The CMP sensor tells the PCM which cylinder is coming up on its firing stroke so it can sequence the injectors — when it quits, the engine cranks but won't catch, often hot, often intermittently. Where the sensor lives changes with the model year, and that's the whole job.
The camshaft position sensor handles fuel sync: it tells the PCM where the camshaft is so injectors fire on the right cylinder at the right time. Pair it with the crank sensor and you have the two signals the engine cannot run without. A failing CMP throws P0340 and frequently its partner P1391 (intermittent loss of CMP or CKP), and the symptoms read like a haunting — random stumble, bucking over bumps or on right-hand turns, a stall going into reverse, and the classic crank-no-start that gets worse as the engine heats up. Heat and vibration kill these sensors, so the failure tends to show up at operating temperature and clear when things cool down.
Before you spend a dime, know which XJ you have, because the sensor lives in a different place across three eras and the procedure is not the same.
Three designs, three procedures:
RENIX, 1987–1990: The sensor is a "sync pulse generator" (a stator) buried inside the distributor. Replacing it means pulling the distributor and re-establishing sync on reinstall. This is the most involved version.
HO with distributor, 1991–1999: The CMP sensor sits inside the distributor housing under the cap and rotor. Mopar 56026882 is the OE sensor on 1996+. You either reach the sensor with the distributor in place or pull the distributor — and if you pull it, you have to put it back in the same clock position or re-time it.
HO distributorless, 2000–2001: The 4.0L went to a coil rail with no distributor. The CMP sensor bolts to the top of the oil-pump-drive (camshaft synchronizer) assembly near the oil filter on the passenger side. Sensor-only replacement here is two bolts and genuinely quick.
Check the synchronizer shaft for wobble first (2000–2001). Before installing a new sensor, grab the shaft the sensor mounts to and check for play. A worn synchronizer with any wobble will chew up a brand-new sensor in weeks. If it moves, the whole oil-pump-drive assembly needs to go in, not the sensor alone.
Buy the Mopar sensor. Cheap aftermarket CMP sensors on the 4.0L have a documented habit of being dead in the box or failing within a month. This sensor lives in heat and vibration — the genuine part is worth the few extra dollars.
Confirm the symptom matches the sensor. P0340 with P1391, crank-no-start that worsens with heat, or stumble/buck/stall over bumps and into reverse all point at CMP. A cold engine that starts fine and dies hot is the signature.
Disconnect the negative battery terminal and set it clear of the post before touching anything.
Optional but worthwhile: test the old sensor with an analog multimeter. Back-probe the sensor output and ground at the connector, key on, and watch for roughly 5 volts of supply. Crank by the key and the needle should sweep 0–5 volts as the pulse ring passes the pickup. A flatline confirms the sensor. A digital meter usually can't show this — the needle bounce is what you're after.
2000–2001 — sensor on the oil pump drive (the quick one)
Find the sensor on the passenger side of the engine near the oil filter, sitting on top of the oil-pump-drive assembly. Disconnect its electrical connector.
Grab the synchronizer shaft and check for wobble. Any play means the oil-pump-drive assembly is worn — stop and plan to replace the whole assembly, not the sensor alone, or the new one fails fast.
Remove the two sensor mounting bolts and lift the sensor off the drive.
Set the new sensor onto the drive so it faces the rear of the engine, thread the two bolts by hand, then torque to 15 in-lb (2 N·m). This is a deliberately light torque — these are small bolts into an aluminum casting.
Apply dielectric grease inside the connector and plug it back in. Reconnect the battery. Done — you have not disturbed sync.
2000–2001 — if the oil pump drive must come out
Only pull the drive if the synchronizer is worn. Before removing it, do not let the engine rotate or you lose sync. Mark the rotational position of the drive relative to the block with a paint pen.
Remove the hold-down clamp bolt and clamp, then pull the assembly straight up, noting which way the pulse ring and the slot in the oil pump gear point as it comes out.
