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merc/force ignition

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dtoole

Regular Contributor
hi all, i was driving to work this morning going over the possible causes for the tach issue mentioned in my earlier thread when i realized that i dont fully understand how the ignition system on a merc works. here is what i have or what i think i have. the flywheel magnets pass over the coils on the stator. this produces an ac electrical pulse. this is where i get confused. the stator and timing base are two separate pieces right? the stator sends the pulse to the power pack where it is stored until a signal is sent from the timing base? does anyone have a good breakdown of how this system works?thanks...david
 
im pretty sure that this is correct. i've doe a little reading since my last post. however i am still confused about the role of the rectifier/regulator. is it regulating battery voltage only or does it also have something to do with the ignition voltage ? the cdi troubleshooting list says to disconnect the two yellow wires from the stator to the regulator and retest for fire. how does the regulator affect the ignition ? thanks.....david
 
Here's what Professor Graham posted on this subject:


The trigger is essentially two coils that produce alternating current when the magnet passes them. A trigger signal is a "pulse" of power that triggers (trips) the switch (SCR or Silicon Controlled Relay) which in turn releases the power stored in the switchbox's capacitor (which came as AC from the stator, was rectified (converted to DC and stored))and sends it to the coil and then on to the sparkplug for whichever cylinder the "trigger" told it to go to.

Alternating current doesn't have/need a ground like Direct current does to complete it's circuit. AC is a positive/negative waveform. When the waveform is (going positive) it's pushing power down the line. When negative it's (sucking) it back.

So with the trigger when it's "pushing" power it trips the SCR (that is set to fire cylinder 1 or 2), the SCR switches to cylinder 1 and releases power. When it sucks the power back (or reverses direction if you like), the SCR is (thrown) the other way (cylinder 2).

The trigger bobbin and the second SCR in the switchbox (with it's other assorted parts - capacitor, rectifier, blocking diodes etc) does the same thing for cylinder 3 and 4 (they could be paired in any combination - 1&3, 2&4 or whatever). (wandering a bit here) So, the 4 wires from the trigger work as follows - with no ground, because it's "alternating current".

If the power travels from the red wire to the switchbox and returns on the red/white back to the trigger that would be a "positive" pulse and cylinder 1 would fire. When the "wave" starts it's negative crossing, the current flow would switch direction and travel from the red/white through the switchbox and back to the trigger on the red wire - a "negative" pulse which switches the SCR to fire cylinder 2 (or 3 or whatever the "paired" cylinder is).

The firing order is dependent on however they have engineered (chopped up) the trigger coils and wired them. The two positive pulses (waves) will be 180 degrees apart and the two negative waves will be 180 degrees apart with the two complete waves separated from each other by 90 degrees such that you would get something like this - (in the example below the "degrees" would represent rotation of the flywheel utilizing a trigger being energized by two magnets - as the first magnet passes the trigger coil it produces the positive portion of the AC wave, the second magnet produces the negative portion - for each of the two trigger/sensor coils)

I can tell you this about tach readout: if both yellow wires from the stator are not hooked up, the tach won't read. (Ah-hem!)

Jeff

PS: The above was for a 4 cylinder Merc.
 
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