battery light flickering

hey guys. I tried searching the forums. (still looking through old threads now) but my truck. 06 2.3 2wd standard cab started last week, the battery light turns on then goes off. I put a new battery in it, in sept. Had the battery and alt checked out by 2 different shops and both checked out and are working fine. It runs good,always starts and its not dieing or draining anything. Everything works as it should. well the batter cables have some corrosion on them but i plan on replacing them by the end of the summer. Thinking about it, it mostly turns on when i spot and goes out a couple secs after taking off again. I'm not sure what it could be hoping someone here might have some input.

My only other issue is when running on the highway, the temp gauge will go from being in the normal middle range almost drops all the way down to the cold but once off the highway it goes back up. doesn't over heat and has not over heated either. I want to think tstat (it probably needs replaced anyway) just wanted to make sure if it cooling off like that if it will hurt the engine?

thanks a lot guys in advance. and sorry for the long post and if it seems a little confusing.

Don't know about the 2006 alternators, but on my 94 there is a separate single wire that plugs into the alternator that lights the indicator when it is going bad. Sometimes this connector comes loose or the tab on it get broken and slips out.

Rangers always run on the cold side.

Reads like you have a bad field in the alternator.
Alternators have 3 fields, when the rotor in the alternator spins the fields generate AC power, each field has a set of diodes that converts the AC power to DC power.
Example:
For a 90amp alternator to produce 90 amps it needs 3 fields producing 30amps each.
Not specifically correct but just an example.

It is quite common for a field or it's diodes to fail, people may notice head lights flickering at idle and then get steady when RPMs increase, no that is NOT normal, lol, it is a failed or failing field.
But since you really don't idle much it isn't an issue, battery stays charged and everything works because the other 2 fields can produce enough power when RPMs are above idle.

Flickering battery light is the same as the head lights, at low RPM the alternator voltage is dropping lower than battery voltage so light comes on.
Best thing to do is to get a Volt meter
Engine warmed up and shut off
Check battery voltage, should be 12.6v if it is fairly new
Start engine
Check battery voltage again, should be above 14volts, alternator is recharging battery after it was drained from using starter motor
Let engine idle for 3 or 4 minutes
Turn on lights, heater fan, radio, leave door open, so high power draw
Voltage should steadily drop then hold at 13.6volts
If it goes below 13.2volts alternator is going bad, if it gets down to 12.x volts it is very bad.


I think the '06 had electric radiator fan, it could be stuck on but long shot.
I think thermostat stuck open is correct.
Start cold engine open the hood and feel upper rad hose at radiator end.
Let engine idle for 3 or 4 minutes and feel upper rad hose again, if it is warming up the thermostat is indeed stuck open.
Upper hose should stay cold until engine warms up to 190deg and thermostat starts to open, that takes 5 to 8 minutes.

Assuming summer time
In the winter people with 4cylinders often put cardboard in front of rad to keep engine bay warmer

Do the alternator test that Ron said, but also snap the throttle open while doing it. If the voltage doesn't' go up or hold steady when the engine revs the alternator is weak.