1996 F150 - short?

Son's 1996 F150 drained the battery in a couple hours on Thursday. We put a trickle charger on it Thursday night and the battery hadn't charged enough overnight to start on Friday morning. After I removed the charger the battery would only make the starter grind a couple times, then the battery was completely dead within 20 minutes (interior lights wouldn't even come on).


I removed the battery and charged it on the bench overnight on the same trickle charger. The next morning I had it load tested to make sure it wasn't shorted out internally - it was fine and tested above its rating. Brought it home and installed it - there was spark jumping from the post to the cable (way to much draw) - hooked it up and the trucked started and ran. Surprisingly when I removed the cable the truck kept running - I thought it needed the battery to run even with a working alternator.

To track down the draw do you set the multimeter on "amps" and hook it between the battery and cable and check the draw (I am assuming about 3 amps right now) and start pulling fuses to see which circuit is causing the excessive current draw?

You will always get a drain at the radio fuse for its memory on those trucks. Second, unplug the alternator, both plugs. Put a test light in lone between the positive battery cable and positive battery terminal to check it. Pull the fuse for the radio, if there is still a draw (which it sounds like there is), the test light will still be lit up. If after you unplug the alternator, and remove the radio fuse, plug the alternator in, see if the test light lights up. I will say that I've had quite a few of these trucks, and most of the time it is the alternator causing a draw. The alternator still works fine, just draws on the excite circuit when off.

Do not remove a battery terminal when the vehicle is running. (it was a poor choice test with a generator, it can be a fatal test with an alternator, and can blow out all the electronics in the vehicle, Ignition, Fuel computer, radio and gauges. The removal of the battery from the circuit causes (in many cases) the alternator to charge wildly into the hundreds of volts! Jim

I think I have heard that you should never unhook a battery with the engine running.

Pull the battery cable while it is running is NEVER a good thing to do. Doing so can/will burn up an alternator plus if you would happen to get between the cable and ground it can also turn hurt or kill you

Here's my copy of the gm Kent- more special tool. Place tool between the batt. and negative cable, close the switch and walk away for 1/2 hr. or longer to allow any modules to go to sleep, hook your voltmeter to each side of switch and on 10 amp. setting, open switch and if it reads less than 10 amps go to milli amp setting. A reading of around 25 ma or less would be a acceptable. Then just start pulling fuses.

You can try to find it with the test light, as you described, in series with the disconnected cable.

That may or may not work. Some of the newer vehicles will need the amp meter test. If the test light stays on, even with the alternator disconnected, try the amp meter test.

Whatever you have drawing is substantial to bring a good battery down that quick. Check the obvious first.

Usually its a shorted out starter solenoid that causes a decharge like that but I don't think its the case this time.

check relays on the left fender, likely either power relay to pcm or fuel pump relay is stuck on.

Set your multimeter on the 10 amp scale,and the leads between a battery post and the cable.Make sure the door is closed and the dome light is off.With modern vehicles with modules and pcms,etc.,your draw could normally be .3 or.4 amps.When you first hook up your leads you will see an upsurge of several amps,and it will taper off to .3 or .4 amps if all is well.If not,start pulling fuses.Actually I would disconnect the alternator first.It can charge,but still have short.I've seen it many times.Mark

With battery hooked up is the alternator getting warm to touch, you might want to start at the alternator 1st
sounds to me with that much draw the voltage regulator / alt is fine place to look.

Setting the multimeter will be the biggest problem. Son tossed the instruction manual after he used once. I figure I have about a 2.8 amp draw - that's why the 3 amp battery charger barely got any power in the battery over night.

Battery every 4 years or so....is it past due? Have you started by removing and cleaning the post and cables, or just saying "It looks clean."


Pulling off a cable with the alternator equipped engine running is a good way to blow out ignition modules, computers, body control modules, fry alternators. . .
If you have a generator equipped engine, then you can pull off the cable. A "generator" does just that. It generates and does not need a source voltage for reference. If you pull the cable on the alternator, you no longer have source voltage and the alternator goes full field and can easily produce better than twice the highest rated out put. So basically your talking upwards of 24 volts and about 200 amps. ZAP.
Your battery acts as a giant capacitor smoothing out voltage spikes. Take off battery and create your own voltage spike. POW!
In olden days, pulling off the cable with the engine running was a good way to check generator output. If the engine died, you had a bad generator. Pull the cable off with the alternator and YOU can kill the engine electronics.

To check for your short, Get yourself a DVOM. (Digital Volt Ohm Multi meter) The old needle swinging analog meters will draw enough current and can also blow out computer circuits. That fancy new digital scale meter is what you need and you also need it to read amps. Most small meters will read 10 amps.

Some of the smaller amp reading DVOM's have an internal fuse to protect the meter from overloading it and blowing the meter up. That fuse is generally not one that is typically found at the auto parts store. So, get yourself and inline fuse holder and a 7.5 amp fuse, I prefer the auto spade fuses, splice that into the pos cable on the multi meter and now you won't be blowing the fuse inside the meter. Instead, you can rplace the external fuse easier than the internal fuse, should you overload the circuit.