Truck electrical problem

[LeanMachine]

I have a 2004 f150 that has had some wonky electrical issues for a while. One of them was battery drain. New battery. Good alternator. So yesterday I pulled fuses one by one, and used a meter to see when the milliamps dropped. I found 2 that were having a fair bit of drain and removed them. Both were related to the headlights. So I let it sit overnight to see if the battery still drains. It does indeed. And I happen to notice that there is a bulb on both sides that the filament is glowing. I believe these are the daytime running lights. They shouldn't even have power with the fuses pulled I don't think. Well I get to testing more things and find that with the negative battery cable disconnected, I read 12 volts between the negative post and the negative cable. That has to mean I have a short somewhere right? I haven't managed to locate one. Any advice on best way to go about finding it?

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[roughneck266]

Start pulling fuses with your meter set up between the negative post and the negative cable. You shouldn't be seeing voltage there. When the voltage reading goes away, you will likely have found your issue.

Also, can you check the amperage reading while it is connected to those same points? YOU should see low milliamps, there, for the ECM, BCM and stuff like that, but it should be pretty low.

[Kingfisher789]

Check the harness on passenger side where it makes a bend Some of those F150 have an exposed stud mounting the evaporator housing that the harness rubs on.

[LeanMachine]

Start pulling fuses with your meter set up between the negative post and the negative cable. You shouldn't be seeing voltage there. When the voltage reading goes away, you will likely have found your issue.

Also, can you check the amperage reading while it is connected to those same points? YOU should see low milliamps, there, for the ECM, BCM and stuff like that, but it should be pretty low.

Thats a good idea. Yes I have done that testing for amps and located 2 fuses with abnormal draw. Will try the same with volts.

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[LeanMachine]

Check the harness on passenger side where it makes a bend Some of those F150 have an exposed stud mounting the evaporator housing that the harness rubs on.

Is this under the hood?

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[Kingfisher789]

Yes under hood

[brushtrooper]

Start pulling fuses with your meter set up between the negative post and the negative cable.

Does this mean you disconnect negative cable from battery and then set meter between negative post and the negative cable? I have never down this and I am trying to learn something new.

[LeanMachine]

Does this mean you disconnect negative cable from battery and then set meter between negative post and the negative cable? I have never down this and I am trying to learn something new.

That is correct. You could do the same on the positive side but you run the risk of creating sparks.

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[roughneck266]

Does this mean you disconnect negative cable from battery and then set meter between negative post and the negative cable? I have never down this and I am trying to learn something new.

Yes.
THis is only to identify which circuit is actually completing the circuit to the battery. Once you have identified which circuits remain alive, you can go to milliamps and check each one individually. There will naturally be some on newer vehicles, because things like digital radios, and the onboard computers will always have a small electrical draw, but it should be very small. Anything over around 100 milliamps is going to be too large a draw over time. If it is dying in a single night, there is going to be something much more substantial than that.

[LeanMachine]

Messed around with it some more and might be more confused than I was before.

Set the meter to volts with the same setup between negative post and negative cable. Pulled every fuse one by one and the voltage never dropped. Ok so maybe the short is before the fuse box?

There are 3 separate cables that connect to the positive battery terminal. I removed them individually and identified one that voltage dropped to zero when removed. It is the cable that goes directly to the fuse box, which makes sense I guess. There are 5 or 6 plugs that exit the fuse box. I removed them one by one and the voltage never dropped. I can't figure out how that is possible. I even tried removing all of them at the same time. No change.

I should add that I did fully remove the cable that connects the battery to the fuse box to inspect it. It looked to be in perfect shape. I did not take the plastic armor stuff off it it but it showed no signs of wear and it is just one heavy wire.

