Need help asap

[austin]

I HAVE A 2009 FORD RANGER 4X4 4.0L when im sitting at a light my idle goes from 900rpm to 500rpm and my headlights will dim and then get bright same for my interior lights and my blower motor speed will go up and then down. I just replaced my battery a few months ago, and today I just replaced my alternator. with my meter hooked up it would charge at 14.5 and the motor would bog down and the alternator would whine. and then it would stop charging and sit at 12.6 and then go back up about every 5 seconds. so I assumed it was the alternator. but with the new ford remanufactured alternator it is still doing the exact same thing. I cleaned all engine grounds so now im out of ideas...any info will help thanks!!!

 

[RonD]

When engine idle is at 700 or above alternator should show 13.5v or higher at battery.
Below that RPM it is basically shut off so you would read battery voltage 12.3v to 12.8v, and with a 1 volt difference lights would look dimmer.

Manual trans idle is 700, automatic 800, that's warmed up.
Cold idle is based on outside/engine temp at startup, 1,100 is about average.

If your RPMs drop below 700 then yes lights would dim a bit, if an alternator has a field that has failed lights may even dim at normal idle.
Alternator has 3 fields that make AC voltage, it is not unusual for one to fail over the years, but the other 2 can pick up the slack for the most part, just not fully at idle.

An alternator won't cause RPMs to drop, if alternator bearing was bad the belt would start to slip, so you would hear squealing, an alternator simply can't cause enough drag so slow down 100+ horse power engine.

So your issue is the erratic idle, could be a vacuum leak or dirty IAC(idle air control) Valve.
In normal start up computer opens IAC valve all the way while cranking engine, when engine starts RPMs should go up to above 1,500, then computer will start closing the IAC Valve to set idle for engine temperature, 1,100 cold and 700 warm(or 800).

If your engine doesn't start like this then clean the IAC valve first, it isn't hard to do.

Anther thing to do is to warm up engine and unplug the IAC Valves wires(Valve closes all the way), idle should drop to 500, engine may even stall, either is good, it means no vacuum leak.
If idle stays high, you have a leak.
PCV hoses on the 4.0l were common leak points, especially the elbows, underside would wear thru.

 

[austin]

even with the new alternator all the same problems are there. the alternator whines real bad when it is charging and is quiet when its not charging. and voltage at the battery goes from 14.5 to 12.6 every 5 seconds which is not normal. and even when driving at 70mph the blower speed changes and lights dim. it has to be an electrical issue correct me if im wrong