How To Check Current Draw On A Vehicle

On occasion, a battery may experience meaning drain long later on the engine has been shut off. The event can often pb to a flat, caused not past a faulty battery or through something obvious like the headlights being left on. It typically happens due to a short circuit that can exist tricky to pin down.

This is known every bit parasitic draw. A certain amount of describe is normal – in the region of 25mA (milliamps) – but if you lot have 100mA or more being lost, something is up.

It could be anything from a dimming dome light staying awake or glove compartment switch sticking on, to faulty aftermarket radio or alarm wiring. In addition, any number of fancy electronics systems that are installed on newer cars could be playing upward, such as your GPS or proximity primal system.

What You'll Need

A digital multimeter is all-time for this test as opposed to an analog one, which is non equally accurately read. A DMM with the ability to measure continuous electric current to 10A will exist fine in most cases, though one rated 20A would be less prone to blowing a fuse. Yous also need a milliamp range for college resolution to pick up minimal current draw as most faults tend to exist of the trickle variety.

Yous can buy fairly inexpensive, half-decent multimeters these days, but bearing in mind that the cheapo, no-name ones are not known for their quality or input protection. Information technology is best to stick with a make name, though you don't necessarily have to go with a top branded Fluke model.

Procedure

Stride 1: Gear up for Test
You should have a fully charged battery before starting – voltage would be around 12.6V for a good ane. Exist sure to shut all doors, turn off the radio and disconnect accessories, such equally smartphones and addition GPS devices. Even a key in the ignition can take systems waking up on newer cars that will initiate draw.

Footstep two: Disconnect the Battery
Popular the hood and disconnect the negative cable from the negative bombardment final. It is always best to do a parasitic draw test from the negative side so there is no chance of accidentally shorting should yous test from the positive side.

Step 3: Connect the Multimeter
Plug the black probe into the common (COM) jack input of the multimeter and the red probe into the amps (commonly A) jack input. Then connect the red probe to the previously disconnected negative cable of the car loom, and the black able onto the battery terminal.

Note: in order to test for current, the multimeter must be continued in serial with the circuit as it needs to judge electron flow, and then to speak. Voltage, on the other paw, is measured in parallel.

Stride 4: Give the Car Fourth dimension to Sleep
Modern machine systems may 'wake up' when you connect a multimeter in such a style as it picks up the power of its batteries. So expect a few minutes before conducting the depict test.

CAUTION: when connected, avoid starting the engine or even turning on the headlights as this volition likely accident a fuse in the multimeter. Vehicle batteries put out a lot of amps due to being low voltage.

Footstep 5: Reading the Meter
Newer, auto ranging multimeters volition automatically select the correct range of a point they pick upwards. On transmission ranging meters, yous will need to select a range, usually either 2A or 200mA. If you get 'OL' on the screen, it means over limit and y'all should select a higher range.

If you see a reading of around 50mA and higher up on a standard car, information technology mostly ways something is cartoon power and you probable have parasitic draw. Just note that if you take a big, modern, luxury motorcar its added computerized systems will likely increase the natural current describe. In this instance, the threshold might exist 100mA.

PARASITIC Describe ESTABLISHED

Information technology is now fourth dimension to meet what is causing the draw. The tried and tested method is to beginning pulling fuses to see which circuit is the culprit. [For testing newer vehicles with complex systems, see below for a different method].

Step half dozen: Pulling Fuses
Locate the fuse panel(south) and commencement pulling out fuses one subsequently the other, taking notice of any driblet in the reading on the multimeter. If in that location is no joy with the fuses, start removing the relays. For those fuse boxes that are within the vehicle, a tip is to lay the meter on the windshield facedown and then you can run across the display.

Footstep 7: Narrowing Down
When the display drops to a nominal mA draw, annotation the fuse/relay position number and check the vehicle's transmission. There is often more than than one circuit on a given fuse, then y'all would and then demand to eliminate each one, such as remove a bulb or unplug a heater.

Step 8: Repair and Reconnect
When the meter drops down to 25mA or less after removing the offending fuse, bulb or unplugging a circuit, you likely have your culprit. Repair the fault/replace the particular and run another test to be certain. If you're confident there are no other faults, reconnect the battery.

