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There are 2 types of GM DFI coilpaks. The Dumb type, where the late OBD-2 type computer controls its operation, and the Smart type, where the ignition module itself controls the operation, and the computer only controls the spark timing output. We use the smart type.
Basically the operation of the trigger is like this, the ignition module, which controls the operation of the whole DFI unit, detects the time difference between the 10 degree TDC notch at startup and now knows that in 66 degrees time (the Reference Angle), that it will be at TDC.
From here it is then programmed to skip One notch and fire on the 2nd. This is now at approx 180 degrees past TDC, (66 ref ang + 60 + 60) OK? Then it skips the next Two notches and then fires on the next (another 180 degrees). It then repeats, sees TDC notches, skips 1, fires, skips 2, fires and so on, very basic, but very clever. The only downfall to this system is that the engine must turn a complete revolution before it can synchronize and then startup…. Big Deal. Most of us probably hang onto the key at least 100 revs after the bloody thing has started…...a bit like reaction times, and snoozing on the line…. Most GM vehicles today with their more exotic crank triggers will start within just a few degrees of rotation, , you should see a GEN 3 crank trigger!
To setup and fabricate the crank trigger, you must first mount the Crank Sensor in a convenient spot. You then position the crank at TDC, then using a degree wheel or similar, turn the crank backwards 66 degrees. Now mark on the pulley/disc or flywheel exactly where the pickup is pointing to.
This is the TDC Reference Point.
Then turn the crank forwards 10 degrees and make another mark .
Then mark the remaining 5 notches at 60 degrees from this mark.
We assume that the smaller the diameter of the crank trigger, the smaller the slots will be, and the larger the diameter, the larger the slots.
On a flywheel installation that we (Peter Jenkins) did, we drew a diagram to scale of the factory trigger we had which was 163 ml. From the centre, we then ran lines through the 5 ml notch, and at the diameter of the flywheel where we wanted to mount sensor, the notch was 13 ml. We simply ran a 13 ml drill through the thickness of the flywheel, and it worked perfect. See photo on Custom 4 cyl's.
When all this is done, there is one last thing to do, and that is, using Kalmaker software, you must change the Variable No. 64 SPK:OUT:REFANG to 66 degrees.
What Reference Angle is all about, is basically, the delay it takes from the timing marks on the trigger to get to TDC. From this figure the Timing Advance for the appropriate load point is then subtracted and then the coil is fired.
The reason for different reference angles are, there are 2 modes (actually there are 3) in which the ignition module runs.
Module Mode, and EST Mode.
Module Mode is when the engine is cranking, and it gives out a fixed 10 degrees spark advance.
The computer has no control of this, and in theory if you were to unhook computer, the engine would continue to run at 10 degrees. (if it had fuel)
EST Mode When the engine reaches 450 rpm, it is considered to be running and switches to EST Mode whereby the computer controls the amount of advance.
There is also another mode which is part of EST Mode called Diagnostic Mode.
Diagnostic Mode is also controlled by the computer and is not to be confused with Module Mode, as it is just coincidental that it also fixes timing to 10 degrees. This is mainly for setting timing.
Diagnostic mode is activated by bridging the diagnostic link in the Service Socket (ALDL) and this switches the computer to a fixed timing output, which is 10 degrees default. It can be changed in software.
Getting back to the reason for different reference angles, when the engine is cranking, it is in Module Mode and ignores computer, thereby ignoring reference angle. The module sees the TDC and 1st notch and then fires on the 2nd notch, in theory you could call this the rising edge as in the case with Hall Effect trigger.
Once the engine is running it switches to EST Mode, and then switches on the 3rd and 6th notches, which again you could call these the falling edges, as in the case with Hall Effect.
Therefore, because the notch is 60 degrees apart and the TDC notch is a further 10 degrees, you would think the Ref Angle would be 70 degrees, but there must be some hysterisis involved and it ends up at 66 degrees. (In a Hall Effect sensor, there is 10 degrees hysterisis from reading the rising edge to the falling edge)
Therefore Reference Angle derives from the mechanical (hardware) angle of the difference between the rising edge and the falling edge.
Basically if the Reference Angle in the software doesn't match the Reference Angle of the Hardware, your engine will run like shit.
With a distributor, if your Ref Angle is wrong, you can time the dissy so that it runs ok in EST mode, but your timing wont be 10 degrees when cranking. If the cranking amount ends up too far advanced, it will have trouble starting. There is more to a distributor than that, that's why you need to read more about Reference Angle
See Reference Angle in 25 Most Used Variables
Under Construction, Photos etc to come..
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