Rick's Free Auto Repair Advice

How to find source of battery drain on Ford Vehicles

Finding the source of a battery drain can be frustrating. Here’s a test to use with a Ford vehicle. If you’ve suffered a dead battery and both the battery and charging systems check out, it’s time to perform a parasitic drain test. The test involves hooking an ammeter in series with the negative battery cable to measure the amperage draw. Professional technicians use a clamp on ammeter.

Here’s what you’re looking for: Most computers have a keep alive mode that keeps drawing current for about 15 minutes after shutdown. But if you’ve got a Ford vehicle, give it a good hour for all the modules to “go to sleep.” That means shut down the engine, hook up your meter, close the doors, make sure all the lights are off and walk away for an hour. Do NOT touch anything! Don’t open doors. Don’t turn on lights! That will wake up the computers. If you don’t follow this advice, it’s not uncommon to see current readings in the 200 milliamp to 1.2 amp range. That’s WAY above normal and it will trick you into suspecting current draws that are perfectly normal – for a computer that’s awake.

Acceptable current draw in sleep mode is anything UNDER 50 milliamps. If your reading are higher than that after an hour, you’ve got a module that either hasn’t gone to sleep or is sticking. Here comes the fun part. You have to pull fuses one at a time until you see the current drop. That will narrow down the troublesome circuit. But how do you do that without opening a door and waking up a computer? (normally you’d mechanically depress the pin switch in the door pillar—but Ford doesn’t use those anymore. The door switch is inside the door attached to the latch mechanism) There are only two ways: 1) hotwire the driver’s door switch so the computer thinks it’s closed. 2) Crawl in through the driver’s window.

Are you having fun yet? Do you really think Ford has a better idea?

 

© 2012 Rick Muscoplat

Posted on by Rick Muscoplat


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