Knowing how to test your vehicle's electrical system for a parasitic battery drain is valuable knowledge! Follow along, and we will show you how this task is completed.
Nothing is worse than getting ready for work, running to your car, and then getting the dreaded click of a dead battery. A parasitic drain from your vehicle's electrical system can result in a dead battery. This can be brought on by many different areas of your car’s electrical system. Aftermarket stereos, lights, phone chargers, and other accessories can ultimately cause a parasitic drain if they tap into your Mustang's 12v power.
Before we begin, we need to have a few tools that will be required for this procedure. This will include a DVOM (digital volt and ohmmeter), a fuse puller or needle-nosed pliers, and some basic automotive hand tools.
The first thing to do is check your vehicle and ensure everything is off. This includes lights, radio, and unplugging accessories from charging ports or cigarette lighters. Make sure all doors, glove box, and trunk are shut. This will help you avoid any false positive results.
Next, we must ensure that your vehicle's battery is fully charged. To detect any parasitic draw accurately, your battery must be at its correct operational state. Many car batteries can usually be tested once at 12.6 volts or higher. If your battery is worn, dirty/corroded, or will not accept a full charge, go ahead and replace this before moving forward with any other testing procedure.If you are unsure how to test your battery, you can always take it to your local mechanic or parts store to have it tested.
You will need to open your hood and locate the negative battery cable. Loosen the cable and make sure it is entirely removed from the terminal. DO NOT use the positive cable for this testing procedure. This can cause electrical shorts if the procedure is not performed correctly.
Connect both of your cables to your DVOM. Insert the red to the highest amp input (typically 20A) setting, and then the black lead on the negative or COM input. Turn the dial on the meter to the AMPERAGE setting. You will want to use a DVOM that has a range of up to 20 Amps and goes down to 200 Milliamps.
Next, attach the DVOM in series between the negative cable and the negative terminal on the battery. If your DVOM has alligator clamp attachments, this will help hold everything in place. You can have a friend lend a helping hand to hold and read the meter.
Now that we have the meter attached, we can go ahead and get a reading. If you are showing anything over 50 milliamps, then you will need to begin your investigation for your parasitic draw. PLEASE NOTE: Most modern vehicles have an average draw of 20-50 milliamps in most cases. This can be from dash clocks or the radio in most cases.
If your reading was over 50 milliamps, you can now begin to test each individual circuit. This task is quickly completed by removing a fuse one at a time. Removing the fuse from the circuit will create an open or break in the flow of electricity. The reading on your DVOM should drop substantially once you remove the fuse causing the issue. (For example, if the reading goes from 2.05 amps to .02 amps.) Start with the underhood fuse panel first (if equipped) and then move into the interior fuse panel. As I previously mentioned, having a friend watching the meter here can help, so you do not have to multitask.
Once you have found the issue, you will need to figure out what circuit is affected. If you check out our Fuse guides on the links below, this can help you break down your Mustang’s electrical system. One example is the interior glove box. The switch can stay on even with the door closed, causing the light to operate. Other common issues can be aftermarket stereo installations or even aftermarket parts such as electric fans that tie into your electrical system.
We hope that this article has helped you better understand how to properly diagnose a parasitic battery drain on your late model Mustang. Always remember that if you do not feel comfortable with any kind of electrical diagnosis of this nature, you can rely on a local shop or professional help to assist you with these issues. Often, these problems can be simple, but depending on the exact circuit, some of these can be tricky backtracking wiring harnesses and reading electrical schematics. For the latest tech info like this one, keep it here with the real Mustang enthusiasts at Late Model Restoration