Parasitic Draw

Measuring Parasitic Draw

Description
Parasitic current draw is the low but constant draw of current on the battery caused by the memory registers in the ECM/PCM, radio, clock, security system multiplex and most control systems.

Normally this current draw is so small that you need to measure it in milliamps (one thousandth of an amp). Newer models draw between 3 mA and 13 mA, but older models can draw as much as 50 mA. And a current draw of more than 50 mA can discharge the battery.

If the vehicle is driven daily; parasitic current draw really isn't an issue; the charging system does it's job and juices up the battery. But it can become a problem if the vehicle sits for a long time, especially if the battery was partially discharged when the vehicle was parked, or the parasitic current draw is high. Left unchecked, the current draw on the battery can drain it to the point where the engine doesn't start.

Procedure To Measure Parasitic Current Draw

1. Make sure all switches are turned off, then remove the ignition key. Wait at least 5 minutes for all current reduction timers to turn off (sleep mode).
2. Loosen (but don't remove) the negative battery cable.
3. Set your ammeter to the 10A scale if it doesn't have an auto range capability.





4. Use an alligator clip to connect your ammeter's positive lead to the negative battery terminal.
5. Hold the ammeter's negative lead firmly against the negative battery post.





6. Slide the negative battery terminal over your ammeter's negative lead, and read the current draw. If your ammeter's range is too high to accurately read milliamps, lower the range so you can.

To isolate the parasitic current draw to a single circuit, remove fuses in the circuit one at a time until the current draw drops. Refer to the appropriate electrical diagram to identify the components on the circuit. Unplug components one at a time to find the cause of the high parasitic current draw.