Slow crank or no-start electrical? Test the three components in order: battery (load + voltage), then alternator output, then starter draw. All three are interrelated.
The 'battery-alternator-starter triangle' is the foundation of cranking diagnostics. Every slow-crank or no-crank issue on a healthy ignition system traces to one of these three. Battery first: voltage at rest (12.6 V good, 12.0 V dead) and load test (parts stores do this free - should hold above 9.6 V under load for 15 seconds). Alternator second: with engine running, battery voltage should be 13.8-14.6 V; output current under accessory load should hold above 13.0 V. Starter third: with a known-good battery, measure voltage at the starter S-terminal during cranking (should be at least 9 V) and current draw (130-180 A peak is normal for 4.0L; over 250 A means seized engine or failing starter). Ground straps: the engine-to-firewall ground strap and the block-to-frame ground are XJ-specific weak points. Add a supplemental 4-gauge ground strap from the battery negative to the engine block as preventive maintenance - cures many mystery electrical glitches.
Written and maintained by an AZ wheeler and driveway wrencher. Always cross-reference your factory service manual — modifications affect vehicle safety and warranty. Work at your own risk.