It has been a little while since I flew the -200.
-200 (JT8D) has heated Inlet Guide Vanes that have two valves (R & L) that allow bleed air in. If one valve sticks closed you would still have engine anti-ice. The cowl valve opens to heat the leading edge of the cowl.
-300, 500, and NG (CFM56) Uses only Cowl engine anti-ice.
When selecting the switch to open the Blue light goes bright showing the valve is in transit and then goes dim saying that the light is in agreement with the switch. When you turn off AI the blue light goes bright then out. If the blue light stays bright the valve has not opened or closed depending on which way you have positioned the switch.