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I searched the forums, but missed this post about how to test the ISV/IAC in-car. So I looked at the wiring diagrams and came up with a scheme that doesn't activate the fuel pump and can be done without the 12V battery installed. These instructions are for an '87 or later car. Not sure if this procedure can be done with the older brushed ISV/IAC.
Requirements: A floating (i.e. the negative output should not be connected to chassis ground) 5-12 volt power source (variable power supply, 9V battery, USB power pack, etc.) and a helper.
1. [Optional] Disconnect 12V battery. Not strictly necessary, but may reduce electrical stress on the LH when the plug is disconnected.
2. Pop the hood / bonnet open
3. Unplug the 35-pin plug to the LH. If you want, you can unplug the EZK also, but it is not necessary.
4. Touch positive lead of the power source to pin 9 of the LH plug. Touch negative lead of the power source to pin 33 of the LH plug. Have your helper listen for a clunk or thunk near the intake manifold.
Caveats: This only tests if the ISV/IAC armature moves. It won't tell you if it has friction from rust, or if the input or output rubber hoses are cracked.
At 6 volts, my ISV drew 0.6 amps. ISV/IAC resistance (between pins 9 & 33 of the LH plug) measures 8.4 ohms. Not sure if these are normal values, but it made a satisfying clunk when energized.
Photo borrowed from Rob Edwards (Note: This is the LH plug from a '90 or later 928. '87-'89 have the cable going into the top side of the connector, but the pinout is the same):
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Alternative in-car ISV/IAC quick test
