Flyfisher said:
Wow...lots of good information there. Maybe you can help further with those of us that are electrical engineers (I'm a civil engineer).
How do I go about "checking the voltage to the module", both connected and disconnected?
How do I "ohm test the CPS"?
I will clean stuff and get a stock coil-to-cap wire (mine is after-market now), and will clean the inside of the cap.
I have had some ground issues in the past..mostly with rear lights and turn signals, and know they can be culprits for multitudes of problems. Will check/clean those.
Thanks again for the good information...I hope I can chase it down!
The large yellow wire going to the ignition module comes from the ignition switch, through more than a few connectors and splices. Unplug the connector to the ignition module and with the key in the run position, do a voltage test to battery ground. Test your battery across the poles. The difference between the two readings will tell you how much line loss you have. A couple of volts is typical.
Put a small cut in the insulation on the yellow wire, plug in the connector, start the motor. Do a voltage test from the cut to the battery ground. Much below ten volts (actually 9 or so) and the coil isn't going to produce a very hot spark. 6 volts or less and the motor will do some serious misfiring. These voltage numbers aren't definitive (or absolutes) but indicators. Low voltage (pressure) or low amperage (flow) will cause low high voltage spark which can cause misfires.
If you pull a plug, open the gap to around 0.060 and ground the plug, start the motor and look at the spark closely, you can see a mostly blue spark crisp with little splatter. Do the same with a plug that has low high voltage and it will look thicker, yellow and splatter.
This test is best done wearing rubber gloves, with your crotch well away from the fender.
The advice you got to inspect the CPS wire was good advice. I covered my CPS wires with plastic cable sheathing and cable tied them to the speedo cable a long time ago (on my last three XJ's). Learned that lesson the hard way.
The Crank position sensor should ohm test to 200 ohms plus or minus 75 ohm's. When they get near the outer tolerance edge they can cause misfires and other intermittent problems. The CPS I have in mine now reads 292 ohms hot, but starts fine and runs good, Renix is more of an art than a science. I'm a firm believer in the if it isn't broke don't fix it philosophy.
The connector between the coil and the module (under the coil), is out in the open and has a tendency to corrode. When it corrodes it can get hot from resistance, which can relax the connector. Worth a look, it only takes a few minutes to separate them and a torx driver. I usually pull the whole module/coil and separate them on the workbench. Grease the screws/threads before you put it back in the engine compartment and it will be even easier to pull and separate next time.