Jim Grant's Tech Tips
Ford Ranger, Idle Speed Problems
Every time I turn my slide bar to defrost, on my 93 Ford Ranger,
my engine revs go from 1000 RPM to approximately 1250 RPM for about 3-5
seconds. This repeats every 40-45 seconds or so. It feels like the truck
wants to 'lunge' forward. Any idea what is causing this?
Ill bet coffee that your Ford Ranger has A/C. Why is that? Most
all vehicle manufacturers activate the A/C system whenever the defroster
is turned on. The reason? The A/C system removes moisture from the air.
This dry air helps to quickly clear the inside glass of moisture. How
could this possibly affect your vehicles idle? The vehicles
computer knows that the A/C system will add additional load to the engine.
To prevent the engine from stalling the computer turns the idle speed
up. What the computer doesn't know, is if the A/C system is low in charge
the A/C compressor will cycle on and off. The result is an idle that goes
up and down when the defroster is turned on. So, to fix a hunting idle
problem with the defrost on may just require recharging the A/C system.