Test Key-Off Drain the Easy Way

Test Key-Off Drain the Easy Way

It's quick and easy. All you need is a DMM's ammeter range of 10A or 20A and regular DMM test leads. This quick method won't allow the electrical system to power down which turns OFF circuits that get upset/reset when they power up again. Check key-off drain this way!

Follow these 4 simple steps.  

(1) Set up DMM to read 10A or 20A whichever your DMM provides.
-- Plug the red test lead into the 10A or 20A jack.
-- Add an alligator clip to the red probe tip.
-- Plug in the black test lead to the COM jack.

(2) Connect the red test lead alligator clip to the negative cable clamp. Make sure there is a solid connection between the alligator clip and the negative cable clamp.

(3) Connect the black test lead (COM) to the negative battery terminal POST and maintain contact with negative post until the test is completed.
-- Slightly loosen the negative cable clamp so it can be disconnected from the negative battery post.

(4) Lift the negative cable clamp up from the negative battery post (as shown in the illustration above). The DMM will indicate the amount of Key-Off drain.
-- Once a DMM reading is obtained reconnect the negative battery cable clamp to the negative battery post and tighten securely.
-- Remove DMM test leads from the circuit.

Conclusion:

The electrical system remains supplied with battery voltage at all times during this procedure. The DMM ammeter function is connected in parallel with the battery. When the battery negative cable is disconnected from the negative post, battery voltage remains applied through the ammeter which now indicates the amount of key-off drain. Be careful to leave the black test lead (COM) connected to the negative battery post while the cable clamp is disconnected from the negative battery post.

Battery power to the electrical system is not interrupted at any time if this simple procedure is followed correctly. Once the key-off drain is known you are done. Reconnect the negative battery cable to the negative battery post before removing the black test lead from the battery post.

If the key-off drain is acceptable it is usually between 0.5 amp (50 mA) but not more than 0.1 amp (100 mA).  Follow a manufacturer's procedure for tracking down the cause. Key-Off drain is often caused by an on-board computer that doesn't completely "go to sleep."

By the way, if you suspect a control unit or other electronic component does not go to sleep, try this. Connect a battery trickle charger to the battery terminals with the ignition key OFF and allow the vehicle to sit for a few hours preferably not in direct sun light. Then using an IR gun, test the surface temperature of every control unit or other electronic component to see which one is a little warmer than all the others. When an electronic component remains ON it will generate a little heat while the other electronic components cool down to ambient temperature.

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