Parasitic draw diagnostic strategies

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Parasitic draw diagnostic strategies

Craig Truglia is an ASE A6, A8, and L1 certified technician who presently works as a service writer for Patterson Auto Body, a repair facility in Patterson, N.Y. A former shop owner and editor of several automotive repair magazines, Truglia combines his Columbia University education with the real-world experience he sees daily in the automotive repair field. Technicians Truglia and Fred Byron took part in diagnosing the different vehicles in this article.

Key On Engine Off (KOEO) battery draws have always been a relatively difficult concept for customers to wrap their heads around. They figure when the vehicle is turned off, nothing is on any more. However, these problems are hardly new, as vehicles have had shorted wiring, switches, and other simple problems for years.

For the last few years now, many parasitic draw issues have become increasingly complicated, thanks to the computer networks on vehicles. This means that tried-and-true diagnostic techniques may actually complicate a KOEO battery draw diagnostic.

Thankfully, even though there are a few new techniques and increasingly complicated systems to be dealt with, the process is still essentially the same in principle..

Pulling fuses. Many technicians believe they do not need a review in the “pulling fuses” technique, but forgetting the basics can cost valuable time. The first step is to test the batteries after having them fully charged. A technician can waste hours diagnosing a vehicle only to find that a shorted battery cell is literally draining power, even after it is charged or the vehicle is run for some time. Make sure the vehicle is working with a known good battery.

In fact, recently a Ford diesel (equipped with dual batteries) came into the shop and the customer demanded that the batteries not be changed, because he wanted to change them himself after the problem was diagnosed. Lo and behold, one battery was measurably warmer than the other because it had a shorted battery cell. His battery was the parasitic draw all along.

After putting on an amp clamp (or meter on series), it is good to know that a battery generally should not be pulling more than 100 mA or so. A nice rule of thumb is 25mA to 75mA.

A fairly simple way to know if a parasitic draw is in acceptable range (because manufacturers do not generally publish specifications) is to divide the reserve capacity by four. So, if the reserve capacity is 120 minutes, if you divide it by four, you get 30.


This means 30 mA is the “acceptable” battery draw according to the battery manufacturer. In reality, a draw that small on a battery of a RC of 120 with a 30mA draw may take many weeks to go dead. However, if you double the number, now it takes two or three weeks. Triple the number, a week or so. It is easy to see how a 120 mA draw can cause a no-start in a few days, and even higher in a day or less.

After confirming that the battery draw is excessive, it’s time to start pulling fuses. When the excessive draw disappears, the fuse that was pulled essentially isolates this system. Then, after consulting an information system and wiring diagrams, you can determine what is connected to that fuse. At this point, simply unplugging whatever switches, modules, etc., are part of that fuse’s circuit generally narrows down what is causing the parasitic draw.

New techniques. The old school way of pulling fuses remains to this day a solid way of diagnosing most parasitic battery drains. However, it can no longer be the only diagnostic strategy in the technician’s arsenal simply because of the increasing amount of computerized management in the average vehicle.

Many observant technicians might have noticed that checking the battery drain the moment the vehicle is turned off is not useful, simply because modules need to “go to sleep.” It usually takes anywhere from three to 15 minutes, but it can be longer. Before the modules go to bed, the vehicle can be drawing several hundred mA for quite some time. Once they have all gone asleep, the bare essentials will s

till be drawing power (PCM, BCM or RCDLR (remote control), the clock, etc.) However, they will draw power at a very low rate, usually below the numbers that were discussed earlier in the article.

Every vehicle is engineered differently, so each model will have different modules go to sleep at different times. Different designs make it necessary for the engineers to adopt different software strategies.

Now, most of the time there is no easy way to find out what the software strategy is. So, there is no way to anticipate if the technician pulls a fuse, killing power to the BCM, that the instrument cluster now wakes up, thereby increasing the KOEO draw the technician is measuring. Therefore, pulling fuses does not always work.

How do we find a parasitic draw when pulling fuses wakes up more modules than it puts asleep? In extreme situations a thermal imaging tool can catch a relay stuck on or a fuse in which something on its circuit is on, because power running through a circuit draws power. However, most of the time, temperature differences are minimal unless a lot of amperage is being pulled. It can be useful in picking up a shorted battery cell, alternator diode, and other odd KOEO draws that can be found by simply pulling a fuse.

