so thanks for reading.

95 F-150, 5.8 E4OD. 157k. The truck's been fairly reliable until today when we attempted to start it and the battery was dead as a doornail. Under 1v according to my multimeter. Lots of corrosion on the neg terminal that wasn't there the last time I was under the hood. Ignition was off, lights off, nothing out of place.

I pulled the battery and within a few minutes the charge picked up to 6.7v by itself. It's trickle charging now and taking charge. Cleaned all corrosion.

I checked the neg and pos terminals on the harnesses and I'm getting 1 ohm with the ignition off. So while I'll probably replace the battery I'm not convinced it's the battery's fault.

Can anyone suggest a strategy to solving this? Old threads suggest pulling fuses while looking at continuity, checking the PCM relay, fuel pump relay, testing the alternator, and a few other things. It sure would help if I at least could start with the most likely source since we need the truck to tow on Wed.

Thanks,

Dave

 

Hello Dave;

Unfortunately, the best way to trace a parasitic draw is to put an ammeter between the neg terminal and the negative cable and check for current. Make sure all the systems are turned off, the key is in the off position, the doors are all closed and the glove box light is not turned on. If you have an under-hood light, make sure it is disabled. If you have more than about a 50mA draw, you need to trace the system. That is best done by removing each fuse, one at a time, and leaving them removed until the drain is found. Then search that system for the draw.

If the draw is less than 50mA, it is an acceptable draw, and you can presume that the battery was the culprit. There are two times when batteries mostly fail. Either extremely hot or cold temps. When batteries fail, they usually fail without much warning. Start good one time, then bam! won't start.

By the way, the corrosion on the terminals and in the cables could be what is causing the truck not to start. Corrosion acts just like a big resistor. It is possible that the corrosion caused the problem, too. Look at the battery cables and check to see if they are swollen up, or the insulation split. If either of these conditions exist, replace the cable, not just the terminal end.

Good luck and let us know what you find.

Dan Harriman
Orange, Texas

 

Thanks for the tips. Getting a new battery for sure. But the 1 ohm across the harness terminals w/ the battery disconnected is screaming of an issue.

I got my wife to help watch the ohmmeter and I found that the #8 fuse in the dash fuse panel is affecting it. If I pull this fuse the resistance goes from 1 ohm to over 9kOhm. That would put the amp draw about normal.

The manual says #8 connects
- Courtesy/Dome/Cargo Lamps
- Electric Outside Mirrors
- Radio/Clock Memory
- Speedometer Memory
- Warning Buzzer/Chime Module
- Overhead Console (Bronco) Sun Visor Illumination.

I've already checked the dome lamp and the rear cab light that faces the bed.

1) Do I need this fuse connected to run the truck? I'm going to reconnect the battery with the fuse out and see what happens.
2) Any tips on how to find the culprit fast?

It's getting late, so I'm tempted to simply punt and drive the truck to a shop just down the road, and hope they will take care of it tomorrow.

Dave

 

i had the same prolem it turned out to be a short where the harness plugs into the alternator ford alternators have a history of shorting and even causeing fires.

 

I'm not sure why that would be remedied by pulling #8 fuse though.

I punted. I pulled #8, put in the battery, started with a jump, and drove it to the shop. With the alternator charging the half-dead battery I saw 13.7v. It definitely doesn't run normally that way since the speedometer sensor is completely disabled - no odometer, no speedo, the tranny doesn't shift at the normal points and it shifts like a kicking mule. But it got me there.

I'll post what I learn tomorrow.

Dave

 

Well the verdict is: battery. Just the battery.

I thought that the 1.0 ohm resistance running thru the #8 fuse circuit was abnormal (since a 1-ohm circuit will easily drain a battery), but apparently it's supposed to be like that. If I had been lazy and just dropped in a new battery without probing the circuits I would have never gotten off-track. This circuit is requires a small amount of current at all times.

Can any electrical types explain to me how a 1-ohm circuit does not drain the battery? Is there a relay or some widget that changes the effective resistance once the 12v is hooked up, or there is significant current flowing?

Thanks,

Dave

 

Actually, resistance doesn't cause a drain on the battery. (If you think of it, if you disconnect a battery cable, you have just created a very large resistance, so large that it will not let current flow from the battery, and thus the battery stays charged up).

Just for reference, here is the formula for voltage/current/resistance in any electrical circuit. It is called Ohm's Law. E = I x R, where E = electromotive force, or volts; I = Intensity or Amperage/current flow; and R = resistance. using this formula will let you know how large a fuse to use, or how big an alternator to use, etc.

When you measured the resistance, all you were doing was measuring the resistance in the conductor, or wire. By the way, you should never measure resistance with a live circuit. The circuit must be open or disconnected.

