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Parasitic Draw Diagnosis: Tips for Best Approach

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Voltage drop testing across a fuse to detect a parasitic draw prevents the disconnecting of the battery and allows all the systems to stay fully powered and functional.

Diagnosing a parasitic draw can be a time-consuming challenge. The complaint is frequently that the engine won’t start after a couple of days of sitting because of a dead battery. Often the battery has already been changed, but the problem remains the same: The engine won’t start after sitting for a few days and needs to be either charged or boosted.

Basics

Before we start a parasitic draw diagnostic, we need to ensure that the vehicle’s battery is fully charged and in a state of good health and that the charging system is functioning correctly. We need to verify that the correct battery is installed and that it will meet or exceed the vehicle’s reserve capacity (RC).

We need to verify that the customer’s driving habits aren’t to blame. Frequently driving short trips or a vehicle that sits for days or even weeks before being started can easily cause a battery charge to be depleted.

We also need to verify that all the customer’s plugged-in accessories are removed from any power ports or audio/video ports and that there aren’t any devices plugged into the diagnostic link connector (DLC). A good visual inspection and a customer debrief are important. 

Once we have verified that nothing appears to have been left on or plugged in, the customer’s driving habits aren’t to blame, the charging system is fully functional, the correct battery is installed, and it has a proper state of charge, we can begin the process of checking and diagnosing a parasitic draw.

What is a parasitic draw? 

A parasitic draw is when an electrical component continues to consume energy from the battery when it shouldn’t, even after it and the vehicle have been shut off. Some battery draws are acceptable and are required to retain memory, settings, security and other functions on the vehicle, but these are designed into the system and the proper battery will be able to maintain and deal with these.

What is an acceptable mA battery draw and how do we test for it?

Most manufacturers publish an acceptable mA (milliampere) draw range specification. General Motors (GM) suggests 40 mA as the maximum normal reading. Nissan considers 25 mA draw as normal. Dodge/Ram considers 5 to 35 mA as typical. Honda has a range vehicle specific of 27 to 42 mA. 

If you can’t find a published specification, there is a rule of thumb that we can use to determine a maximum mA value. It involves using the factory-recommended battery’s RC value and dividing it by four to come up with an mA battery draw value. If the RC of the factory battery was 140, then using the math: 140 divided by 4 equals 35. Using this formula, the rule of thumb value for this battery would be a maximum 35 mA draw. 

There are two common methods used to detect a parasitic draw: disconnection and voltage drop. The disconnection method involves the installation of a digital volt ohm meter (DVOM) set to the mA scale installed in series between the disconnected negative battery terminal and the negative battery cable end. The excessive draw is then located by slowly removing one fuse at a time until the circuit that was causing the draw was identified. Using this method, though, can hide the issue because the interruption of power (battery disconnected) could cause the module, relay or device that is the cause of the excessive current draw to shut off, release or go to sleep. This could mask the issue until it acts up again. 

The voltage drop parasitic draw detection method allows all the circuits, modules and communications networks to remain functional, powered and intact because the battery is not disconnected for this test. 

A voltage drop is created by the internal resistance of a circuit when the current flows through it and the material that the fuse is made of causes some resistance and creates a small but detectable voltage drop when current flows through it.  We can measure this voltage drop using a DVOM set to millivolt (mV) and use it to identify the circuit that might be the cause of our excessive current draw.

There are many different types, sizes and ratings of fuses being used on the modern vehicle, and there are charts that can be found online or in our information systems to display what the voltage drop across each type of fuse will translate to as an mA reading. 

The modern-day parasitic draw testing procedure that I follow is a combination method. This procedure is a multi-phase blended approach that is effective at finding and identifying a parasitic draw on modern vehicles. 

The Combination Method 

Phase 1: Start with proper vehicle preparation and the use of an inductive amp clamp.

Step 1. Prepare the vehicle; perform the needed battery tests and verify that the issue is not caused by a customer driving pattern. Perform a full module scan and note any codes. Many European vehicles can set codes for parasitic draws. BMW offers a diagnostic “Energy Management test” that may identify the area of concern.  Remove the scanner and verify there are no aftermarket accessories connected to the auxiliary power ports, audio/video ports or the DLC. Open your information system and note the location of all the fuse blocks on the vehicle being tested.

Step 2. Road test the vehicle and activate as many accessories and devices as possible from the infotainment systems, HVAC, power windows, etc. The idea is to operate as many components as possible, to verify the proper operation and to observe if anything doesn’t function as designed.

Step 3. Return to the shop after the road test and turn the ignition off, open all the doors, hood, liftgate and manually close all the latches and ajar switches. Using the key fob (if equipped), lock and set the vehicle’s alarm. Ensure the vehicle’s transmitter key fob is taken away from the vehicle so that the security system won’t detect it and possibly wake up the systems, or not allow them to fully power down. 

