This is actually called the ISCU (Idle Stabilizer Control Unit) and it is
located behind the right rear tail light in 86-91 Vans.  These do go bad over
time.  I have two in stock right now that are good, tested on a bench, units.
 I would like $100 each for them.  New they do run somewhere around $200.  
Let me know.  I would also like your old unit as a core.

Thanks,
Ken Wilford
Van-Again
John 3:16
http://www.vanagain.com
Office: (856)-765-1583
Fax: (856)-327-2242