'98 ranger 4x4, 4.0 ltr, manual tranny. The problem is when i push in the clutch to stop the idle drops to around 500 rpm on the tach and if driving at night the lights dim. Sometime the engine will pick back up and run at regular idle speed but its mostly around 500 rpm. What could it be tps, alternator, Iac??? no codes have been pulled, just trying to get an idea, thanks alot

 

Could be a number of things & there could be more than one problem here.

With the lights dimming, it would suggest you need to have a electrical system load test done.
Most autoparts stores will do it at no charge, with their portable tester. It can properly load the system & check for a weak battery or alternator.

A weak electrical system can cause all sorts of unusual problems, so get it checked out first & make any repairs needed & let us know how it goes afterward.

If the idle is still low, there are other things, includng the ones you mentoned, to check out.

 

ok, i cleaned the IAC, load tested the alt, replaced it with a new one, idle, problem seems to be solved but the IAC is making a loud humming noise, does this mean its bad too??

 

I would try and clean the IAC to see if the noise goes away. Just take it off and try and clean any carbon buildup out of it with solvent spray.

 

Was it making any noise before you cleaned it???? If not, what did you clean it with?????

It will not tolerate some strong solvents, as they can get after rubber, plastic, or composit parts.

If you choose to try to clean it again, try a solvent thats safe for the above materials.
If the noise is intolerable & re-cleaning doesn't help, replace it.

 

noisy iac is almost always a symptom that its bad or going bad. If the throttle plate is set properly, the truck will idle (when warm) at the proper rpm with the iac closed, but many vehicles have dirty throttle bodies, etc. that make it so the iac needs to be partly open to idle at the proper speed. If the iac sticks, it can cause a low idle, especially as the engine speed is coming down from driving (such as at the stoplight you mentioned). cleaning the iac usually only lasts for a few days or weeks, so I'd suggest you buy a new iac and your idle speed problem will be fixed. if you are feeling adventurous, you can do the complete 'set the idle speed' routine, but its really not needed.

the iac should be cleaned with carb cleaner (which is a pretty strong solvent) the iac is just a solenoid controlled plunger valve, and the plunger can get a bunch of crud in it. the solvent won't hurt the iac, in my experience. i'm on the third iac on my truck, and haven't managed to hurt one yet by cleaning it, but, like i said, it usually doesn't last very long before it starts to stick again.

if the truck idles at 500 rpm, the lights will dim, because the alternator won't put out enough current to keep the voltage up at that slow of speed. that is not an indicator of an electrical problem, it just supports the laws of physics.