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Discussion Starter #1
Hi, I'm new to these forums and need some help.

Situation:
  • I recently bought a 1990 Saab 900 turbo, knowing it had a faulty turbo (not boosting properly)
  • I took it to local garage who have a bunch of really good mechanics who know way more about Saab’s than me
  • They checked wiring and connections and came to conclusion that the APC solenoid valve (part 7517477) was faulty
  • They tested and confirmed this theory by taking an APC valve from a working Saab and installing it into my car... which fixed the boost issue - hence why it seems logical to assume that the original APC valve is the problem. Unfortunately I couldn't keep the borrowed APC valve...
  • I obtained a second-hand APC valve (couldn’t find a new one anywhere), got the mechanic to install it, but this didn’t fix problem - it was same as with the original
  • I got the second hand APC valve back from the garage and went through the current/blow test (as described on pages 291-6 and 7 of the Saab manual (by Bentley). The valve passed this test ok - seem to work as it should.
  • I then tested the original one and this also passed this current/blow test
Questions:
1. If the APC valve appears to work correctly (mechanically speaking) but doesn’t work properly when installed in the car, is there something else that could be wrong with it that will cause it not to boost properly?
2. Can other tests be done on the APC valve to troubleshoot it?
3. Is it possible that both the original and replacement APC valves have the same issue? Is it just bad luck that both don't work properly, or are issues with these solenoids common?
4. Should I try to source more second hand APC valves until I find one that works just as well as the borrowed one, or is there a way to fix the faulty ones and resolve the issues?

Thanks, in advance for any insights and help you can give!

Cheers, Steve
 

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Good place to start. I believe solenoid not the issue.
 

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Discussion Starter #3
That's what I thought too, but the fact that the boost problem was resolved when we borrowed an APC solenoid from a working Saab and installed (as part of the troubleshooting process) can only mean that the solenoid is the problem, right?? I just cant see how it can be anything else. But am seriously confused by it all so totally keen to hear about what else it could be...
 

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That's what I thought too, but the fact that the boost problem was resolved when we borrowed an APC solenoid from a working Saab and installed (as part of the troubleshooting process) can only mean that the solenoid is the problem, right?? I just cant see how it can be anything else. But am seriously confused by it all so totally keen to hear about what else it could be...
Is the car in base boost? I gather that the needles moving nowhere into the yellow? If you Take a long extension and crack the knock sensor while the car is running it should make the APC solenoid chatter. The knock sensor is dead center on the top part of the motor through the intake. I’d also confirm the APC controller doesn’t have a dirty connection sometimes water gets inside of the connector and corrodes the contacts.
These are things they can be tried for free by you. I also recommend reading the link I had posted and going through that check list.
 

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What do you mean by 'turbo not boosting' and 'turbo worked normally.'
What are the boost gauge readings? When it 'worked OK' did it boost into the red zone? Does the needle ever go into the yellow?
The solenoid can't prevent it from boosting at all
 

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Discussion Starter #8
Thanks all. When I said it was 'not boosting properly' I didn't mean it wasn't boosting at all - just that it wasn't delivering the sort of power you would expect. Very sluggish, needle moves into the orange just (but not red) and with no obvious acceleration grunt. And the only reason why the APC has been singled out as the cause is because the boosting is much better (and behaves as it should - according to the mechanic, who knows Saabs very well) when they took one out of a different Saab 900 (which was boosting correctly) and put that in mine.

I'll be getting my car back from the garage this weekend and seeing what I can do so appreciate the suggestions and checks I can do, especially the link to the check list (thanks Second Saab).
 

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Defeat the APC (I forget which hose) and see if you get scary overboost.
Set base boost.
 

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Remove either the C or W hose... mostly the same difference, except removing W creates a vacuum leak all the time, and W creates a leak only under higher boost. For the length of the test, it probably doesn't matter. Just don't blow up your engine.

There is no good way to test the solenoid... it can click and seal as it should, but that doesn't mean it works properly. It's 30 years old. Nothing lasts forever.

Unfortunately the system is basically analog, there aren't many areas to test nor many quantifiable results. If the inputs to the APC box are solid, you basically need to trust the box and the output (the solenoid) are working or parts-swap with known good ones. I would assume at this point the shop did test the inputs?

If you're reasonably certain it's the valve, you just gotta keep hunting til you find a replacement. Ebay, Saab dismantlers, junkyards.
 

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Solenoid Valve CAN be rebuilt . First meter the thing to see if the two coils are intact If open or shorted it's Dead Unless you have fresh coils to fit? The inner valve a wee disc is shunted back and forth by the coils magnetic fields. The valve seats are brass.. these can/do wear unevenly for a poor seal And the disc can crack and shatter. One razor saws the valve body at it's obvious glue joints A bit tedious but actually quite a simple procedure. Then the thing pulls apart to reveal the innards. Epoxy reglues it.
 
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