P0878: Transmission Fluid Pressure Sensor/Switch D Circuit High

What does P0878 mean?

Transmission fluid pressure switches C and D report whether a clutch circuit is pressurised. They are more widespread than A and B: GM's 6L80/6L90 and 6T40/6T70 six-speeds put switch C on the 2-6 brake and D on the 1-2-3-4 clutch, Allison uses them for its third shift valve and its reverse switch, Ram's RFE gearboxes for the clutch pressure switches in the solenoid block. Ford reports pressure faults under its solenoid codes. P0878 means switch D reports no pressure although pressure is commanded. GM checks the 1-2-3-4 clutch above 700 kPa and sets it after 6 counts on the 6L80 or 8 on the 6T70, without MIL. Allison 2007 sets it when the reverse switch stays open in R or fails its check after engine stop, type A.

The code tells you which monitor failed, not which part is broken. Test before replacing parts.

Symptoms

• Code stored in the gearbox module; on GM without MIL, on Allison with MIL
• Slip or shift shock in gears 1 to 4 (GM) or in reverse (Allison)

Common causes

• High Signal wire or connector open (gearbox pass-through, harness)
• Medium Pressure switch does not close (diaphragm, seal)
• Medium Clutch pressure does not build: low fluid, leak, regulator valve
• Low Gearbox module input

Likelihood is a general estimate across vehicles; your make and model may differ.