Idle speed control valve ispection/service

[Doctor J]

The idle speed control valve (ISC, ACV) may have several problems; two of these I would discuss now.
One problem is when the valve is getting dirt and sludge build up on the shutter and the second problem is when the bearings of the rotary shaft are getting gummed up. Both problems are show up as incorrect idle speed; on my car I experienced low rpm after cold start; some customers were complaining about high idle speed after warming up.
When ignition is off and the outside temperature is about 60 degrees F, the valve has a medium gap (see below)

When ignition is ON but engine is not starting the valve has narrow gap (acts like a choke)

When the is running cold the valve opens wide allowing engine to run at elevated speed (1500RPM), as soon as engine warms up the idle speed drops.

On the three pin connector, the center pin is connected to the +12v source when ignition is on
Upper and lower pins are connected to the ground by 2 transistors inside the computer
When the lower pin is connected to the ground, the valve opens (cold idle)
When upper pin is grounded the valve is closed (that pin is always grounded with ignition on engine off)
Therefore if the cold engine does not have high idle speed, do the following:
Unplug the 3 pin connector from the ISC
Connect + terminal of 9 volt battery to the center pin on the valve
Connect – terminal of 9 volt battery to the bottom pin on the valve
If idle does not change the valve is stuck and must be diagnosed for dirty shutter or dirty bearing or open elctrical coil (rare)
If engine increases RPM the problem is outside of the valve (bad coolant sensor for example)
If throttle valve was cleaned by carburetor aerosol spray it may flush the sludge from the shutter to the bearing located behind the bimetal coil (picture below) therefore cleaning by a brush or cotton swab may be a better choice

Normally the valve should react to the voltage applied instantly with “solenoid” clicking sound, if it rotates slowly it needs to be checked. (This information is also helpful when shopping for used valve!)
if the bearing needs to be "rejuvenated" a drop of ATF may help at least for the short time