Converting from CDI to CDM ignition

[Redbarron%%]

OK I confess I have created a *astard ignition motor going from a CDI with switchboxes to late mercury CDM modules for (maybe ) improved reliability.
I have a couple of issues, however and maybe someone can help me out.
First the CDM modules are readily available on Ebay as is the three cylinder harness to mount them to the motor.
The issue is with the bias circuit and the trigger coils.
As I now understand it the old CDI had the bias circuit built in the switch box where the new CDM has it buitl in the trigger coil. What is the bias, you ask? Again as I understand it the bias circuit is a negative voltage added to the trigger voltage to bias the point where the trigger fires the SCR in the pack down as the voltage rises due to increased rpm from the trigger magnet pickup. The arrangement of the magnets in the flywheel and the coils in the trigger is different between the CDI and CDM ignitions, however the pulse from either will fire the CDMs, but the timing may be wrong with the mixed parts.
Since with my setup there is no bias voltage generated in the trigger coil circuits the timing may be advancing too far or triggering incorrectly with the unbiased output from the coil.
Does anyone know what is in the new trigger that creates the bias in the CDM units?
I suspect that the voltage generated by the trigger coils could be rectified in a bridge and somehow subtracted from the voltage applied to the CDM.
Again as I understand it a 0.60 positive voltage at the trigger input to the CDM will fire the SCR and a voltage of ~25 volts negative is applied at higher RPMs ti keep the faster rising voltage as the RPM increases from triggering the SCR sooner and advancing the timing too much.
I have a miss at high RPM that may be related to the lack of the bias circuit in the FrankenForce ignition system I have created.
I hate to get a late flywheel and trigger to test the theory and also to go back to the old CDI system.
Anyone have any ideas or knowledge about just what circuits are in the new CDM trigger?
I had assumed that too much voltage out from the CDI triggers would not be a problem as long as it would fire the SCR, but at the time I was unaware of the bias issue.
Sorry for the long technical post, but what to do?
Thanks,
JD
PS.
The new style CDM triggers cannot be tested with an ohm meter test and I think that the additional circuitry is why.
I have seen a circuit that has a resistor network from the pick up coil (3) and a voltage divider network to back to back 5.1 volt zener diodes to limit the voltage to 5.1 volts plus and minus and this may be all that as needed as there is not room in the trigger for a lot of electronics. I may try making this circuit on a perfboard to install between the trigger and the ignition harness.

s.

 

[jerryjerry05]

No idea.
I always thought the OEM, Prestolite was the better system.
I've found, if it's gonna fail? no matter how expensive or who makes it? it's gonna fail

Good luck with the project.

 

[Redbarron%%]

Thanks for the reply, Jerry.
It seems that there is a compensation circuit built in the 1996-2000 trigger coil that is not in the earlier units as the newer one cannot be resistance tested.
I think that the signal conditioning circuit does the bias compensation within the unit where the older system doe it within the switch box.
The issue is as the rpm increases the voltage from the coils increases and the point where the trigger voltage reaches the 0.6 volt level the trigger the SCR in the CDM advances. The voltage ramps up as the magnets turn and at some point it reaches the trigger voltage and that point advances with the RPM.
I wish I could find a circuit drawing of that circuit.
I may have to design a digital pulse circuit that gives a pulse out when the transition from N to S on the magnets.
Of course easier might be to just put the old switch boxes and coils back on the engine.
I have attached a page showing the basic CDM circuit, but there is nothing about the Trigger conditioning board.


View attachment CDM Circuit.pdf

 

[jerryjerry05]

Over the years, I found that the only upgrade I ever liked was the one someone posted on here.
It took the old ignition and changed it to an automotive coil.
Yup, changing back to the old would be easier

 

[Redbarron%%]

Right now I am getting ready for a trip and it is also hot as HHHH here in Navarre so this project is on the back burner for now.
I have ordered some Hall effect sensors to play with if I decide to proceed with the modifications.
I wish I could find a circuit drawing or schematic or theory discussion on the bias circuit to have some idea of what the factory did originally.
I think that the "new" trigger coil for the CDM modules has a forth coil inside that probably generates the bias voltage and some kind of rectifier bridge and charging cap to produce the negative bias voltage to match the output from the timing coils installed.
Perhaps the trigger coils could be tapped and fed to a rectifier bridge to do the same in the old trigger external to the bridge.
Knowing what was done in the old switch box would be nice!
One difference between the coils is the common is grounded with the new trigger and "floating" in the old one going into the switch box.