Ignition Off Draw (IOD)
DESCRIPTION
Ignition off draw refers to power being drained from the battery with the ignition turned off. A normal vehicle electrical system will draw from 5 to 20 milliamps. A vehicle that has not been operated for an extended period of time (approximately 20 days) may discharge the battery to an inadequate level. Battery drain should not exceed approximately 20 ma (20 milliamps 0.020 amps).
The 20 ma are needed to supply ECU memory, digital clock memory, and ETR (electronically tuned radio) memory. Excessive battery drain is caused by items left turned on, internally shorted alternator, or intermittent short in wiring.
If the IOD is excessive (over 20 milliamperes), the defect must be found and corrected before replacing a battery. In most cases the battery can be charged and returned to service.
TEST PROCEDURE
Testing for higher amperage IOD must be performed first to prevent damage to most milliamp meters.
Verify that all electrical accessories are OFF. Turn off all lights, remove ignition key, and close all doors. If the vehicle is equipped with electronic accessories (illuminated entry, high line radio), allow the systems to automatically shut off (time out), up to 3 minutes.
1. After determining that the underhood lamp is operating properly then disconnect bulb.
2. Disconnect battery negative cable.
3. Connect a typical 12 volt test light (low wattage bulb) between the negative cable clamp and the battery negative terminal.
- The test light may light brightly for up to 3 minutes or may not light at all (depending on the electrical equipment). The term brightly being used throughout the following tests, implies the brightness of the test light will be the same as if it were connected across the battery.
- The test light must be securely clamped to the negative cable and battery terminal. If the test light becomes disconnected during any of the IOD test, the electronic timer function will be activated and all tests must be repeated.
4. After 3 minutes, the test light should turn OFF or be DIMLY lit (depending on the electrical equipment). If the test light remains brightly lit do not disconnect it. Remove each fuse or circuit breaker until test light is either OFF or DIMLY lit. This will eliminate the higher amperage draw.
- If test light is still bright after disconnecting each fuse and circuit breaker, disconnect the wiring harness from the alternator. Refer to Alternator Testing in this group. Do not disconnect the test light.
- After higher amperage IOD has been corrected, low amperage IOD may be checked.
- It is now safe to install milliamp meter to check for low amperage IOD.
5. With test light still connected, securely clamp an ammeter between battery negative terminal and negative battery cable.
- If the test light or the milliamp meter circuit is broken the various timer circuits will start. Do not open any doors or turn on any electrical accessories with the test light disconnected or the meter may be damaged.
6. Disconnect test light. The current draw should not exceed 0.020 amp. If it exceeds 20 milliamps isolate each circuit by removing circuit breakers and fuses. The meter reading drops once the high current problem is found. Repair this section of the circuit, whether it is a wiring short or component failure.