Another strategy is to check for a voltage drop across a fuse. This avoids having to pull the fuse in order to see if it is pulling power. There are fancy formulas for finding out X voltage drop, on Y fuse, equals Z draw. The higher the mvV (or fuse rating), the more mA the voltage drop reading reflects. A good resource for these is However, just like scoping ignition, sometimes it is not necessary to know every in and out of these things. If every fuse does not have a significant voltage drop because nothing on the circuit is on, but one or two suspect fuses do, that narrows down where to look, right?

Last, there is the “Chesney Method” of finding parasitic draws. It is done the same way as testing for voltage drops, but the DVOM is put in ohms mode. Supposedly, after subtracting for ohms of resistance in the leads (simply touch the leads against each other to get a number), every 0.1 ohm equals a little more than 10 mA on meters with 10 megohm of impedance, and only 5 mA on meters with 20 megohm impedance. However, most technicians are not electric engineers and do not know what impedance their meters are. It is best to do an easy baselining on the vehicle with an amp clamp to see what one should expect.


If the technician likes to do algebra, he can figure out how many mA the KOEO draw is without pulling any fuses out of the vehicle. Using a Fluke 88 DVOM and an amp clamp, headlights pulling 6 amps on a 2003 Toyota Solara measured about 10.1 ohms. If there are 6,000 mA in 6 A, by dividing 10.1 ohms by 6,000 mA makes it where every mA should theoretically equals 0.00168 (or 0.002 to make it easy) ohms. So, a 100 mA KOEO draw should work out to 0.2 ohms, 500 mA to 1.0 ohms, etc.

In the real world, every vehicle has different specifications and depending upon where the technician takes his measurements, the ohms will be different. So, using this technique to get an exact reading is not recommended if the technician is not a mathematical genius. Nonetheless, he can simply just observe that the vast majority of fuses have near identical resistance, while the ones with significant KOEO loads will have much higher resistance. It is not rocket science to discern which fuse has the parasitic draw on its circuit.

Much has been made of the “new” techniques for finding parasitic battery draws. However, just like the tried-and-true technique of pulling fuses, the principle is the same. The technician is just looking for the fuse that is coupled with the circuit that has the draw. This is merely a different technique of testing the fuse. Instead of testing the fuse indirectly by pulling it out and observing the draw elsewhere, the technician tests the fuse directly.

Some people think that attaching a meter to the fuses can be a tad annoying and time consuming. For this reason, it is best to exploit such techniques when the pulling fuses technique causes the whole “module wake up” issue. Otherwise, it is probably more trouble than it’s worth.

1997 Jeep Grand Cherokee parasitic draw. The following is a to-the-point case study that reviews the principles set forth in this article.

This Jeep came in with a parasitic draw of about 200 mA. Like many drivers with mid-1990s Jeeps, she had “no money” to put into the vehicle, even though she daily visits the C-store to buy cigarettes.

On a vehicle this old, the “pulling fuses” technique works just fine. The dome light fuse, when removed, dropped the parasitic draw to around 125 mA. Being that she wanted to save as much money as possible, she did not care whether or not the timer or something else was responsible for the battery draw. She simply wanted the fuse left out.

However, the vehicle still had a draw. After pulling another fuse, the draw dropped to about 30 mA. The engine started up and ran normally with this fuse pulled, but it was evident that it was for the instrument cluster, because it did not work!

After consulting the wiring diagram to see if there was anything else on the same circuit with the power to the cluster, it was evident that the cluster was the gateway module to a primitive on-board vehicle network. The datalines from the cluster also connected to the ABS, BCM, PCM and airbag modules.

At this point, the customer was not willing to pay for the next stage in the diagnosis (which would have been unplugging the modules, starting with the airbag, ABS and BCM ones first.) So, she took the car as is, knowing that she would have to let it run every few days or the battery would go dead. However, it is easy to see that even on a 1997 vehicle, data networks have already become a relevant part of a technician’s diagnostic routine.   ‚óŹ

For more from this author, see:

OBD-II Mode 6: Making it part of your diagnostic arsenal

ECU soft resets: Killing power to on-board modules does wonders in diagnosis and repair

Lambda diagnostics: Solve those system lean problems fast

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