Re: APC solenoid is a clue - Saab 900 Bulletin Board - Saabnet.com
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Re: APC solenoid is a clue
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Posted by Mark J (more from Mark J) on Wed, 20 Aug 2003 13:28:24 Share Post by Email
In Reply to: Re: APC solenoid is a clue, scottgf, Wed, 20 Aug 2003 09:00:57
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When APC system is offline, wastegate "sees" true manifold pressure and opens at base boost setting; so boost is regulated at about 5-6 psi. Strictly mechanical regulation of boost just like a regular turbo car without APC.

When APC is online, APC system "hides" true manifold pressure from the wastegate by bleeding pressure from the wastegate hose via the APC solenoid. So true manifold pressure may be much higher than what the wastegate is "seeing". The pressure in the manifold may be 10 psi, but the wastegate is only "seeing" 5 psi (and therefore remains closed) because that is all the APC system is showing it. This is normal operation. When true manifold pressure reaches a preset limit (stock is ~11 psi; between yellow and red border) APC stops bleeding pressure from the wastegate hose and wastegate opens regulating boost.

The test that you did by disconnecting the wastegate hose makes it so that the wastegate never opens at all - totally unregulated boost - actually quite dangerous. But it serves to tell you that the turbo itself and engine/fuel management works OK and that the APC system is not working. HTH.

- Mark
1985 SPG
1978 99T

posted by 12.144.1...

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