The idle speed solonoid on the manifold is supplied constant 12 volt + power when the ignition is on and is activated by two hydrauilic switches located on each side of the tranny on the bottom with one wire connecting both switches and the solonoid. One switch is for reverse and one for forward. The fluid pressure in the tranny activates one of the switches and grounds out the wire to activate the solonoid. Your 3 position solonoid on top of the tranny has no effect on the fast idle solonoid on the manifold. To test, get out your test lamp and figure out if you have 12 volt + power at the solonoid, if not check fuses etc. to get it fixed, if you do, then simply supply ground to the other terminal, if the solonoid activates it is probably OK and your problem is possibly a broken wire from the tranny, easy to check, just follow the wire, or more likely a bad switch on the transmission. Have someone sit in the car with it running, it must be running to supply fluid pressure to activate the switches. Place the car in a forward gear and check continuity between the switch and ground, if you have no continuity you probably have a bad switch, not that uncommon. You must place the car in reverse and check the other switch also. It would be unlikely to have both switches bad. I forgot which switch is which, not that hard to figure out though.