3 Things Anyone in a Mechanic Program Should Know about Parasitic Draw

auto mechanic

A parasitic draw is an automotive problem that can lead to a car’s battery draining very quickly. As special features and gadgets are added to cars, the standards for checking for a parasitic draw are likewise changing.

A common situation for a car owner with parasitic draw is finding their engine won’t turn over even though their battery was fully charged the last time they used their vehicle. This might happen a few days in a row or just once before they decide to bring the car into the shop. When they do ask for help with their seemingly un-chargeable battery, you will need to know what to do.

1. An Auto Mechanic Knows There Could Be Many Causes of Parasitic Draw

Parasitic draw refers to a situation where the battery is drained by one or more circuits continuously after the car is turned off. There is a normal level of draw because of all the electrical systems in a car. However, if there is a parasitic draw, as an auto mechanic you should know that it is likely due to a faulty circuit somewhere under the hood or the dash. If the car has any after-market add-ons, like stereos or security systems, then those should be your primary suspects.

2. How to Do a Parasitic Draw Test Without Damaging Your Equipment

After you complete a mechanic program you will use all sorts of equipment to work on cars. When finding out if a car has a parasitic draw the tool you will use is the amp meter, but you want to be sure not to damage it. Here’s how you perform a parasitic draw test safely.

  • First, turn everything off, but make sure the battery is fully charged
  • Before you turn on the amp meter turn it to the highest setting on the direct current (DC) amperage scale, then connect it.
  • Some mechanics recommend using the negative cable to connect the meter because you could short the system out on the positive side accidentally.
  • Move the setting lower until you get a reading. If the current isn’t within range of 50mA, start looking for the faulty circuit.
  • Pull out fuses one-by-one in each fuse box. When you pull the fuse of the problem circuit the meter should go down to below 50mA.


Each fuse is pulled out and pushed back in to test for the source of a parasitic draw
Each fuse is pulled out and pushed back in to test for the source of a parasitic draw

After testing, you may need to consult a wiring diagram to fix the problem. Again, to keep your amp meter intact make sure the car and all its electrical systems are off while you test.

3. New Cars Might Have Higher Draws or “Key-Off” Loads

Cars change, but your main duty as a car mechanic is the same and as important as ever: find the problem and fix it. To do this, it’s necessary to keep your knowledge up to date. Recently, some mechanics have noted higher than 50mA draws because of increased electrical demands in newer vehicles. This is so common that some vehicles now come with solar chargers to keep the batteries charged. Other mechanics have also noticed that newer cars sometimes take longer to turn off so you might have to wait up to 30 minutes before beginning testing.

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