I'm wondering if somebody can help me understand what i've been seeing. I have a 1999, F250 with a 5.4L.

My truck sits for, say, a month between me driving it. During that time, the battery will drain and die down to the point where it won't turn the engine over.

When I have the dead battery connected to the truck, even if I jump start the truck, and the truck starts up, it won't keep running. Here's what happens:

1) I turn the key, the starter solenoid clicks a bunch of times, but there obviously isn't enough charge to turn the engine over.
2) I hook up some jumper cables, jump the truck, it starts right up.
3) I disconnect the jumper cables, and the engine will start to struggle, and slow down, sounds like it's misfiring, and eventually dies out.
4) Reconnect jumper cables and repeat. Once the jumper cables are removed, the truck will die out again after, maybe, 15 seconds.

Now, if I take the battery out and charge it for an hour, then throw it back in, the truck runs fine, it doesn't die out, and there's no problem at all.

I guess my question is, even with a dead battery, after the truck's running, why wouldn't it *keep* running, regardless of a dead battery. As far as I can tell, the alternator is working fine.

Anyone have any ideas?

 

Your alternator is shot and I'm sure you killed the battery too. It would still run with the battery dead unless the alternator is out, luckily on a 5.4 its easy to change right on top. Also don't store your vehicle and drive it once a month that's just horrible for it. If you have a multimeter you could hook to the power wire off the alternator and ground to test if its making anything.

 

Yeah, bad alternator and bad battery.

And your best investment if you are going to keep using the truck this way is a battery tender, keep the battery alive while it's sitting. NOT, repeat NOT a trickle charger, but a battery tender, this is a "float" style charger that keeps the battery healthy.

 

Running the truck with a low or bad battery can ruin the alternator.

 

It's not a guarantee that the alternator is bad at this point, it just doesn't put out enough amperage at idle to charge a completely flat battery and run the truck. I bet if you turned off all accessories and reved the engine to say 1500 RPM while someone else disconnected the cables it would keep running. Replace the battery and put a battery tender on it, check the alternator ( I suspect it's still good) and carry on.

 

I totally agree with Clutch73. Your alternator has been shot and that has affected the battery. Try to not keep the truck idle and drive it atleast once a week. Check the alternator again.

 

I'd go with clem above. Alternator MAY be bad at this point but I'd try this first. Put your battery on a charger and let it get back to full charge...it may take several hours. Reinstall in truck and start it. Make sure it continues to run ok. Remove the ground wire from the battery and see if the engine stalls/quicks on you. If it does, alt. is gone. If it keeps running the alt. is providing the voltage necessary to run the truck and is prob a good alt. DO NOT USE THE ALTERNATOR TO RECHARGE A DEAD BATTERY! You chance ruining the alternator.

 

Both this and what Clem said. An altenator will not charge the battery enough at idle.

 

Thanks for the help, guys.

I'll start checking out my alternator to see if it's having any problems. That sounds like the best, logical path to follow.

 

Yikes! I wouldn't recommend that procedure at all. That is a sure fire way of damaging the ECU or BCM on a car or truck. Computers don't like power surges or low voltage with high current.

What I recommend is to charge the battery fully, start the truck. Take a DC voltmeter (not a test light) and measure the voltage at the battery terminals. You should see 13.8-14.4VDC at idle. If 12.3VDC or less, then your alternator isn't charging. If you have 13.8 or more VDC, turn on your headlights and air conditioning. Measure the voltage at the battery terminals again. If the voltage maintains at 13.8VDC, your alternator is good and charging the battery, even under load.

Good Luck!

 

DO NOT disconnect positive or negative battery cable with truck running, an open circuit alternator can generate high voltage spikes that can destroy other electronic components like the ECM etc. Only way to test an alt is with a voltmeter and a load on alt, whether in the truck or on a bench tester.

 

^^^^^^^^^^^^^^^^^^^^^^^^^^
This.... Read the following and see section 2 on Load Dump:

http://www.ti.com/lit/an/snva681/snva681.pdf

You can quite easily take out a lot of your electronics by doing this...

 

I've never seen a vehicle not run when the alternator is good, even with a bad battery. The alternator alone should allow enough current to keep the engine running, even at idle.

I agree with the others that the alternator/regulator isn't operating properly.
Ive had the same issue with the Expy before and 4 alternators later and 320k, it runs great.

 

This sounds like the classic symptom of a shorted diode in the alternator draining the battery. Solution is to disconnect the battery, then test/replace the alternator. Then check the fluid level in the battery and make sure that it is OK. Then charge the battery. A slow overnight charge is best for a battery that has been drained like yours. Then hook up the battery. At this point, if your battery is OK, you are done. If the battery is not OK, replace it. If you still have a problem, do the "key off battery drain" tests. If you don't know how to do this, a search should provide all the info you need.

A good, fully charged battery should not drain to a no start level in 30 days. I have left my truck for as long as 90 days and it still cranks fine.