What you should measure is how much current the vehicle is using when the key is in the off position, all the doors closed, and all elctrical loads off. A small amount of current is always being drained from the battery by the memories in the computer(s), clock, auto A/C, radio presets, etc. This current shouldn't be more than about 50 to 75 mA, that is milli-amps. The battery has enough energy in it to overcome that small drain, but if the drain is excessive, the battery cannot compensate for it by itself, and goes dead. It will need a trickle charge if stored a long period of time between running the vehicle, or it needs to be run every day or so to keep a good charge on the battery.

Getting back to your original question as to why 1 Ohm drains the battery, it can't. However, if you have too much resistance, such as in a corroded terminal or cables, that can keep your vehicle from starting, or cause one or both headlamps to burn less brightly than it should, or keep a horn from blowing, etc., but that is another subject that we can elaborate on later if anyone wants.

Hope that helps some, and I am glad that all that was wrong with the truck was a bad battery. This is the time of year that they go bad...in either very hot or very cold weather. They usually give no notice that they are about to take a crap, either. Then BAM! It gets ya!

Happy motoring,

Dan Harriman 84 F150 lwb
Orange, Texas

 

I hate to say this, and don't know a better way to express it, but the above post by kcsgxl amounts to, for the most part not much more than out of context gibberish, and facts misaplied to reason. And it's too extensive to go over point by point.

dgeesaman.........THE REASON YOU SHOW A LOW RESISTANCE READING IS ATTRIBUTED TO THE FOLLOWING.

When you first hook up your battery, there is a high current draw for only a split second, and then it drops to nil and stays there. This is why you see a low resistance reading.
This can be verified by disconnecting your negative battery lead and inserting an ampmeter between that lead and your negative battery terminal.
As you touch the ammeter to the battery terminal, you will see it jump high for a split second, then drop to millivolts, and holding it connected for a long period of time will confirm for you that it stays at millivolts. This is with everything off.

It's as simple as that !!

kcsgxl is right in stating that you can't really trouble shoot that type of problem or circuit with an ohmmeter.

 

OK, what ever you think.

However, you check volts with a VOLT meter, AMPS with an AMmeter, and resistance with an OHM meter.

The main thing is that he got his truck doing what it is supposed to do.

Drive safely,

Dan Harriman 84 F150 lwb
Orange, Texas

 

I'm familiar with Ohm's law. I know that 50mA is about the max drain on most vehicles, so when the battery was dead I pulled it, put it on a trickle charger (no outlet where the truck was parked), and went back to the truck. The truck had hardly been used, so I figured there was probably a reason the battery died, and I spent some time trying to find the reason.

I would've used the ammeter if there was a battery in the car, but since there was no battery there I used the ohmmeter. So when I saw 1 ohm I was reluctant to put a brand new battery in (parts stores were closed by then anyway) in case there *was* a bona fide short circuit. I know the math well enough that 1 ohm on 12v is a pretty good amount of current. I figured that this may be perfectly normal (as you now have described) but I didn't take the chance and I needed the truck to be working for today.

So the root of the question: is the low resistance caused by capacitance in that circuit? Such that initial current flow charges the caps and causes resistance to go up to a normal stable level?

Thanks all.

Dave

 

When you took the resistance measurement you had the ohmmeter hooked up in series with the wire running to fuse #8. Your ohm-meter would be functionally hooked up as an ammeter where the meter's internal battery would be the power source. An ohmmeter is supposed to work this way and give you accurate readings in ohms *if* the circuit is a pure resistance. If the circuit was not totally resistive this might account for the odd readings.

I use an ohmmeter to troubleshoot electrical circuits and sometimes get oddball readings.

The best way to look for parasitic loads is with an ammeter in series with the battery as Dan described. Isolate one circuit at a time until you find the offender.

With luck, the corroded battery terminals just prevented the old battery from getting a proper charge and it just died.

Lou Braun

 

dgeeman, I could see you have the proper logic and were doing things properly.
And I'm sure that you know that in all practicality that you can't get good resistance readings at real low ohm situations. I think you did good and had valid concerns.

We went over this phenominum in another thread about a month or so ago, and I don't remember if anyone had a difinitive answer to what causes it, but I doubt it was capacitance.

I do know that when testing for this phenominum with an ammeter, it's easy to miss or verify because it drops to millivolts so fast. Then won't reoccur till the battery is disconnected for a while.

EDIT: As an afterthought, I remember checking for a capacitance situation, but ruled it out because varing time delay waiting for the reset to draw the current again didn't change the amount of the surge. Of course whatever is causing it probably has a decaying cap in it as part of it's circuitry.