I just recently aquired a 2003 Ford Ranger 4.0l . When the engine is on and left idling all the lights interior and exterior flicker not bad just enough to annoy me. When throttle is given it stops. When the accesories and ac turned all the way up the flickering becomes almost non existant.

I checked the alternator and it was going bad so I replaced it also the belt was bad so I replaced it. The Belt tensioner does have some play in it when running but not enough to hear any audible slippage ( I also dont know the nomial tolerances) I also checked the ground cable underneath the driver side and it was in good condition

I could go ahead and replace the idler pulley and tensioner tomorrow Id just like some thoughts and suggestions on some possible resolutions.

thank you all in advance for any help given.
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what does your terminals look like? corroded? and are they tight?

 

check the wires. if theres any corrosion or anything that could be your problem. may just be bad connection, something could be loose somewhere

 

the truck just started throwin codes now evap transmission solenoid etc.. fun I appreciate the help but it is a lost cause for me at this point lol back to the stealership.
thanks for yalls help anyway