I have a 1990 F150. The RED battery light on the dash stays on constantly. The alternater guage shows the alternator is charging. I have replaced the battery. Put the truck on a code analizer and no codes were shown. Anybody know what might be causing this?

 

It sounds like the alternator. If you take the truck to a parts store, you can have the battery and alternator tested. They can tell you which one is bad. What does the voltmeter gauge read? It should be at about 11-12 volts when not running, 15 at idle.

 

The dash voltage guage reads about 14 when at idle and goes up to about 16-17 when the engine rpm increases.

 

Here's an easy test to see if your alternator is undercharging. Right after you shut the truck off, take a screwdriver or something magnetic and hold it close to the alternator pulley. If it is attracted to the pulley (magnetism) then the alternator isn't working properly. I don't know if it is opposite for overcharging (repels from pulley). I've never dealt with an overcharging alternator before.

Trevor

 

11-12v on key on engine off? Mine is like below 10....But, my battery IS 4+ years old . At idle its around 13-15.

 

I'm not so sure you have this right, I am going to check this the next time around, but i do know this, magnetism is made when the unit is charging, so therefore, when the alternator is running, place a screwdriver or something of a steel magnetic object at the center of the back end or slip ring end of the alternator an it will stick if the alternator is charging, if the steel object does not stick the alternator is not charging. Broken Wire

 

I believe I am right. I have experienced this before (it was a while ago) and got this tip from THE best mechanic I have ever known (happens to be my dad - 25+ years doesn't normally lie) But I could have mistook what he told me. I still believe I am right though. I don't think that the pulley itself shouldn't be magnetized if everything is working properly. Next time I'm talking to dad, which should be tomorrow, I'll ask him again.


Trevor

 

unplug the alternator at the three small wires and start up the truck, if the light is not on, have you alternator tested by a compitent rebuilding facility, the light wire goes into the alternator voltage regulator and is powered on the key side of the ignition circuit, goes through the light to the voltage regulator, the regulator grounds the circuit, and therefore your light is lit, but it should only be lit with key on engine off or key on engine running with an alternator problem. hope this helps you, Broken Wire

 

Brokenwire, what I said is true with the key in the off position. There is no residual magnetism in an alternator, but there is in a generator. If the PULLEY is magnetic with the key off then there is a leak through the rectifier bridge or diodes (diode trio on a Delco) Sorry if I didn't make this clear the first time.

Trevor

 

Used to have the same problem with the cheap rebuilts U-Haul gave us when I worked for them. If the battery holds acharge and you are not having a problem starting there is a bad diode in the alternator. Gotta replace it. Also the factory volt meters are not calibrated so you can't use that except to give you an idea if the alternator is working. For exact voltage you'll either have to buy an aftermarket or use a multiemeter on the battery it self.

 

I have a 94 F150 2WD 5.0L. After I jumped my son's car, the red battery idiot light is locked on. It does not go out even if I have a charger connected during run. Using a digital VOM battery voltage is 13.8 prior to start and 14.8 idle due to alternator input, dash gauge looks to read a little higher than that. I have an internal-fan alternator with a replaceable regulator and a non-replaceable diode. The "pulley magnetism" test I read elsewhere on this site was slightly positive (screwdriver shank slightly attracted to the alternator pully); this would indicate an undercharge condition.
My VOM readings imply plenty of alternator charge voltage. If the diode was bad wouldn't the VOM get ugly about the DC voltage supplied? I'm ready to take the alternator to the parts store for checkup at this point since I can't find any alternatives on this site. Will advise results.

 

jbarker,
I am going to make some assumptions here, if I am wrong help me out, I am going to assume we are talking about your F150 and not your sons car, I am going to assume that the idiot light is off when the ignition is shut off, and the idiot light is on when the engine is running, with the engine running, unplug the 3 small wire connector and see if the light goes out, if it does the regulator has popped its field circuit in the regulator and the regulator needs replaced, if the light is still on, the wire that goes to the idiot light from the alternator is shorted to ground, your rectifier is fine with a voltage of 14.8 running, battery charger will not help to take the light out, the rectifier in this unit can be replaced also, it is not a non replaceable rectifier assembly, As far as the magnetism thingy, well....., that is a whole other story, your feel of slightly positive is a great guess as if the unit is functioning incorrectly at best, what you just saw was exactly what I was trying to point out above, magnetism goes hand in hand with steel, that is also what makes alternators turn on, and that is what makes a one wire alternator work better than others, if there is no magnetism in a one wire alternator, they don't turn on at all, rotors become a spinning slug with nothing to put out. Generators do not need magnetism to turn on, they are excited by a hot powered ignition source through the regulator, which powers the field circuit in the generator and produces output, alternators can be built without an ignition source to turn them on, so what makes them turn on? magnetism..... so who took the magnetism away when you shut off your alternator in your truck? You don't, it is there or the rotor in the unit is junk. Broken Wire, Hope this helps

 

Hey brokenwire,
thanks for the tip, I removed the leads from the alternator and the light went out, also voltmeter on the dash dropped down (as expected). I had noticed that the voltmeter had been actually risen since yesterday, which lends further credence to a bad regulator.
Thank you very much for your prompt help.
jbarker

 

FYI, the first thing that probably should have been done, is a voltage test. It's the best way to check the charging and starting system.Forget this magnetism stuff....

You may have found your problem already, but I'm putting this in for extra info.

With the vehicle off, use a voltmeter and see what kind of voltage the battery is putting out. Should be around 12.5-13 volts.

Have someone start the vehicle while you're watching the voltmeter which would still be connected to the battery. See how much the voltage drop is, if when starting the vehicle the voltage drops into single digits, the battery charge may not be appropriate, or the starting system is requiring lots of extra power from the battery (caused by bad cables/connections...etc etc). It should be 9-10+ volts.


Next, once the vehicle is running, look at the voltage at idle, should be about 13.5-13.8ish. Then raise the vehicle RPM to around 2k...should be up to about 14.4ish....doing this test will indicate if the alternator is charging correctly, and if the general starting procedure from the electrical system is working correctly (doesn't find everything that could be wrong, but it's the place to start).

 

Hey Brokenwire & Mustang:
Just a close-the-loop followup on this battery charge light problem. I did check voltages running and non-running and everything looked OK. I finally changed out the regulator. The battery light stayed on! But now for the bizarre twist, after a couple days it went out and stayed out. Maybe the bulb burned out, I don't know. Anyway, all voltages look good, the starter turns great even at zero degrees and the battery is clearly getting, and holding, good charge.
Thanks again for your comments and input on this issue.
Jim Barker