The CPS on the 4.0L HO reads the flywheel reluctor ring from inside the bellhousing — when it fails from heat, the engine stalls and won't restart until it cools. This guide covers diagnosis and replacement on 1991–2001 XJs.
The crankshaft position sensor tells the PCM where the crank is in its rotation so it can time fuel injection and spark. When it goes, the PCM has nothing to work with and shuts everything off. The failure pattern on the XJ is almost self-diagnosing: the engine runs normally when cold, stalls once it's been at full operating temperature for 20–30 minutes, won't restart for 30–60 minutes, then fires back up like nothing happened. That's thermal failure — the sensor works when it's cool and quits when it gets hot.
If that's your XJ, the CPS is the first thing to address before going after cam sensors, ignition components, or fuel system parts.
**RENIX vs. HO:** This guide covers the 4.0L HO (1991–2001). RENIX engines (1987–1990) use a different 2-wire sensor with a different connector and different failure behavior. If your XJ is an '87–'90, confirm the correct part before ordering.
**Check the connector before you buy the sensor.** The 3-pin plastic connector on the CPS pigtail is notorious for cracking from heat cycling and developing internal corrosion at the terminals. A failed connector produces the same symptoms as a bad sensor. Before ordering parts, trace the wire from the sensor up the engine block to the connector and look at it closely. If the housing is cracked, brittle, or the terminals show green corrosion, order the pigtail at the same time as the sensor — or you may fix nothing.
**Buy OEM or Mopar.** Cheap aftermarket CPS sensors have a documented pattern of short service life on the XJ. The sensor lives in a hot, vibration-heavy location. The extra $15–20 for a genuine part is not where to economize.
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| Part | Vendor | Est. price |
|---|---|---|
| Crankshaft Position Sensor — OEM Mopar (4.0L HO, verify with VIN) | Quadratec / Mopar dealer | — |
| CPS Wiring Connector Pigtail (replace if cracked or corroded) | Quadratec / Mopar dealer | — |
| Dielectric grease | Any auto parts store | — |
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.