P0320: How to Diagnose & Fix — Ignition/Distributor Engine Speed Input Circuit Malfunction

The distributor-mounted Thick Film Ignition (TFI) module is a chronic weak point on these Broncos. Because the module is bolted to the distributor and baked by engine heat, its internal electronics break down over time, causing the engine to miss, stumble, and stall once fully warmed up, then restart fine after cooling for 20-30 minutes. The module tends to fail once its temperature exceeds ~257F. This exact defect was the subject of a class-action lawsuit (Howard v. Ford) that Ford settled, covering an estimated 22 million Ford vehicles built 1983-1995. It is one of the single most common driveability complaints on the platform.

The crankshaft position sensor (CKP) on the 4.7L (and 4.0L) Grand Cherokee commonly fails from heat-soak: it works when cold but its internal resistance rises when hot, so the PCM loses the crank signal and the engine won't restart until it cools 30-60 minutes, or it stalls randomly while driving. A cranks-but-no-start after a hot soak is the classic signature and a top WJ search.

The 3.9/5.2/5.9 Magnum uses a camshaft position sensor inside the distributor plus a crankshaft position sensor on the bell housing. These sensors are heat-sensitive and fail intermittently — the engine stalls or dies when hot and refuses to restart until it cools, or cranks-but-won't-start with no other symptom. Corroded/loose sensor connectors cause the same behavior.

The crankshaft position sensor on the 4.0L I6 and 2.5L I4 XJ is a classic thermal-failure item. Internal components and wiring degrade with age and heat, so the sensor works cold but drops signal once the engine reaches operating temperature. Without a CPS signal the PCM cuts spark and fuel, producing a stall or a crank-but-no-start that mysteriously 'fixes itself' after the engine cools. It is one of the most frequently discussed XJ failures in enthusiast communities and typically strikes around 125,000-175,000 miles.

First-generation Proteges (and 1.5L/1.6L second-gen cars) use a camshaft-driven distributor that houses the ignition pickup. With age these distributors suffer internal bearing wear and moisture intrusion past the seal, which corrodes the pickup and shaft. The result is a car that runs fine cold but stalls once hot and refuses to restart until it cools, or a sudden loss of spark and a crank-no-start. Documented cases include a 1990 Protege that ran ~10 minutes then abruptly stalled until cold (fixed by a dealer distributor replacement) and a 1.8L DOHC no-start where new coil, wires, cap and rotor did not cure it until the distributor was replaced.