Just tonight, I'm driving home and when I slow down, I notice the battery light comes on, then as soon as i accelerate, the battery light goes back off, then, when ever I'm not using the accelerator, the light flickers back on. Also, the battery gauge on the right side of the cluster is about half way(It used to be much higher).

I have a 1997 F-150 XL 4x4. I don't believe the battery has ever been changed, or if it has been changed, it hasn't been in the last 5 years.

Of course, when sitting in park, the light comes on and when I rev it, the light goes off until it idles down. Also, the lights seem to all be alright, no dimming as I see it. I've also started it up in this condition a few times in the past hour and it's all been ok.

I want it to be the battery but fear it could be the alternator. Since it's too late to take it to my local battery shop for testing, I have to wait till the morning to test the battery.

 

If you have a voltmeter and check the battery voltage while the car is running and it is between 12.8 and 13.5 volts the Alterntor is probably working, check it with the head lights on and it will put a load on the alternator. If you Read 12.5 or less the Alternator is gone.

 

Im gonna go with Alternator on this one.

 

one more for the Alternator

 

Alternator problem. but i would check out the battery and the alternator both. Check the connections too.

 

This is very common for the f150. I am on my 4th alternator in 10 years; probably because I have so many after market electrical equipment.
I warn you there is sometimes very little time between the light flicking on at idle and total failure of the alternator (actually it is the regulator but they are sold as one piece). When it fails the truck will discharge the battery in no time and you will get stranded.
Get an alternator with a lifetime warrantee if you plan to keep the truck for a while. This is not a hard fix.

 

Agree with WLF, gone through the same damn thing on my 98

 

Can't tell either way.

An alternator needs enough voltage applied to it to energize and operate. An alternator has no permanent magnets. It spins a rotor inside a magnetic field produced by electromagnets. Without enough battery power, the alternator will not energize and you will not know which is the culprit.

You need to take it in some place (like Autozone) and have them test it with their BEAR.

Anytime the batt is below a full charge, you cannot test an alternator for proper function. First you must charge or replace the battery and then test.

 

I say it's a bad Battery.

 

It is a shot in the dark until you check the voltage at the battery with the engine off, engine idling, and have someone rev it up.

Do those three measurements and post back.

Adrianspeeder

 

i say take your battery to autozone and get it tested for free

 

Batteries can be tricky. They need to be tested under a load. They can show 12V or better and as soon as they are loaded the voltage drops off to near 0. Any autoshop can check your battery and let you know if it is good (even Walmart). They should be able to tell you how many cold cranking amps it can deliver.

This happened to me in the summer in the middle of Dallas. Battery died and gave no indication that it was going bad. I was barely able to drive it to the parts store to get a new one.

 

I am willing to bet money on the alternator but I do agree with what is being posted here. If you go to AutoZone or NAPA or just about any auto parts store they will test the alternator and battery in your truck and let you know which one is the problem.
Since the dash light is based on voltage and goes on when the voltage drops below 12 volts, and since the light goes off when accelerating I feel confident it is the alternator. A battery with a weak or dead cell would be bad all the time, not just when idling.

Let us know and good luck with the fix.

 

The altenator may be bad, but with a battery that old, I would replace it before it lets you down somewhere.

 

5 years on a battery thats about all youll get. alternator isnt that hard to do on 4.6 5.4.