BlueArkie

I have a '92 F-150, 351 V-8 that has a problem I can't figure out.

The battery will discharge to zero volts in about 36 hours, sitting in the driveway. A new battery didn't help, and when I checked the current draw on a freshly charged battery, it was only drawing 150 milliamps. As I figure it, 36 hours X .15 amp = 5.4 amp hours, not something that would draw the battery down to zero.

Disconnecting the alternator made no difference and there is nothing else in the electrical system that points to a problem like this. Short of tearing the harness apart to isolate different possible loads, any ideas on troubleshooting this one?

Thanks...

imp

When standing, the only current drawn from the battery should be "keep-alive" current required by the PCM (computer). That is usually much less than the 150 milliamps you mention, more like 5 to 10 ma. Looks like some component somewhere is drawing current, not a huge amount, as in a short circuit.


I would charge the battery fully, disconnect it completely, measure it's fully-charged voltage, then let it sit disconnected for a day or so. If it is discharging internally (entirely possible), it's voltage will steadily drop over time. If you find that to be the case, replace the battery. Now, if you are able to disconnect one battery cable, place a D.C. ammeter in series with the battery post and cable end, you should see a constant draw of about 10 milliamps, ALL electrical accessories must be OFF. If you see more, begin removing fuses, one by one, while noting if the current flow ceases. The place where current is being "wasted" can be found in that way, knowing what purpose each fuse serves, and the cause isolated. imp