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Re: Load or no-load? Posted by Ari [Email]  (#2847) [Profile/Gallery]
(more from Ari) on Mon, 9 Jul 2001 14:26:09
In Reply to: Re: Load or no-load?, denis, Mon, 9 Jul 2001 14:10:31 Members do not see ads below this line. - Help Keep This Site Online - Signup |
A pressure transducer is a sensor that measures pressure. In this case, there is a transducer (sensor) that measures the air pressure inside the intake manifold - basically, it converts boost pressure to a voltage for the computer to read.
Yes, it could affect the APC, since in some years the APC computer used measured boost pressure to regulate the APC solenoid.
I would expect that pressing the brake pedal to the point where the brake lights go on would cause the APC voltage to go to 0 volts. But it's interesting that you have a '94 convertable. That is the absolute last year of the Old Generation 900 body style, but the newer engines were out. Do you have a DI (Direct Ignition) cassette in your car -basically, is there a distributor cap? I'd have to check my manuals for which Fuel Injection system you have in that car - it's one of those 'in-between' model switches. The more modern engines have some pretty sophisticated systems for regulating things, so the absolute rule of 'base boost with brakes on' may not apply to your car.
As a '94, I know for a fact that the APC system regulates boost pressure by 'pulsing' the APC solenoid. It sends lots of quick pulses to the solenoid. If it sends lots of short pulses, the solenoid is more off than on, and you'd get a lower voltage. If it sends longer pulses close together, the solenoid would be more on than off, and you'd read a higher voltage. Saab may have re-engineered the APC mapping to allow more boost with the brakes on - that would explain 10 volts with brakes on. That also explains a lot of strange voltage readings - the APC is pulsing the system to get exactly what boost it wants to allow.
Yes, that runs counter to the earlier answers in the post. I was working from the assumption it was an older (say '87) system.
I guess I'll have to ask the question - what is the car doing wrong? I don't remember your older post. It's entirely possible that you're chasing the wrong problem - that 'partial' voltage readings at the solenoid are normal for your year, and that the problem lies elsewhere. I'll assume that you don't get full boost. Base boost is about half-way into the yellow. Do you get more than base boost, but you don't always hit the red like you'd like to? If so, do the simple stuff. Run a couple of tanks of premium (93 or 94 octane) fuel. See if that helps. Track down and fix all vacuum leaks - vacuum leaks can cause lean running, which can lead to knock. How do the spark plugs look? Do they look good (light grey) or is the engine running lean (white). Is the timing correct (if adjustable in your year). After that, you go to harder stuff to check, like Fuel Pressure and fuel delivery rates.
Let's go back to the beginning. What's wrong?
, Mon, 9 Jul 2001 13:14:25, Mon, 9 Jul 2001 14:26:09 <-- Viewing This Message
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