P0870: the Jeep overdrive hydraulic self-test did not get its response

The generic description for P0870 names transmission pressure sensor/switch C. In the 2002 Jeep WJ/WG 45RFE/545RFE manual, the corresponding procedure is an overdrive-clutch hydraulic pressure test failure. That application-specific description is essential: it identifies what the controller was attempting when the fault appeared.

A commanded test, not merely a pressure reading

Shortly after a forward shift, and periodically afterward above the specified engine-speed boundary, the TCM briefly applies pressure to clutch circuits that are otherwise unpressurized. It expects the appropriate switch to close. Two missed closures set this fault. This makes the missing switch response meaningful, but does not prove whether pressure was absent or the response was lost electrically.

The factory procedure gives related line-pressure, ratio and pressure-switch faults priority. It then separates line-pressure feedback from the overdrive switch circuit using dedicated transmission simulation equipment. A stable simulated input but unreliable sensor feedback can redirect the investigation toward the sensor or its connection rather than an internal clutch.

When internal inspection becomes relevant

If the external input path responds correctly, the manual proceeds to fluid-pan contamination and internal valve-body investigation. Debris changes the repair decision; simply fitting a pressure switch would not address its cause. Conversely, an open or shorted switch wire can imitate a hydraulic failure.

This is a professional diagnostic overview, not a pressure-testing or road-test procedure. Verification must establish the OD circuit's response after the identified fault is corrected. The described strategy is limited to the cited Jeep transmission manual and should not be assumed for every P0870 application.

Source: DaimlerChrysler 2002 WJ/WG transmission diagnostics, physical PDF pages 123–128. The original PDF is also available as WG/02wget.pdf in the factory-manual archive.