Fluctuating neg. to ground on ignition coil
Most replacement ECUs do not need the current limited by a resistor. They get power from the run wire and no longer use the 5th terminal in the connector.
To be clear, most newer ECU's no longer need the power through the 5 ohm half of the dual resistor. Look at the connector on the ECU; if there are only 4 pins, then you should not need the 5 ohm half of the dual resistor.
The 1.3 ohm value of the low resistance part of the ballast is double what it should be. It is supposedly to be 0.5-0.6 ohms cold. This is a typical problem with aftermarket ballasts. When hot, that 1.3 ohms will rise to probably 5 ohms or more (based on ballast measurements I have done in the past.) That is far more than the 2-2.2 ohms of the original ballast when hot.
If the ECU has only 4 pins, then I would:
- Disconnect the 5 ohm side of the resistor and see if the system runs
- If so, then use just a low resistance single ballast to coil + of the correct value. I like to buy OEM ballasts on eBay or the MSD 0.8 ohm cold resistor. OEM Part number 2095501 is one old PN.
This may not solve every issue for the OP, but it needs to be corrected.
The coil resistance is good but is not 100% proof that the coil is all good. I have had one that measured 1.5 ohm but was producing weak spark. I replaced it with a Pertronix 40011.
And just for info: All the above info is from when I solved the exact same issue (rough idle) in the '62 Dart in my Avatar. Wrong ballast resistor and weak coil.
