series with the circuit to test for current as it needs to gauge electron flow. On the other hand, voltage is measured in parallel.
Once the multimeter is connected, allow the car time to sleep. Modern car systems may 'wake up' when a multimeter is connected due to the power of their batteries. It can take up to 20 minutes for the vehicle's modules to enter sleep mode. During this time, ensure that the vehicle is locked, and if it has a hood latch, make sure it is closed. For keyless starts, move the fob away from the vehicle.
Additionally, you can open a door and close the latch with a screwdriver to gain access to the fuse block in the car. This will allow you to disconnect fuses and components to further diagnose the circuit causing the parasitic draw.
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Identify the circuit responsible for the draw
To identify the circuit responsible for the draw, you can use a multimeter or a clamp ammeter. A multimeter is a measuring instrument used to measure electric current, voltage, or potential difference. Before you start testing, ensure that the battery is fully charged. Some circuits will remain live for a while until the modules that supply them go into "sleep" mode. This can take anywhere from 30 to 120 minutes, depending on the manufacturer and the module.
Once you have the multimeter, set it to measure amps and connect it in series between the negative battery terminal and the disconnected negative battery cable. Next, pull the fuses one by one until the amperage draw on the meter drops. This will help you identify the circuit causing the draw.
You can also use a clamp ammeter to measure current draw. Clamp the ammeter around the wire you want to measure and set the dial to the proper spot. You can clamp individual circuits as well as the conductors leading out to the meter.
Having a wiring diagram and knowing the locations of connectors, relays, and modules can be extremely helpful when performing this test. This information can help you understand the design of the electrical circuit and diagnose any problems.
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Frequently asked questions
A parasitic draw is when your vehicle is drawing power from the battery when all the lights and other electrical components are off. A small parasitic draw under 1 Amp will take a few days to drain a battery; over 1 Amp could drain a battery overnight. A certain amount of draw is normal – in the region of 25mA (milliamps) – but if you have 100mA or more being lost, something is up.
You can use a digital multimeter to test for a parasitic draw. First, prepare the vehicle by parking it in a safe and well-ventilated area, ensuring all lights, accessories, and electrical systems are turned off, and waiting for at least 20 minutes to allow the vehicle's modules to enter sleep mode. Next, set up the multimeter by switching it to DC ampere (A) mode and setting the range to a suitable value higher than the expected current draw. Then, disconnect the negative battery cable and connect the multimeter leads in series between the negative battery terminal and the negative battery cable. Finally, connect the red lead to the battery terminal and the black lead to the battery cable.
Once you've identified a parasitic draw, you can proceed to disconnect sections of the harness or components in that circuit to pinpoint exactly what's causing the draw. For example, if the circuit causing the draw has a driver's heated seat, you can unplug the heated seat relay and see if the draw goes away. If it doesn't, you can move on to checking other components in the circuit.
