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Posted by dave n (more from dave n) on Sun, 4 Feb 2001 17:09:34 Share Post by Email
In Reply to: Theory of C900 ignition system, Rob M, Sun, 4 Feb 2001 14:40:46
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The spark stays the same. Ok, dense fuel mixture at higher boost makes it harder for spark to jump across plug. The time the spark happens relative to flywheel position changes with pressure/vacuum. The timing change is purely mechanical. The distributor vacuum canister advances the timing at vacuum but delays it with pressure up to 3-4 psi. Any pressure above that doesn't retard the timing further. I know, the distributor is fixed to engine so how does it rotate itself to change timing?? The distributor housing is fixed to engine but pressure actually causes sensor inside distributor to rotate. IF you want to see something for yourself, there's a check you can do with a multimeter. You attach multimeter at ignition amp to read the DC volts signal from distributor hall sensor. THen you could remove the distributor from engine and turn it by hand with key on (engine off!). The meter will go from 10v to nothing and back as you turn it. That's the signal telling coil when to fire. Another thing I realized: there is no vacuum on the distributor canister when car is at idle. You probably know these things but I like to confirm what I "think" I know! By the way, this ignition system isn't unique to saab. Similar systems were used in most european cars of similar model years.
dave n

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