If the crank did not move, drop the new drive back in matching your marks, finger-tighten the clamp bolt, install the sensor (15 in-lb), reconnect, and verify sync.
If the engine was rotated, re-establish sync: pull the coil rail and the #1 spark plug, rotate the engine by the crank damper bolt until you feel compression at #1, then continue to TDC (0°) on the timing scale. Rotate only in the normal direction — never backward. Use a toothpick through the housing's access hole to index the pulse ring, then seat the drive so it drops flush (if it won't seat, the oil pump slot isn't aligned — lift and realign).
Verify with a capable scan tool's SET SYNC routine — the reading should land IN RANGE at 0. Most DIYers don't have a DRB; a modern bidirectional scan tool with the cam/crank sync relearn for this platform does the same job. Then torque the hold-down clamp bolt to 17 ft-lb (23 N·m). The SET SYNC number is fuel sync only and has nothing to do with ignition timing, which the PCM controls.
1991–1999 — sensor inside the distributor
Mark the distributor before you touch it. Paint a line across the rotor tip onto the distributor body and a second line from the distributor body to the block. These marks let the distributor go back in the exact clock position and save you a re-time.
Remove the distributor cap and rotor. On many of these you can reach and lift out the CMP pickup with the distributor still in the engine — do that if you can, since nothing moves and sync is preserved.
If the sensor isn't serviceable in place, pull the distributor: remove the hold-down clamp bolt, then lift the distributor straight up, noting that the rotor turns slightly as the helical gear disengages. Do not crank or bump the engine while it's out. Swap the sensor (or the whole distributor — many owners replace the whole assembly, since a fresh Mopar distributor sidesteps the fiddly internal pickup).
Reinstall by dropping the distributor in so the rotor returns to your reference mark; the gear will pull it the last few degrees into your line. Install the clamp and torque the hold-down bolt to 17 ft-lb (23 N·m).
If the engine was rotated with the distributor out, set #1 to TDC on the compression stroke and drop the distributor so the rotor points at the #1 cap terminal, then confirm. An improperly indexed distributor surges and light-bucks around 2,000 rpm under light load at temperature — that's your tell that the clocking is off.
All years — finish and verify
Apply dielectric grease to the connector and seat it fully. Reconnect the battery.
Start the engine and let it idle. Clear stored codes with a scanner. On OBD-II XJs the codes to watch are P0340 and P1391.
Verify under heat and load. The failure mode shows up hot — drive to full operating temperature and run it 20+ minutes, including a few right-hand turns and a shift into reverse, the conditions that exposed the original fault. A clean hot drive confirms the repair; a cold start tells you nothing.
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Why it works
Cures one of the most maddening intermittent faults on the 4.0L — the hot crank-no-start and the over-bumps stumble both trace back here
On 2000–2001 it's a two-bolt, sub-hour job with no sync to disturb if you replace the sensor only
Genuine Mopar parts are inexpensive, and catching a worn synchronizer before it kills the new sensor saves a second round
Trade-offs
The job is not the same across model years — getting the procedure wrong for your XJ wastes an afternoon
Anything that disturbs the distributor (1991–1999) or the oil pump drive (2000–2001) brings sync and re-clocking into play, and verifying fuel sync properly wants a capable scan tool
Cheap aftermarket sensors fail early and a worn synchronizer destroys a good one — skipping the wobble check means doing this twice
Tools required
Socket set with 13mm and 1/4" drive
Torque wrench reading in-lb (the sensor bolts are a light torque)
Torque wrench reading ft-lb (for the distributor or oil-pump-drive clamp)
Distributor wrench or stubby ratchet (1991–1999, helpful for the hold-down)
Analog multimeter (the bouncing-needle test the digital meters struggle with)
Wooden toothpick (2000–2001 pulse-ring alignment)
Dielectric grease
Paint pen or marker
Parts
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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.