Step 4. Attach an inductive amp clamp capable of reading down to 1 mA to the negative battery cable. Set the amp clamp to the mA scale and note the reading.

Step 5. Allow the vehicle to go to sleep. Most vehicles will start to go to sleep after about 10 minutes, with periodic wake-up cycles as the vehicle powers down. Falling fully asleep may take as long as two hours and will be different for each make and model. Typically, the vehicle will have less than a 75 mA draw during this falling asleep period.  

Note: An engine-off natural vacuum evaporative test can occur during this time and cause a draw. Some vehicles may be equipped with an HVAC afterblow that may power the HVAC fan to blow for a specified length of time. These conditions are normal.

Step 6. After everything has gone to sleep, the expected mA draw displayed by the amp clamp should be less than the max mA draw that the manufacturer specifies or if no specification is available, less than the RC calculated value. 

• If the value is less than the vehicle specification mA draw or the RC calculated mA draw, the issue isn’t present at this time or the issue is intermittent. You may have to reinitiate the test again to see if you can duplicate the problem.

• If the value is above the vehicle specification mA draw or the RC calculated mA draw, then we will need to continue testing using Phase 2. 

Phase 2: Voltage Drop Testing

Step 7. Using our DVOM, set to the mV scale, perform a voltage drop test across each fuse, one at a time to determine which fuse has the parasitic draw or excessive current flow (you can ignore negative readings). Start by first testing all the smaller Micro2, Micro3, Low Profile, Mini, Regular and Maxi blade fuses.  You can look up the mV to mA conversion charts online to aid in diagnosing how much current is flowing across each fuse, but a simple like vs. like comparison (10-amp mini vs. 10-amp mini, for example) should point out the larger current draw quickly. We will be leaving the larger J-Case type fuses (the ones with the plastic covers) for Phase 3. 

Note. If the vehicle is equipped with a battery junction block that provides access to mega fuses that feed other fuse blocks, start a voltage drop testing here first. The mega fuse with the highest voltage drop indicates the fuse block to start our fuse-by-fuse voltage drop testing looking for the fuse that is supplying the power to the parasitic draw circuit. 

Note. If the vehicle is using J-Case type fuses, I don’t recommend prying off the plastic covers to perform a voltage drop test. Move on to voltage drop testing all the smaller fuses in all the fuse boxes first, and if that doesn’t yield results, we will have to move on to Phase 3.

Note. When testing the Micro3 fuses that have three legs, the center pin is the common voltage supply, so remember to test both sides of these fuses. 

Note. Some fuses may have an initial voltage drop that fades away, and some fuses may always have a nominal draw that is supplying power for memory retention. This is normal.

If during Step 7, we find a single fuse that has an excessive voltage drop, do not remove the fuse, as it might cause the draw to go away and it might not return. Locate the fuse in your wiring diagram to identify the component or components to which the fuse supplies power. If the fuse is supplying multiple devices, disconnect them one at a time until the draw is gone. If all the devices in the wiring diagram are disconnected and the draw remains, be suspicious of an aftermarket device being installed somewhere in the circuit. If individual fuse testing doesn’t identify a voltage drop, go to Phase 3.

Phase 3. The old method last

Step 8. If we haven’t located the parasitic draw yet by using the voltage drop test, we will use our previously installed inductive amp clamp in the final step. We will start removing all the J-Case fuse(s), circuit breaker(s) and relay(s), one at a time while monitoring the mA draw on the inductive amp clamp, to verify if it drops down to or below our vehicle specification mA draw or RC-calculated mA draw. Many of these J-Case fuses will supply high power to individual devices, for example, the anti-lock brake system (ABS) module. When the excessive draw is removed after a J-Case fuse or other device is removed, a careful review of the wiring schematic will show which fuse or device is supplying power. Disconnecting these device(s) one at a time should identify the probable cause of the excessive draw.

Note: Before removing anything from the fuse block under inspection, take a quick photo with a cell phone to ensure stuff gets plugged into the correct spot and correctly oriented to the original position.

Step 9. Many times, the actual fuse block itself can be a module and have components, relays and processors that are integrated internally. If the previous steps have isolated the parasitic draw to one fuse block and all the other testing has failed to show or produce the device that is causing the parasitic draw, the final step is to remove/unplug the suspected fuse block. If the draw is now gone, the issue is likely in the fuse block itself. 

Drawing a conclusion

Typically, identifying a parasitic draw takes time, a detailed approach, and patience from the tech and the customer to be successfully diagnosed. There isn’t a magic bullet or special tool that will quickly and easily point a technician at the source of a parasitic draw, especially if the parasitic draw is being caused by a non-factory installed device. But this combination approach to testing should help isolate even these hidden aftermarket devices to a specific circuit and help in finding the source of the parasitic draw.  

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