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[wsteffen]

It will always read a voltage unless you disconnect everything including the computer, radio, etc. You need to use a amp meter to determine what the drain is. I had a similar problem and it was the battery from a parts store, their brand. Decided to do away with it after they replaced it twice and bought a battery from Walmart and never had another problem. They claimed that therre was nothing wrong with the battery both times, but I recorded voltage over a 24 hour period. Normal drain on a battery in a newer vehicle is between .4 and 1 amp.

[LeanMachine]

It will always read a voltage unless you disconnect everything including the computer, radio, etc. You need to use a amp meter to determine what the drain is. I had a similar problem and it was the battery from a parts store, their brand. Decided to do away with it after they replaced it twice and bought a battery from Walmart and never had another problem. They claimed that therre was nothing wrong with the battery both times, but I recorded voltage over a 24 hour period. Normal drain on a battery in a newer vehicle is between .4 and 1 amp.

I think you are misunderstanding that I am getting voltage on the negative circuit. I tested the same thing on my 2019, to make sure that voltage on the negative circuit wasn't normal, and indeed it reads zero.

Did some more digging online and became suspicious of the body control module. I'm not 100% sure this truck even has one but online seems to suggest it has one behind the back seat. Someone mentioned a water leak compromising theirs. Then I happened to remember seeing a crack in the light on the top of the cab above the bed. I went out to see if I remembered correctly and not only is it cracked but it is broken where the mounting screws go in so that I could just lift the whole thing off. So it is certainly plausible that if there is a bcm on the back wall that it could have been compromised by water leaking in through the bad light housing. Will investigate more when I get a chance.

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[kingranch]

there is some small fusebox that my brother had to replace on his 2012 ecoboost he was having intermittent elec problems like no start etc.. it sits up top i think to the right and has the relays inside

[RifleBowPistol]

Like the first guy said, have a test light or a amp meter hooked between the negative cable and the negative battery post. Then one by one, pull fuses till either the light on the test light goes out or the amp meter goes to zero, depending on which you are using. I usually use a test light, that way I can see it from a distance. Once you figure out which fuse or fuses are causing the drain. Then look at a wiring diagram for those fuses. Then put the fuses back in place. Then disconnect every component on the circuit that is causing the drain, till you get the draw to drop to zero, or not. If it does not drop, by disconnecting every component on that circuit, then start disconnecting connectors that have that circuit in it. Say you see the fuse supplies power out to three different components, you disconnect those and you still have a draw, then look for electrical connectors that will disconnect each of those three circuits. You can start as close to the fuse box as possible or closer to the components. Narrow it down till you find what area the short or draw has to be.

Then if all of that fails, pull the GEM module/fuse box and remove it from the truck. Disconnect the battery first. Once you have it out, look at the connectors that are on top, when the box is in the truck. Commonly on those trucks, they have water leaks, from around the windshield. When the do, it will always drip on the steel bar behind the dash, and always run across to just above the GEM module/fuse box and drip onto the top of that box and F up all types of things. Hardly get two with the same symptoms. I have been finding that problem since 1996, I still find trucks just recently that have that problem.

To know if it has been getting water in the box, you will look for green or white corrosion and water lines, down in the bottom of the plugs on top of the box. Once you see those conditions, if you are brave, you can take the box apart, see all the layers of buss bar, going on inside and then the module board. You should find signs of corrosion in various places inside the box.

Don't cheat, follow the correct diagnoses procedure, you might actually have some other problem, but most likely that's your problem. Works best when I get to stand there drinking a beer watching you, get completely lost trying to figure out something you thought you knew, but then find out you don't know much at all. I get to sit back and chuckle, cause 35 years ago, that would have been me, having know idea what I was doing.
About 90% of the time those trucks have strange electrical problems, or battery drains, power windows not working, power locks not working, ECT, ECT, ECT. It's the GEM module that got water into it. Ford does not sell them anymore, has not for a long time, the aftermarket does not sell them, too many different part numbers for the aftermarket to even think about making and selling them. Best bet is to get on E-bay and search for a good used one. Get the numbers off of yours and start searching.