not in front of my book but I believe mine is supposed to be "battery voltage" (approx. 12.5 volts) across that plug when key is on and engine off. Engine has to be cold I think also. I would check the tightness and cleanliness of the plug that the Idle control module plugs into between the two firewalls. I just experienced that problem and it was a real gremlin for me. Again, not sure if your 86 560 is same as mine but I think they are very similar on this part of the car. The test for the ICV was listening for the audible click when turning key on but the buzzer was too loud on mine and could not test it that way so I turned the key on waited for buzzer to go silent and plugged and unplugged the Idle control valve and cold hear click then. I thnk your problem is either in the ICV [idle speed air valve]or the Idle control module.

Good luck.

__________________
dloveless

1984 500 SEL [131K]-
1991 GMC Sierra 4X4 XC [162K]- War Pony.
2004 Jeep Liberty Limited [8k]- wife's sled- Belle.