P0891: the Jeep transmission relay output is powered too early

The 2002 WJ/WG 45RFE/545RFE diagnostic manual sets P0891 when the TCM sees more than three volts at its transmission-relay output terminals before commanding the relay on. The check occurs during ignition transitions into Run. This is unexpected electrical power at a particular time, not a finding of excessive transmission fluid pressure.

A stuck relay and a backfeed are different faults

The listed causes include a relay stuck closed, a control circuit shorted to voltage and an output circuit shorted to voltage. Each can leave the monitored output powered when it should be off. The distinction matters because changing the relay will not remove voltage entering through damaged wiring.

The factory isolation sequence checks the control and output circuits with the relay and controller disconnected as specified. Those results establish whether voltage is coming from the external harness or remains associated with the switching components. Such electrical isolation belongs to a technician using the correct diagram, rather than improvised relay bypassing.

The code's timing also separates it from P0888 in the same manual. P0888 concerns an output that remains low when the relay is requested on; P0891 concerns an output already high before that request. They are opposite command-versus-feedback situations.

What confirms completion

Record whether the fault is current and examine intermittent connections when it is not reproducible. Verification should demonstrate the expected off-to-on sequence after correcting the identified cause. The three-volt boundary and ignition-transition logic are specific to the cited Jeep transmission procedure, whose detailed steps are marked for the five-speed application.

Source: DaimlerChrysler 2002 WJ/WG transmission diagnostics, physical PDF pages 152–154. The original PDF is also available as WG/02wget.pdf in the factory-manual archive.