Re: 89 YJ 258 with no spark, sometimes...
When you say it won't start, do you mean it won't crank over, or it will crank over and not start?
If it cranks and doesn't start, try these Quick tests, With the key in the 'Run' position,
1. Check for 12 volts at the Harness Connector Red wire to the module. (Module unhooked, you are testing the harness).
If you don't get 12 volts here with the key 'On', you have a loose fuse, bad fusible link, loose connections, bad ignition switch, or bad factory tach.
2. Check for 12 volts at the Harness Connector White wire to the module. (Module unhooked, you are testing the harness).
You will need a second person for this one, you will need to test the circuit with the key in the 'Run' position, and with the key in the 'Start or Crank' position.
If you don't get around 12 volts here when the engine is cranking, jump directly to step #4.
If the solenoid checks out, it's your wiring harness or connectors.
Don't forget to hook your module connector back up!
3. Check for 8-12 volts at the Red wire in the Ignition Coil Connector (Ignition Coil Unhooked).
If you have around 8 volts, your resistor wire is fine. If you have 12 volts, you don't have a resistor wire.
4. With the Starter Cable Disconnected (so the vehicle doesn't crank with the key in the 'Start or Crank' position)...
Test for 12 volts at the 'I' terminal at the starter solenoid while the starter is cranking.
If you don't get around 12 volts here, your solenoid is fried.
You don't have to do these all at once, just remember where you left off...
And in most cases you can just use a test light instead of a multimeter...
Have your alternator checked.
If you haven't hooked something up that is putting the hurt on your battery...
When you get in, unhook the ground (negative) battery cable. Make sure there is nothing hooked up to the negative battery terminal.
Take a voltage reading about 10 minutes after you park the vehicle.
Let the vehicle sit all day, and take another voltage reading.
If the battery went down sitting still with no cables and no load, it's the battery. (now ask yourself what killed the batteries...?)
The battery discharge problem is one of two things,
Your battery is fried. Unlikely with two different batteries.
(overcharging will kill a new battery in the way you describe, so the alternator is still suspect)
You alternator has a bad rectifier or voltage regulator.
Take it into the local 'AutoJerks' and have them test the Rectifier Diodes.
My guess is your problem lies here...