As posted on T-D by MINIMOPAR 02-09-2005
Thanks
Randy
Naturally Aspirated
Join Date: Jan 2003
Location: Chicago, IL
Posts: 1,954
Here's how it works: the cruise servo uses vacuum to operate. On 1988 and later cars, they added a second vacuum source from the turbo. If you look at your vacuum lines from the cruise servo carefully, you will see a "Y" connection, two check valves, and one line going to the brake booster and the other to a "T" near the airbox. When you are in vacuum, the brake booster is the source. When you are in boost, a line that is connected to the turbo compressor housing is routed to the airbox. Right before it vents into the airbox, it goes through that same "T". If you take that T out and look inside, you will see an aluminum insert. That insert uses the boost from the turbo to generate a venturi in the T. The venturi effect creates a vacuum just behind the end of the aluminum insert, which is where the line from the cruise ties in. So there you go: vacuum made from boost. *Whew*
I think they added this because the little Mitsu goes into boost so easily that the cruise wouldn't work on the highway.