Constant Current Drain Test

Charging System Schematic:

Check for current drains on the battery in excess of 50 milliamps with all the electrical accessories OFF and the vehicle at rest. This test can be performed one of three ways:
- Use a "clamp on" current probe to the battery positive or negative cable.
- Use an in-line ammeter between the battery positive or negative post and its respective cable.
- Use a 12 volt test lamp between the battery positive post and the positive cable. If the lamp illuminates, then there is a drain somewhere in the electrical system.





Typically, a drain of approximately 1 amp can be attributed to an underhood lamp, glove compartment lamp, or rear deck lid lamp staying on continually. Other component failures or wiring shorts may be located by selectively pulling fuses or disconnecting fuse links to pinpoint the location of the current drain. When the current drain is found, the current will drop below 50 milliamps or the test lamp will go OUT. If the short is still not located, after checking all the fuses and fuse links, the drain may be due to the alternator/regulator. Disconnect the alternator output wire from the output stud and the regulator connector. If this eliminates the drain, check the charging system to locate the problem.

NOTE: A voltmeter is sometimes used for this test but it will react to a very small normal drain caused by "always-on" electronic systems such as Starter Interlock, Anti-Theft Alarm, Illuminated Entry, etc., which are so small they cause no problem. The test lamp shows only drains which are large enough to cause a problem.

NOTE: When the Message Center or electronic instrument cluster is initially powered-up (after a battery disconnect), the "computer" may be energized for as long as 1 minute causing a 0.25 amp current draw before returning to the normal 0.010-0.012 amp current draw. Therefore, it is important to allow at least 1 minute to pass (after ammeter hookup) before observing any current measurements.