Alternator fault?: if you've gone through all fuses and relays (and checked for other fuse boxes), often the alternator tin can exist at mistake. One of the diodes may accept shorted out and caused electric current to drain dorsum to the battery. Test information technology with the multimeter before and after unplugging.

Parasitic Draw Test on Newer Cars

Some of newer vehicles tin can be a real pain to diagnose parasitic depict test with a multimeter due to all the added electrical circuits, computers and control modules. You have electrical mirrors, heated retentiveness seats, GPS, trackers, and a slew of gadgets. Some of these systems are interlinked, exacerbating the problem.

Even car keys tin indirectly crusade current drain. Proximity keys are designed to wake up a car's locking mechanism when they are within close proximity then that y'all can unlock and open up the door without the fundamental. This is smashing in normal conditions, say, at your house, but tin can be an upshot in exercise. If yous park in an area where lots of other cars have these systems, for instance, the computer may wake up every time their drivers walk by your car. In this scenario, you may need to disable the sensor. More on this issue…

Old vs New Method

The old method detailed above – disconnecting the bombardment, putting the multimeter in serial and pulling fuses until yous find the draw – might not work as effectively on these cars. This is because the very disconnecting of the battery may temporarily set the effect past sending the system to slumber along with the mistake. You don't want this; you want the fault to keep drawing current and then you can diagnose it on your meter.

An alternative is to thus diagnose parasitic depict by measuring voltage drop beyond the fuses. You tin practise this in-situ equally auto fuses have open test points on them, so no need to remove them or disconnect the battery. What yous are looking for is a voltage drop in the millivolts range. It is caused by an increment in resistance brought about by the heating of the fuse through movement of electric current.

Different size fuses will prove a dissimilar voltage driblet, but all of the same rating will have roughly the same resistance, and then information technology is easy to get a reference past testing 1 10A fuse against the next. Every bit an example, a 10A fuse with 1A flowing through it might show a voltage drop of 5mV. If you were to measure the aforementioned fuse with no electric current flow, information technology would be more than like 0.1mV or 0.2mV. Quite pregnant and easy to spot.

Process

Bulldoze the car and operate all circuits – wipers, heater, seats, windows, GPS, lights, sabbatum-nav etc – and and then render and remove the key.
Consult the manual on how long it takes for the various systems to sleep.
Be certain to tape down the commuter door switch that operates the dome light and leave the car unlocked so you don't end up switching on whatever systems upon reentering.
Carry out the testing of each fuse and note any spurious voltage drop.

Other vehicles: the above likewise works for motorcycles and other vehicles, with a like process.

Key to Better Battery Life

Along with existence a nuisance and potentially leaving you stranded, even minimal describe can wear downward a vehicle'due south bombardment over time, often leading to performance issues and perchance a slow death. The key to practiced battery life is to nip any parasitic draws in the bud and likewise ensure the battery keeps charge higher up 12.4V.

Vehicles that only run into occasional employ are more prone to losing charge, where sulfation further compromises performance and longevity. Those driven daily accept the benefit of the alternator to maintain accuse, and when you test the voltage and electric current often, you can choice up whatsoever potential issues early.

Determination

Parasitic draw on one's vehicle can be a nightmare and often happens at the about inopportune time. But it needn't be the mammoth job you lot initially envisioned and is wholly within the realm of the layman, as long as there is a multimeter to manus. You tin can take smashing satisfaction in diagnosing the fault yourself and potentially save hundreds of dollars in the process.

See here how to check battery accuse

Choosing a Multimeter

There are a slew of multimeters on the market place, from inexpensive Chinese-made testers and Craftsmans up to high-finish Flukes. A basic device is all you'd need for a parasitic depict examination with a multimeter. Just make sure it can examination amperage, has an machine ranging mode, and is at to the lowest degree a known brand.

US brand INNOVA makes decent multimeters for automotive tasks, such as the 3340 or cheaper 3320, but even a standard electrician'due south multimeter can exercise the trick. Do spend a bit of time going over the specs to ascertain functions and the electronic input protection. This is particularly important for the multimeter and your health. Both meters mentioned are non fused on the 10A input, for example, whereas the Fluke 77 IV is.