Parasitic Draw Testing

After the key is off and the vehicle has been sitting long enough for all modules to enter sleep mode, some current still flows. The BCM stays partially awake for security. Memory circuits retain data. The clock keeps time. A small amount of draw is completely normal. The question is whether the draw is within specification — or whether something is staying awake that should be sleeping.

Even 150 milliamps of excess draw will kill a battery overnight on a vehicle sitting in a customer driveway. The customer brings in a no-start complaint. You test the battery — dead. You charge it. The vehicle starts. You send it home. It happens again in two days. Without finding the parasitic draw, you are just treating the symptom.

Less than 50 milliamps is the general acceptable range on most vehicles. Some manufacturers with many modules specify higher. Always check the manufacturer specification before making a determination.

Connect a digital milliammeter in series with the battery negative cable. Now wait — and this is the part most technicians rush. It can take up to 45 minutes for all modules to enter full sleep mode on some vehicles. Do not open the door, press any button, or disturb the vehicle in any way. Any activity wakes modules and restarts the sleep timer. Once the reading has dropped to its lowest stable point and stayed there, that is your actual parasitic draw. Above specification — pull fuses one at a time while watching the draw. The fuse whose removal drops the reading identifies the circuit. Then disconnect individual components on that circuit to find the specific fault.