Code P2799: Transmission Auxiliary Pump Circuit Malfunction
Code Meaning
The Transmission Control Module (TCM) has detected an anomaly in the transmission auxiliary pump control circuit. The electrical signal is outside the parameters specified by the manufacturer, indicating a problem in the transmission fluid management system.
Probable Causes
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Auxiliary Pump Failure: Mechanical or electrical failure of the pump.
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Wiring Issues:
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Open circuit or short circuit in the wiring harness
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Wire damage (chafing, corrosion, breakage)
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Connector Faults:
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Broken, bent, dislodged, or oxidized pins
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Poorly engaged or contaminated connectors
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TCM Malfunction (less frequent): Internal problem with the control module.
Common Symptoms
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🔶 Check Engine Light illuminated (or “Service Engine Soon”)
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Sluggish gear changes
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Transmission overheating
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Entry into limp mode
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Abnormal noises from the pump
Diagnostic Procedure
Preliminary Steps
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Advanced diagnostic scan:
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Read associated fault codes
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Check TCM real-time data
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Visual Inspection (Priority)
| Area to Inspect | Check Points |
|---|---|
| Wiring Harness | Damaged, pinched, or melted wires |
| Connectors |
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At the auxiliary pump
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At the TCM
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Intermediate anchor points | Oxidation, mechanical play, sealing |
| Pins | Deformation, corrosion, partial insertion |
Electrical Checks
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Continuity test:
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Between TCM connector terminals and the pump
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Resistance test:
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Compare with manufacturer values (typically 5-25 Ω)
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Short circuit check:
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To ground
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Between phases
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Voltage measurement:
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When activating the pump via diagnostic tool
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Pump Validation
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Direct power supply (12V) to test mechanical operation
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Flow measurement if possible (depending on accessibility)
Repair Protocol
| Identified Cause | Solution |
|---|---|
| Faulty Wiring | Repair or replace the harness |
| Damaged Connectors | Clean contacts or replace the connector |
| Failed Pump | Replace the auxiliary pump |
| TCM Software Issue | Update the module firmware |
| TCM Failure | Reprogram or replace the module |
Important: After repair, clear the fault codes and perform a validation drive cycle. Verify that code P2799 does not reappear.
Additional Explanations:
The auxiliary pump ensures hydraulic pressure at idle or low speeds in modern transmissions (notably dual-clutch gearboxes). Its malfunction can cause jerking, loss of oil pressure, and damage to clutches or actuators. Prompt intervention is recommended to avoid costly collateral damage.