A parasitic draw is current the car uses with everything shut off. A little is normal, since modules need to keep their memory. Too much and the battery's dead in the morning.
Here's how to test for it and find the circuit causing it.
Set it up
Put an ammeter in series at the battery negative cable. Close the doors, or close the latches with a screwdriver if you need to get to the fuse box, so the car thinks the doors are shut.
Wait for the modules to sleep
This is where people mess up. Right after you shut the car off, the modules stay awake for a while and draw more current. If you read the meter right away, you'll see a big number that isn't the real draw.
Leave the meter connected and wait for the time in service information. And don't break the connection. If power gets cut and restored, the modules wake back up and you have to start the wait over.
Compare it to the spec
Service information gives the maximum allowed draw. Specs are often around 0.050 A (50 mA), but use the number for the vehicle you're working on.
Find the circuit
Once you know the draw is too high, pull fuses one at a time while watching the meter. When the draw drops, that fuse's circuit is the problem. Then check everything on that circuit.
There's a faster way too. Every fuse has a tiny bit of resistance, so you can measure the millivolt drop across each fuse without pulling it and use a chart to convert that to current. That way you don't wake anything up.
Common causes
- A glove box, trunk, or courtesy lamp stuck on from a bad switch
- A module that never goes to sleep
- A leaking alternator diode
- A stuck relay keeping something powered, like an air suspension compressor
Cheat sheet
- Ammeter in series at the battery negative
- Doors closed or latches tricked shut
- Wait for the modules to sleep before you read it
- Compare to the spec in service information
- Pull fuses one at a time, or measure the voltage drop across each fuse
Common questions
How much parasitic draw is normal?
It depends on the vehicle. Specs are often around 50 mA, but check service information.
Why is my parasitic draw reading so high right after shutdown?
The modules are still awake. Wait for the sleep time in service information before you take the reading.
Can I open the doors during a draw test?
Not really. Open doors keep lamps and modules awake. Close the latches with a screwdriver so the car thinks they're shut.
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