Re: ? regarding base boost and cruise - Saab 9000 Bulletin Board - Saabnet.com
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With the cruise control engaged (not just on), the APC solenoid should be de-engergized, which means base boost. In older cars, this was accomplished by sticking a pressure switch on the output of the cruise control pump. The cruise control controls speed by running a vacuum pump. When the pump pulls vacuum, it makes an actuator pull the throttle. So if the CC is pulling vacuum, the vacuum switch will open, and de-energize the APC solenoid - base boost.
However, if you manually accelerate past the CC set point, the CC pump will stop pumping - to it, vaccum means 'speed up', and the only way it can slow down is to not pump and release the pedal. When the CC pump stops running, the vacuum in the line dissapears, and the pressure switch in line with the APC closes, enabling full boost.
So you should get base boost when the Cruise Control is running things, but if you override the CC by pressing on the throttle and going over the CC set point, you can get full boost.
Now, with Brian's 96 Aero, it appears that the CC itself can command more than full boost. I find this surprising. BUT - the 96 Aero has a full-up Trionic system with more sensors than an F-16. I suspect it doesn't have the same CC system of older cars. All I can guess is that Saab came up with a different set of safety overrides for boost and CC in the newer cars. This is just a guess - I'd have to check the wiring diagrams for a '96, and even then the function is probably buried inside the Trionic brain, instead of a clunky pressure switch.
posted by 140.157.4...
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