Re: Update - Saab 900 Bulletin Board - Saabnet.com
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C900 Bulletin Board
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Re: Update
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Posted by Cmyles [Email] (#1126) [Profile/Gallery] (more from Cmyles) on Thu, 2 Jul 2020 22:01:21 Share Post by Email
In Reply to: Update, Rose quartz 87 [Profile/Gallery] , Thu, 2 Jul 2020 19:54:37
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Well, rethinking this, one of two things is happening here: 1)The engine is knocking and the turbo/APC system is working perfectly or 2) the engine is not knocking and the lack of boost is due to a turbo/APC defect.

If you actually hear it knocking when it refuses to achieve maximum boost then either there is a combustion problem or the engine has a mechanical problem. Changing out APC components won't help.

If you aren't certain that you're hearing knocking but full boost can not be achieved then the problem may be in the Turbo/APC.

One overlooked feature of the APC system in 1987 era C900's is the cruise control vacuum switch. When it senses vacuum (from the CC vacuum pump) it opens the circuit between the APC box and the solenoid and limits the car to base boost. A defective CC switch or bad connection will open that circuit all the time. Also, People have been known, on early C900 turbos (like your 1987) to accidentally swap the electrical connectors between the CC switch and the upshift light switch, which are very similar in appearance and right next to each other but work differently. The CC switch has a red cap and the upshift switch has a blue cap but the caps can get lost. You can eliminate the issue for test purposes by disconnecting the 2 pin connector from the CC switch (it has two yellow wires with white stripes) and inserting a jumper into that connector (so that the yellow/white wires are permanently connected together), then see if you have full boost. All that stuff is on the left inner fender right near the APC box.

When you switch the ignition on (to run) you should hear one click from the solenoid. If it does click then blow through the hose that connects to the center nipple of the solenoid and you should hear air coming out the hose that's going into the induction pipe. Next turn the key off, disconnect the hose to the wastegate and blow through the center hose again and air should come from the wastegate hose only.
If the solenoid does not make one click when the key is turned on then it's not connected or grounded, circuit problem.

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