Ford F150 - Key-off battery drain

trailpass · Posted: Sun May 13, 2007 2:12 pm Post subject: Key-off battery drain

I have a 1996 F150 4.9L 6 cyl 4x4. I have having an issue with the truck intermittantly having a dead battery. Thought it might be a dragging starter at first so I replaced that and the relay on the fender. New battery last year fully checks on a load tester at the shop.

When the key is off and the doors are closed (no dome light on) I disconnect the neg terminal and check using a volt meter between the neg terminal and the neg battery connecter and there is ful volatge - should there be? I pulled all the fuses, pulled all the relays in the box on the left fender and at the dash. Also pulled the the wires to the alternator. None change the voltage. Any ideas?

Thanks I am really running in circles on this one.

Chris C.

jwt873 · Posted: Sun May 13, 2007 2:30 pm Post subject:

To check for battery drain, you should have the meter set to Amps not Volts..

How many milliAmps do you see between the battery terminal and the disconnected battery connector? If you're seeing more than 100 - 125 milliAmps, then the drain is excessive.

trailpass · Posted: Sun May 13, 2007 4:08 pm Post subject:

OK that makes sense. I measured .03 amps jumping between the neg battery terminal and the neg cable. I think that is alot.

Pulled the fuses again, and the relays and the alternator. also pulled the cables off the starter relay on the fender. no change.

Steve83 · Posted: Mon May 14, 2007 11:09 am Post subject:

Ford specifies 500mA (0.5A) maximum parasitic drain, so you're well under the limit. Find a shop that has a high-frequency battery charger/tester (most dealerships are now required to have one for battery warranty) & find out what condition your battery is in.

Also remember that some drains are intermittent, and can depend on the driver. I once found one in a Trans Am that had a big speaker box in the back. The installer had put a 3" wood screw through the carpet to hold it in place, and it went into the back glass pulldown motor. So it would cycle every ~2 min as the motor cooled off enough to try to pull, and would only last ~3 sec until it overheated again. Another was on a modified truck whose brake pedal was sticking. Sometimes, the brake lights would stay on, killing the battery IF it was parked for a few days. Sometimes, they wouldn't come on until the vaccum in the booster leaked down, causing the pedal to move slightly.

The point is: it might not be obvious, or constant. You have to really pay attention to what YOU do to cause it.
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. '99 CV RIP

Spare Tire · Posted: Mon May 14, 2007 5:53 pm Post subject:

Any 'mods', radio, lights, computer....check them. If you have time, pull a few suspect fuses overnight until you eliminate/find it. OR, take it to the dealer and see if a diognostic can be ran.

Or, do you have a mischevious neighbor or an x-wife?
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Johnny Walker 'Blue' and a Montecristo #2
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trailpass · Posted: Mon May 14, 2007 5:58 pm Post subject:

No ex-wife, but a very mischeivious wife... hmmm

Actually after a bit more trial and error I am thinking that maybe the alternator is periodically not charging the battery. Voltage reads good on the dash all the time, but that does not indicate what is being sent to the battery. Maybe it is not a draw after I turn the truck off, but one not being replenished sometimes while it is running.

Is that the voltage regulator that sends current to the battery when it needs it? Is that part of the alternator or a separate component?

jwt873 · Posted: Mon May 14, 2007 6:15 pm Post subject:

Steve83 · Posted: Mon May 14, 2007 10:04 pm Post subject:

Yes, the dash voltmeter shows (basically) what the battery is getting. That's why Ford started using them instead of ammeters. But "good all the time" is pretty vague - does it show the SAME all the time? It should show ~13V Key On Engine Off (KOEO) and ~14-15V KOER (Running). At idle with everything electrical running, it shouldn't drop below ~12V. Use a real meter (even a cheapo) in the cigar lighter to get real numbers and confirm what the gauge is showing.

The voltage regulator IS attached to the alternator (on the outside, even though it's referred to as an "internal regulator"), but it controls the current flowing to the alternator's windings. That controls what the alt. puts out (not just to the battery - to the whole truck).

To test the alternator, look on the back & find the rectangular plastic cover of the VR. There's a covered screw on one side, and an UNcovered one beside it with the words "GROUND HERE TO TEST" molded into the plastic. Use a jumper wire to BRIEFLY short that to a good ground (like the alternator case), then check the voltage with a meter while holding the engine around 2K RPM. Don't do this for more than a few seconds because the voltage SHOULD approach 18V. If not, the alt is bad. If it does, but it's usually below 14V, the VR is suspect.

Also, when you're checking for the parasitic draw, don't forget to check the alternator. Sometimes a diode will go bad, and the alternator will CONSUME electricity.