Hey-

Last time I was here I was requesting advice on swapping gears in my 95 F150 Mazda transmission. I did change the rear end gears to 373 I think ( I can't really remember now), and the towing was great...thanks for the advice. Unfortunately, one week later I got hit and the truck was totaled.

Now...I got an 01 F150, and I noticed when I hammer it and it gets to arounf 55 MPH proly at about 5 thousand RPM, the battery idiot light comes on, sort of flickering until it shifts into the next gear, at which point it goes away.

Any ideas?

Is this normal?

Thanks as always

Josh

 

your battery is probably weak and so is your alternator im betting. they tend to go hand in hand sometimes. when my alt was first going out the light would come on, then go away. then come on again and go away yet again over and over. pulled the alternator and looked at my brushes, one was wore down so bad it wouldnt even touch anymore.

 

Thanks Klitch. I'll have to check the brushes. At least that sounds like a good scenario to me. Let me see if I get this...at a higher RPM, my engine would be trying to draw more from the alternator, which may be on its way out, therefore unable to supply the necessary power. In turn this ends up tapping directly from the battery?

THanks Again-

Josh

 

close, the alt brushes are spring loaded down to the armature on the alt, the armature has groves in it to change the voltage to dc. As the brushes wear down the spring pressure is less causing the brush to loose contact with the armature at higher rpms. Simply put the alt is on its way out.

 

and prolonging this will work the battery harder which we all know they dont like.

 

Before checking the brushes, I'd check the alternator belt, at high revs, a loose belt will 'flap' more, and possibly slip, which is another possibility.

 

Ya, get a helper to rev the motor in park while you watch the belt. If the belt is "flappy" at higher RPM's that could be your problem and should be adjusted anyway at the tensioner.

The battery idiot light just comes on when current flows through it. Meaning one side is hooked to the battery, the other to the alternator. Ideally the same voltage is coming from the alt as is the battery so the light stays off(law of opposing currents or something). If one is stronger or weaker, current will flow and the light comes on. That's why the battery light is always on if you have the key on "run" but don't start the motor.

Don't wait until the light is on all the time, that's when your motor will just stall in the drive-thru line at McDonald's and you'll get a "click" when you turn the key and nothing more.

If it's been doing this for a while, I'd err on the safe side and replace the battery, chances are it's been worked pretty hard and batteries don't like that at all. Keep the old one for a spare or keep it in the back to boost someone else's car or something.

 

OK

So maybe one of 2 things.

The belt is loose-

The alternator is on its way out-

Both of which will cause additional strain on the battery. I understand that a change in current between the battery and the alternator will make the light go on, but would not the gauge also reflect the change in current? I'll have to check again tomorrow when I'm out, but I am pretty sure that the gauge doesn't go up or down (the battery gauge) when the idiot light goes on (which again is only when I've got it hammered up in let's say at least 5,000 RPMS).

I'll look at the belt and plan to get a battery this week. If the alternator is on it's way out, then I suppose that would be the next thing to try replacing as well. Thanks everyone-appreciate the brainstorm

Josh

 

Great info all! I'm beginning to show the same symptoms on my 98 F-150 and although I'm not a mechanic, I've worked on a lot of Ford vehicle electrical systems before, and I was kind of lost as to where to start with this problem.

So, are there any tests that can be run to see if the alternator is going bad, short of removing and disassembling it? How reliable are the "electrical systems checkups" that some service stations and mech shops provide?

Thanks in advance!
Steve

 

The only way to safely perform a full load test is to remove the alt and have it tested. With a test meter you can get an idea of the condition of the system by connecting the leads to the battery while the engine is off, the voltage should be 12-12.5VDC, if the voltage is low recharge the battery. Leave the leads connected and turn on the headlights, the voltage should drop to 11.5-12VDC, if the voltage drops below 11VDC then the battery has a bad cell(s). turn off the lights and start the engine if all you get is a click and the voltage barely moves check the battery connections. While the engine is running the voltage should be 13-14VDC, slowly raise the engine rpm and watch the voltage it should stay 13-14VDC if it goes low or fluctuates rapidly the alt is dying.
Also while the engine is running if you disconnect the positive battery terminal and the engine dies the alt is dead. (some people claim that doing tis can fry some of the electronics but I have done it on my truck at least 20 times and on other vehicles many more times and never had a problem.)

 

That's great, thanks a million! Your help gives me a start and the results should provide me with a direction in solving the problem. The battery light definitely seems to come on at higher RPM's, and not when I'm stopped or anything like that. Only when my engine RPM's drop below about 1800 does the light go out and stay out. I know that I need to take care of the problem very soon... I don't want to be stuck someplace when the alternator decides to stop working/charging a bad battery...

 

A MIL that goes on and off means only one thing - A high load misfire

 

jbrew, can you elaborate what you mean by high load misfire?? or explain that in non-mechanic terms please?? Thanks.

 

The volt gauge would not necessarily represent the problem accurately. That gauge measures volts only, which is a fairly poor representation of charge or discharge conditions. Considering the stupid thing doesn't even have lines or numbers it's practically worthless.

On older trucks the battery light was a battery icon with "AMP" in the middle, which makes a lot more sense. In fact, older cars all used to have amp meters rather than volt. Positive amps indicated more current was being produced by the car than consumed. Negative amps indicated the generator wasn't keeping up and a discharge condition. You might consider installing one in your truck anyway, they're dirt cheap(seen them under $10) and easy to install with only very basic wiring required.

The crapping-out alt might still produce the 14 volts or so at the high RPM range, but that's 14 volts and virtually no amperage which is what counts. Worn brushes, where very little metal is contacting the spindle, the small amount of metal cannot carry very many amps.

If your Ford moron-proof volt meter fluctuates slightly at idle or when you step on it, that's no cause for concern (it is a real gauge, just w/o any marks or individual numbers). Usually if I'm stopped in Drive the volts drops slightly, to about the "O" in "normal", and the second it comes off idle it rises back to the "M", where it usually hangs out.

The old school test of the charging system was to hook up a multi-tester to the battery and start the motor. With the engine at warm idle, note the voltage. Get a helper to switch on highbeam lights. The voltage will by 2-3 volts. If it doesn't recover(not necessarily to what it originally was w/lights off, but at least above 13v) within 3 seconds, the alt is pooping out. For example, at idle if I pull the "flash-to-pass" and hold, the volts drop down to that "O" again, and then if I'm in the garage I can hear the alt "click on", followed within a second by the gauge rising back to "M".

jbrew I think is referring to a common reason for the "check engine" light to come on-the common misfire. Often times under high RPM's(which are typically only reached under hard acceleration or towing in a low gear, both of which are high-load times), misfires occur in engines in need of a tune up. The sudden rise in cylinder temperature during these times sometimes shakes deposits loose or may foul a plug. More commonly, the coil/coil packs/COP's(they're all different forms of the same thing- to spark the plug) are failing and cannot provide enough "juice" for the plugs at high engine speeds. Of course there are dozens of other misfire issues, clogged FI's, a dirty throttle body or carb, fuel line blockage, the list goes on. Though I don't think a misfire will cause the battery light to come on normally.

 

Thanks RaWarrior