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SAAB DIC TECHNICAL INFORMATION FOUND Posted by saabsince 93b [Email]  (#972)  [Profile/Gallery]
(more from saabsince 93b) on Sun, 9 Mar 2014 09:34:07
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I found an interesting technical description of the Saab direct ignition casette on the Pico website. Not often that you can find information of what goes on in there. Pico makes automotive test scopes. Here is the link that I quote below: http://www.picoauto.com/tutorials/primary-vs-secondary.html
SAAB CDI Ignition
This particular system is different from the conventional magnetic inductive system and is called Capacitor Discharge Ignition (CDI). CDI was used on a few vehicles in the late 60s early 70s, but is now seeing a revival in this innovative system.
The ignition pack consists of individual coils that are mounted directly onto the spark plugs and are housed in a ‘cartridge’ , located between the engine's camshafts. As well as housing the ignition coils and spark plug connectors, the cartridge also contains the capacitor and the charging transformer, plus some other circuitry, as Direct Current (DC) voltage cannot be multiplied by a transformer until it has been converted into Alternating Current (AC) voltage — usually by means of a oscillator.
The 400 volts contained within the capacitor will be discharged into the appropriate coil from signals received from the Electronic Control Module (ECM). This is mainly where this system differs from the conventional ‘Coil per Cylinder’. The 400 volts are discharged to the positive terminal of the coil and the coil negative is a permanent earth. Where as a typical system will supply 12 volts to the positive terminal of the coil and 400 volts is seen on the negative side from inductance.
All of the connections in and out of the cartridge are at 12 volts or less and primarily terminate at the ECM. When the engine is cranked, the ECM will fire cylinders 1 and 4 together and cylinders 2 and 3 as wasted sparks, with every Top Dead Centre (TDC), until the ECM has determined which cylinder is on the combustion stroke (as opposed to the exhaust stroke). The ECM takes its reference from a Hall effect sensor that is located to the rear of the front pulley.
To aid starting when low cranking speed is seen, the ECM will continually fire the plugs up to 60° after TDC with a continual arc across the spark plugs’ electrodes. This process will continue until the engine speed reaches 850 rpm.
If the engine fails to start and the ignition key is returned from the crank position, a sequence of sparks are fired across the plugs to free them from any fouling and to clear any excess hydrocarbons left in the cylinders. Great care needs to be taken when working on secondary ignition systems and even more so on this particular one!
All the example waveforms used were recorded using a Pico automotive oscilloscope. Other manufacturers’ equipment will have different voltage ranges but the resultant picture should be very similar. Please remember that using a higher voltage range will result in the waveform appearing to have a lower amplitude, although the overall voltage will be the same.
In the next tutorial we will be looking at Fuel Injection.
This tutorial was first published by !WEB! The Institute of the Motor Industry !CD! The Institute of the Motor Industry
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, Mon, 10 Mar 2014 12:47:42, Sun, 9 Mar 2014 12:24:06, Sun, 9 Mar 2014 11:21:51
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