Replace the weak Renix coil with a high-output GM HEI-style coil. Cheap, manageable, noticeable on Renix-era 4.0L starting issues.
The 1987-1990 Renix 4.0L uses a low-output ignition coil that, after 30 years, often produces a weak spark — long crank, rough cold start, occasional misfire. The fix is well documented: swap to a GM HEI-style coil (Accel, MSD, or stock GM Delco). The Renix module fires the coil through a single primary trigger, so the GM coil is a direct electrical swap with a bracket and connector change.
The gain is in driveability, not horsepower. Cold starts crisp up, idle smooths, the truck pulls without the occasional stumble. Many Renix owners report the truck feeling like a different vehicle after this swap plus new plugs/wires and a TPS/MAP calibration.
HO trucks (91+) have a stronger coil and electronic spark control via PCM — they don't need this mod. Renix-only.
Cost is $30-80 plus a wiring connector. The Cherokee Forum 'Renix tuneup' sticky is the canonical reference and includes the C-101 connector cleaning that every Renix owner should do as a free upgrade.
| Part | Vendor | Est. price |
|---|---|---|
| Accel GM HEI coil | Accel | ~$55 |
| MSD blaster GM coil | MSD | ~$75 |
| Coil mounting bracket | fabricate | ~$10 |
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.