TFP Switch Assembly

TRANSMISSION FLUID PRESSURE SWITCH ASSEMBLY






A gear range sensing device call an Transmission Fluid Pressure Switch Assembly (PSA) is used by the PCM/TCM in order to sense what gear range has been selected by the vehicle operator. The PSA is located on the valve body and consists of five pressure switches combined into one unit. The PCM/TCM applies system voltage to the PSA on three separate wires.

These three circuits are either grounded or open, depending on which gear range has been selected, and on which combination of the five switches have pressure applied to them.

When the vehicle is in PARK, with the key ON and the engine OFF, the normal state of the PSA will be Drive 2. When the key is ON and the engine is running, the normal state of the PSA will be in PARK/NEUTRAL.

There are two possible combinations of the switches within the pressure switch manifold that do not represent an actual gear range. If the PCM/TCM detects either of these combinations for more that 0.5 seconds, Code 28 will set. Code 28 will not set, however, if a valid gear range combination appears at the wrong time.

While Code 28 is present, the PCM/TCM will take the following actions:
- Assume Drive 4 for shift pattern control
- Use Drive 2 pressure table
- Inhibit 4th gear operation
- Inhibit TCC operation

If the PSA resumes normal functioning, the transmission will resume normal operation after the next ignition cycle
Other possible default codes: 68.