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Short vs Parasitic Draw, Unobtrusive Testing of Draw

Mechanic Under Car

There are two common electrical problems in a vehicle. One is a "short" and the other is a "parasitic draw." One results in non-functioning equipment, the other in a dead battery over time.

What is a "Short?"

A short is when the electrical system for any circuit is overloading. This usually results in popped fuses, melted wiring, or broken equipment as the excess electricity finds places to go. Finding a short is relatively easy. It's usually located somewhere near or on the circuit of the now-faulty equipment.

What is "Parasitic Draw?"

This is when electricity is drawn from the battery even after the vehicle is shut down and all circuits should be shut off. You can think of it as being like when you've left the headlights on and the battery begins to drain because the engine is not running to replenish it. Unlike that scenario, however, the cause is not obvious and could be almost anything.

There are several ways to test for parasitic draw. We talked about one last year, which is a more old school method that works well on older vehicles without modern computer equipment in them. Read that here.

A better method, brought up by Big Block 409 in a forum discussion about parasitic draw, involves using a multimeter on engine fuses to hunt for draw when the vehicle should be shut down.

The method is relatively simple. The top of most fuses have small probes where the tips of a multimeter can be placed to measure resistance across the fuse. Although their original purpose is to diagnose problems with the fuse itself, these points can also be used to unobtrusively test the circuit the fuse links as well.

With the vehicle completely shut off and the key removed, after about ten minutes or so, all circuits in the car should be without power. Leaving the battery connected and taking proper safety precautions, you can test each fuse in the engine compartment individually. If any of them are showing up on the meter, they have power and are thus the source of your parasitic draw.

From there, finding the item causing the draw is a matter of just testing the circuit for a ground or other problem that could be causing the drain.

This video has more.