The control rod is part of the mechanism in the IP that tells the ELR (idle controller) what's going on in the IP. Those are extreme layman's terms since I'm too lazy to fire up the CD manual to find the one page in 70 of the IP survey that discusses the control rod.

If you ask me, the main function of the control rod device is to make it difficult to replace the stop valve.

FYI, Adsit (www.adsitco.com) has used IP's for ~$400 and rebuilt ones for about twice that.

Sixto
91 300SE
87 300SDL
83 300SD