Date: Tue, 6 Jul 1999 15:26:50 GMT
From: (Goran Larsson)

In article <7lse3i$1mk$>, IZZO <IZZET3nopsamAIL.COM> wrote: > But, WOW, i didnt know that the turbos were so complicated. The turbo need something to regulate the turbo pressure. If you don't have anything to limit the pressure the pressure will try to reach infinity (more pressure -> more power -> more exhaust -> more pressure -> more power -> more exhaust -> more pressure -> more power ...) until something breaks. That "something" usually costs a lot of money. To correct this problem turbo cars have a gate, called a wastegate, that can open a channel to short circuiting the turbo. Older turbo cars had a wastegate that was preset to open at a certain pressure, usually done by having a spring holding against the turbo pressure, when the pressure goes above the spring force the wastegate opens. When Saab introduced the APC system in the early eighties they instead controlled the wastegate using electronics. To make it possible to control the mechanical wastegate from the electronics they use a elecromagnetic vent connected with three tubes. The tubes are connected to the low and high pressure sides of the turbo and to the wastegate. When the vent connects the high pressure to the wastegate it opens (if enough turbo pressure is available,) when the vent connects the low pressure to the wastegate it closes. The electronics modulates the elecromagnetic vent (older APC units used 12Hz) and can by varying the make/break ratio control the opening of the wastegate. Doing it this way allows a much more possibilities of controlling the turbo pressure than a simple spring loaded wastegate. One possibility that Saab has used since the introduction of the APC is to have a microphone listening for engine pings and reducing the turbo pressure if the engine start to ping du to low octane fuel or whatever. Older Saabs have the APC functions (all analog electronics) in a special APC box. Newer Saabs, starting with the direct ignition in 1989 or so, have the APC functions incorporated in the engine management system. > I have a '98 900 SE 2.0 turbo...should i be expecting this problem as > well??...and furthermore, is there something i can so like preventtive > maintanence?? The electromagnetic vent should be problem free, but some cars have had problems with them. Why that is I have no idea, my old 9000 '88 never needed a new one. The design of the vent is simple, but the environment it works in is kind of hostile, one side at low pressure, one side at high pressure and high temperature, and then have to wiggle back and forth 12 times a second... There is no preventive maintenance that can be done that I know of. -- Goran Larsson hoh AT approve DOT se I was an atheist, http://home DOT swipnet DOT se/hoh/ until I found out I was God.

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