Dan,
It sounds like you have a big problem with EMI. Your ignition system is an excellent source of EMI.
EMI comes in two forms: radiated and conducted.
Radiated is in the form of radio waves. Your ignition system is pretty close to what Marconi used as a transmitter at the birth of radio.
Conducted EMI is anything that is transmitted though wiring (AC power wiring, I/O wiring, ...)
You might try a simple experiment to determine which of the two forms of EMI is causing you the problem.
Here's the experiment: power your PLC from batteries. Borrow a couple of car batteries. If you know someone that has RC cars borrow enough LiIon batteries from him to get to 24V. Keep the wiring from the batteries to the PLC as short as possible and tightly twist the wires together. Disconnect all I/O wiring from the PLC, do not connect the PLC to ground. All you want is batteries, PLC and very short wires.
If the problem goes away, then your problem is most likely the result of EMI that is conducted through your AC wiring and power supply.
If the problem does not go away, then your problem is probably radiated EMI. Think radio. I/O wiring is a pretty easy antenna.
A long time ago there was something called radio. If you can find an old battery powered AM radio, see if you can "hear" the sparks when the radio is tuned to a dead spot on the AM band. If you can "hear" the ignition system then you know that you have issues with radiated EMI.
Once you figure out how the EMI is getting to the PLC you can get to work. I suggest that you solve the problem at the source (the spark generator).
Do you know an old Ham Radio operator? Many of these guys are experts at solving EMI problems. Ask around.
Best of luck,
Gary D
