First of all, I'm new to this forum

Tonight when I was driving home, my battery light came on, but I didn't think too much of it because I only had to drive 2 more miles. Well my lights started dimming, the transmission felt like it was slipping(pressing the pedal and hearing engine rev up, but going no where), and my steering got quite difficult so I pulled over. The battery was a few years old so I had a friend take me to get a new one. I put the new one in and the truck works fine, but the battery light stays on unless I rev the engine to high rpms. Will this light dissappear after a few more runs with the truck or should I check other things like the alternator?

 

Howdy and welcome!

For sure check the voltage at the battery while the engine is idling. Should be at least 13.5 and 14 is better. If you just see 12something, the alternator isn't charging the system.

A voltmeter is just a few bucks at harbor freight or any homestore if you don't have one.

Adrianspeeder

 

I would think there would be a voltmeter on a 94. But if your voltage is down, try checking your contacts before you buy a new alternator. Where I live it is very common for them to corrode and the battery not charge right.

 

Chances are the alternator is shot, the battery light will only come on when there is no voltage output from the alternator. A wiring/corrosion problem is possible but not very likely. Most auto parts stores will test your charging system for you for free.

-Jon

 

I'd trust the idiot battery light over the idiot volt gauge. Doesn't tell ya much.

Adrianspeeder

 

A fully charged battery, 12.6 volts or so registers to the "N" in "normal on my gauge, that's with the key on but motor off, that trips the light With it idling, battery at about 13.7 volts, it reads at the "a" in normal. Just off idle it goes a tad higher and peaks there. I figured the low battery light comes on when the voltage is below 13 volts or so, meaning the battery is charging either very, very slowly or not at all.

The "amp" light just means low voltage. Clean the battery contacts with a mix of baking soda and water, give 'em a spritz of wd-40 and put them back on. See if that helps. But chances are the alternator is gone bad or the brushes are worn out.

 

The simplified version of how the battery light works is that there is voltage applied to both sides of the bulb, one side from the battery and one side from the alternator. Its called the law of opposing voltages, when there is voltage present at both sides there is no current flow so the bulb doesn't light, when there is no voltage output from the alternator the current can flow through the bulb making it light up.

-Jon

 

Well put!

 

Hmm, never thought of it that way but it makes perfect sense. So I guess if your battery crapped out and it's voltage dropped, the bulb would also light.

 

No, it wouldnt work that way, my previous description really was the simple description. It only will light if alternator output voltage is less than battery voltage. If there is no output from the alternator stator through the diodes then the power backfeeds to ground through the field coil and transistor. I was just playing with all this stuff a couple weeks ago converting a 100amp Ford Taurus alternator into an onboard welder for my offroader I should have used a GM alternator, there a lot simpler to modify.

-Jon

 

People are often confused because the "battery" light doesn't have anything to do with your battery being bad. The ones that say "ALT", or the older ones that say "GEN", were much more descriptively accurate.

 

Mine says "amp" inside a battery icon, kinda misleading I suppose.