codekiddie
Active Member
I'm changing to a Simon BBC electronic ignition and need to get rid of the ballast resistor. Doing electrics doesn't phase me (pun intended), but I still have a question :?
I have read on here various threads about bypassing the ballast resistor for new coils requiring the full +12V, and most suggest simply obtaining a feed off of one or other fuses that is only live when the ignition is on, for example the windscreen wipers fuse.
So far so good, and I am sure it will work just fine, (and I'm also sure that some of you will tell me that it does), but I can't for the life of me see how my tacho is then going to operate :?
In the following drawings I have shown the +12V routes in RED.
The following is the current setup:
The +12V feed from the ignition switch (35) goes to the tacho (30), via the white wires, then on to the ballast resistor (31), via a white/yellow wire, and finally to the coil (32), via another white/yellow wire. The pulses generated by the points in the distributor, (33), opening and closing is what registers on the tacho as they break and make the circuit.
The following is the modified wiring to give a permanent +12V to the coil (albeit from the ignition switch directly and not via a fuse, but electrically the same).
Now the tacho effectively has +12V on both sides of it, and certainly wont be seeing any break/make pulses from the points/electronic ignition, so surely wont work at all.
This version simply shows the ballast resistor shorted out.
The coil will get close to a +12V feed, via the tacho, and the tacho should also work, now being able to 'see' the points opening and closing again. My question, (eventually), is has anybody else done this without any problems with the tacho, since the lack of a ballast resistor would increase the current flowing through it.
I have read on here various threads about bypassing the ballast resistor for new coils requiring the full +12V, and most suggest simply obtaining a feed off of one or other fuses that is only live when the ignition is on, for example the windscreen wipers fuse.
So far so good, and I am sure it will work just fine, (and I'm also sure that some of you will tell me that it does), but I can't for the life of me see how my tacho is then going to operate :?
In the following drawings I have shown the +12V routes in RED.
The following is the current setup:
The +12V feed from the ignition switch (35) goes to the tacho (30), via the white wires, then on to the ballast resistor (31), via a white/yellow wire, and finally to the coil (32), via another white/yellow wire. The pulses generated by the points in the distributor, (33), opening and closing is what registers on the tacho as they break and make the circuit.
The following is the modified wiring to give a permanent +12V to the coil (albeit from the ignition switch directly and not via a fuse, but electrically the same).
Now the tacho effectively has +12V on both sides of it, and certainly wont be seeing any break/make pulses from the points/electronic ignition, so surely wont work at all.
This version simply shows the ballast resistor shorted out.
The coil will get close to a +12V feed, via the tacho, and the tacho should also work, now being able to 'see' the points opening and closing again. My question, (eventually), is has anybody else done this without any problems with the tacho, since the lack of a ballast resistor would increase the current flowing through it.