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E46 M3 SMG cog light error code 55 (0x37)

This is a review of common Code on E46 SMG pump that are due to failure of pump to provide correct system pressure.

55 (0x37) Switching-on frequency of hydraulic unit

The smg pump is typically turned on to build/prime system to a certain pressure window. You will notice this most often when vehicle is has been parked for a while and you open door for first time, at that moment you will typically hear the smg pump turn on and prime system. Most of this fluid pressure is stored for later use in accumulator.  The accumulator will supply fluid when needed for multiple shifting and clutch operations before electric motor/pump needs to be re-activated. This error code will present itself when DME calculates that it is having to turn on smg pump to frequently over a short period of time just to meet minimum pressure requirements.

There are a number of factors that can lead to this, most common problem in order of failure.

1-Electric motor is failing, during this time motor will provide low torque and not perform fast enough to build pressure in allotted time. It will eventually lead to complete motor failure.

2-High power relay(salmon relay) is damaged. This normally is related to item (1) where the additional load is causing high current to wear out relay.

Over time the relay will become unreliable and not follow DME instructions by either staying in an ON or OFF position. this could trigger two specific codes based on bellow.

  • When stuck in ON position it will cause smg pump to keep running continuously and eventually overheat and burn out SMG pump motor.  (this could also Triger 54(0x36) error code to be stored)
  • When stuck in OFF position it will cause smg pump to not prime to build up required pressure.  (This could Triger 53 (0x35) error code to be stored)

3-SMG fluid level is low causing air to get to pump and not allowing any pressure build.  If you have this problem recommend you bleed system to remove residual air in hydraulic system.

4-External Leak in hydraulic system causing loss of fluid leading to low pressure. Check for leaks. common area of smg slave cylinder or actuator leak will allow fluid level to get lower than minimum. When this happens the smg pump will starve not be able to build pressure.

5-Defective pressure sensor will cause an incorrect system pressure reading to DME and cause pump to be turned on more than needed. The actual system pressure will be higher than stated to DME but pump will be triggered to stay on longer as the feedback reading to DME will state low.  (This could trigger a number of other codes 53(0x37), 53 (0x38) and 53 (0x39))

6-Pressure accumulator not within speciation. When accumulator is not in spec. it will not store enough reserve pressure causing pump to activate more frequently to allow you to initiate a clutch/shift request.(Run specific test for accumulator to determine health)

7-Wiring harness damage or fuse damage, this could occur if relay is stuck ON overheating wire harness and damaging insulation.

8-Internal leak in hydraulic system, worn out parts in smg system will allow pressurized fluid to escape back into reservoir. some areas are

  • Solenoid valve seals damaged allowing pressure to leak
  • Pressure relieve valve worn and releasing pressure too early or leaking.
  • Solenoid Valve damaged, sealing surface damaged and allowing leakage.