Tracing low current draw
 

My 2004 f150 heritage developed a low current draw. The battery would run down after about 3 days.
I hooked up the VOM to the negative post and cable. It settled on a 0.52 amp draw.
I then began the normal process of pulling inside fuses to isolate a circuit.
Fuse 15 - 5A - stop lamp switch - shift interlock - this accounted for 0.20 amp draw. This is necessary to shift out of park.
No other fuse affected the current draw. Next the under hood fuses and relays.
None of these affected the current draw.
After putting everything back as it was, I went to move the truck to check wiring areas.
The engine started and ran as normal. I put it in reverse which seemed harsh, but would not go forward in any drive position.
The check engine light, and overdrive indicator light flashing. Shut down and check codes.
P0743, po755, p1747, p0443, p1451, p0403, p0135, p0155.
Disconnected both battery cables and jumped them together (sometimes suggested to reset computer defaults). No change.
Has anyone else had a similar problem or solution suggestion?
thanks,
dave-TX

STOP! You already missed the testing procedure for CORRECTLY checking for parasitic current draw. It is MANDATORY to wait 40 minutes after touching ANYTHING before taking any current measurements. You have to wait for the battery saver module to time out. Otherwise, you'll end up out in the weeds trying to find something wrong with a circuit that was not tested properly.

Check fuse F23 under the hood. It is not supplying power to all of its loads.

I am thinking that I left the neg cable off overnight, before I began testing.
When first connected the amp jumps to 1.6, but after a few seconds settles to 0.52.
I assumed that it was charging the battery saver at that time.
Good point on fuse 23 - I don't know what HEGO stands for, but it is related to the transmission.
I will try to verify that first.
Thanks for the tips

HEGO is the O2 sensor. Not transmission related.

(Heated Exhaust Gas Oxygen)

Like I said, it is mandatory that you follow the correct procedure when checking for parasitic draw, otherwise you WILL end up in the weeds! The procedure you described is wrong.

F23 supplies power to a whole crap-load of stuff under the hood including the transmission solenoids, O2 sensor heaters, and everything else (and probably more) that you are having codes for.

yea you need to be able to let the computers go to sleep in their normal cycle.
then hook up the DVOM (in amp setting)
then disconnect the vehicle while keep the DVOM in series with the truck.

its usually easiest to set this up with a battery disconnect switch, and then you can clip your meter in line, open the switch, without trying to juggle three or four things at once

No, you need to insert the ammeter in-circuit and then WALK AWAY. If you disturb ANYTHING after that, go back to step one.

That can work as long as you keep the sequence in the proper order and obey the timeout requirement.

You risk popping the fuse in the DVOM turning it on, before the car is fully asleep

Also a lot of meters will time out and power off after 5-10 minutes.


Speaking from experience as a professional, the steps are:

Sleep
DVOM
Disconnect car (while keeping it in series)

Yes, certainly the preferred method as well as noting that's what the factory service manual prescribes. The hard part is explaining the procedure to someone who's already out in the weeds... ;)

Thanks to all who responded.
Yes I disconnected all and left it overnight.
A mechanic friend also suggested jumping the disconnected ground and positive cables to insure all residual charge is cleared.
I checked all conditions before proceeding.
Do any of you get RockAuto's newsletter? I like the "mistakes and blunders" section.
Been doing this a lot of years and I still get bit by the dumb bug.
Fuse slots 21,22, & 24 are not used. When I put the fuse for 23, I put it in slot 24. Kick self.
Thanks again. My problem traced to a short in the trailer brake module.

congratulations!

for sure, wasn't trying to get into a pissing match with you.

My philosophy with helping people, is explain it best/accurate way, and if they want clarification or need help understanding they should ask for additional support.



Glad you got her figured out....

I forgot the first step in a dealership environment is to locate and isolate/remove any aftermarket wiring, components, phone chargers etc