Powertrain, generic (extended)

P28ED Electric/Auxiliary Transmission Fluid Pump B Overspeed

Check engine lightOn
EmissionsRelevant
Drive cycleNot required
Limp modeNo

Pump B is a second electrically driven transmission fluid pump that works independently of the engine, for example to hold pressure during start-stop or electric driving, or to feed clutch cooling. P28ED says its motor runs faster than commanded or faster than its permitted maximum. That typically happens when the pump loses its load because it draws air, at low fluid level or with a blocked suction filter, or when the speed estimate of a sensorless motor goes wrong. No manufacturer documents P28ED publicly. Analogous checks exist for single pumps: GM logs P179A on the 2016 Cadillac ELR when the pump motor exceeds its maximum speed of 6500 rpm, Type A, and Ford's 2019 plug-in hybrids flag a speed above 105 percent of command for 3 s under P0B0D. CARB requires functional checks.

Likely causes

Low fluid level or air drawn in, so the pump runs without load
Blocked suction filter or inlet
Faulty speed feedback or control in the pump controller

Symptoms

  • Transmission warning or check engine light on
  • Start-stop or electric driving disabled
  • Whining from the pump area

Repair estimate

DifficultyMediumWorkshop job
Parts and labour€50–1,500
Time0.5–4 h

Typical range across vehicles; the real cost depends on what the testing finds.

Parts involved

aux transmission pump btransmission fluid

Related codes

P28EEP277CP27A3P276D

References: gsi.ext.gm.com, fordservicecontent.com, law.cornell.edu.