Question about 1992 Saab 900
APC Solenoid Valve is making this repeated knocking sound. Its from a 1988 Saab 900 Turbo 16V, it sounds like its opening and closing really fast. and when I unplug the wire it stop. What could this be?
The APC valve regulates boost pressure (via the wastegate) to avoid over-pressurising the engine. It ensures that a safe boost pressure isn't exceeded. Don't run with it disconnected or you risk blowing the engine!
Problem areas can include: knock sensor on the engine block (also poor quality fuel, dirty engine causing knock); engine ECU.
You need to provide more information about when this happens:
hard accelerating? (likely poor quality fuel, dirty plugs/engine causing knock) and general driving similar.
idling? possibly a knock sensor or ECU related problem
What rpm? what boost gauge pressure?
as an outside chance, maybe a sticky wastegate
Posted on May 30, 2009
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Posted on Jan 02, 2017
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Apr 27, 2015 | 1997 Saab 900
NOT EXACTLY WHAT YOU ASKED BUT VALUABLE INFO I THINK YOU'LL AGREE?
There are several common cruise control failures on Saabs. The pedal switch failures are the most common problem. What generally occurs is that the pedal switches become mis-adjusted or the ears get broken off the switches causing a no contact condition. Another common problem is that the cruise vacuum hose leading from the vacuum pump under the false bulkhead begins to crack where the vacuum fitting is located at the firewall.. The third most common failure is an intermittent functioning cruise control module. This can often be identified by noting that the cruise will work intermittently (900 & 9000 only).
Another Issue: On turbo models, there is a vacuum controlled switch, located near the pump (red cap) that cuts the signal to the APC solenoid valve when the cruise control is on (switch opens). If the switch is faulty (I broke mine on rough road), it can keep turbo boost at the "basic" level even if the cruise is turned off. The APC boost gauge will only go half-way into the yellow region, and performance will suffer. Either replace the vacuum controlled switch or pull the pigtail connector below the switch and short the ends of the yellow/white wire going to the APC solenoid valve and
proper turbo boost should return. Be aware that if this second fix is employed, your turbo is not limited in boost when in cruise control.
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