[ischmitz]

The WSM describes a test procedure: You need an oscilloscope and measure the voltage on pin 1 and 2 while the car is idling. I think you need a special test plug (male female with test leads) to get to it while the it is connected to the ISP. You should see a square wave pattern with about 100 Hz. The duty cycle should change when you switch on loads such as high-beams, AC compressor, etc while the idle stays constant. At the same time you should be able to hear the ISP humming. If there is no humming it is bad. I haven't heard of them going bad very often.

A simple test is to disconnect the two-lead plug during idle. If the engine dies it sort of tells you that the ISP is working fine.

The idle speed positioner is an electrically actuated valve. The inner part of the axle rotates letting more or less air pass depending on the duty cycle of the driving signal. If you take it out and bridge it with a tube your car won't start because it gets way too much air.

For poor idle I would first check the idle micro-switch on the throttle body and make sure it engages properly. Your 89 does not have the CEL yet. So you won't notice if it fails to engage when you lift off the throttle. A poorly lubed throttle body mechanism is all it takes to cause problems here.

Next check and adjust the idle base mixture: There is a bypass adjustment allen screw in the AFM. You need to adjust with a CO meter. Some say the basic adjustment it too rich and backing out the screw by 4 turns improves idle stability.

The idle speed itself can not be adjusted on a 964. It is defined in the DME code.

Hope that helps,
Ingo

Hope that helps.
Ingo