P0988: the Jeep fourth-clutch pressure switch reports the wrong state

The 2002 Jeep WJ/WG 45RFE/545RFE manual defines P0988 as a 4C pressure-switch sense-circuit fault. While the engine is running, the controller compares the switch state with the current gear. The manual gives a 4C switch turning on in second gear as an example of a state that should not occur.

State plausibility differs from an active pressure test

P0988 evaluates whether the reported fourth-clutch state fits normal operation. The neighboring P0987 procedure checks response to a deliberate hydraulic test. Keeping those two checks separate helps explain why a circuit may respond during one test yet still produce an implausible state in service.

The factory chart gives transmission-relay codes priority. Missing or unexpected shared power can corrupt the interpretation of a pressure-switch input. It then uses a dedicated simulator to separate the external 4C sense circuit from the transmission assembly, followed by checks for an open, a ground short or unwanted voltage.

The intermittent branch contains mechanical clues as well: an incorrectly installed sump filter and a dislodged reverse-carrier snap ring. These are documented possibilities, not reasons to dismantle a transmission without supporting evidence. A fault that appears under acceleration needs its original operating context preserved rather than deliberately reproduced by an owner.

Verification

A workshop should demonstrate that the 4C indication agrees with the expected gear states after the identified cause is corrected, and complete the manufacturer's transmission verification. This account is limited to the cited 2002 Jeep system; the generic sensor/switch E wording does not establish the same clutch assignment on other transmissions.

Source: DaimlerChrysler 2002 WJ/WG transmission diagnostics, physical PDF pages 168–171. The original PDF is also available as WG/02wget.pdf in the factory-manual archive.