What does it mean?
P1784 is a Powertrain (engine, transmission, fuel system) fault code. It indicates: A/T Additional L/R Pressure Switch Circuit Malfunction .
Symptoms
- MIL (check engine lamp) is on and the transmission drops into limp mode
- Reverse is weak, delayed or not available at all
- Long delay before engagement when moving out of park
- Harsh engagement into first gear and slipping when pulling away
- Engine speed rises without the vehicle accelerating
- Symptoms are worse when the fluid is hot
Likely causes
- Low and reverse pressure switch stuck open or closed within the switch assembly
- Insufficient pressure in the low and reverse circuit, so the switch never closes
- Open or shorted wire between the switch assembly and the control module
- Corroded case connector letting fluid or moisture bridge the switch terminals
- Worn low and reverse clutch pack or a leaking seal ring bleeding off pressure
- Low fluid level, contaminated fluid or a clogged filter
- Control module input circuit fault
How to fix it
- Check the fluid level and condition and look for clutch debris in the pan or on the magnet
- Watch the pressure switch states in live data while moving the selector slowly through park, reverse, neutral and drive
- Measure resistance and voltage at the switch circuit at the case connector
- Check the wiring between the case connector and the control module for opens and shorts to ground
- Fit a pressure gauge and test line pressure in reverse and in low
- Replace the pressure switch assembly or valve body once the wiring is proven; if pressure is genuinely low, the low and reverse element needs internal repair