Audi R8 Spyder transmission control unit M15705 420.927.109.B

Part Number: 420.927.109.B (420927109B)

Hardware Code: M15705

System: R-tronic (6-Speed Automated Manual)

Manufacturer: Magneti Marelli

The 420.927.109.B is the dedicated Transmission Control Unit (TCU) for the first-generation Audi R8 Spyder equipped with the R-tronic gearbox. This unit acts as the “brain” for the automated manual system, managing the hydraulic actuators that operate the clutch and gear selection. The “B” revision is specifically calibrated for the $5.2\text{L}$ V10 engine mapping, ensuring the shifting characteristics match the torque curve of the Spyder.

Technical Role & Function

• Clutch Management: Controls the hydraulic pressure to the release bearing to mimic a human driver’s clutch pedal movement, but at much higher speeds.

• Shift Actuation: Orchestrates the movement of the internal shift forks via the hydraulic power block.

• Blip-on-Downshift: Coordinates with the Engine Control Unit (ECU) to “rev-match” during aggressive downshifts in Sport mode.

• Creep Mode: Manages the partial engagement of the clutch at low speeds (though R-tronic is notorious for high wear in this mode).

Critical Diagnostic & Installation Notes

The R-tronic TCU is a sophisticated component that requires precise “marriage” to the vehicle to function correctly.

1. Component Protection & Coding:

   • This TCU is part of Audi’s anti-theft system. If you install a used unit, the car will likely start but will not engage gears (or will stay in Neutral) until Component Protection is removed via Audi ODIS (online dealer connection).

   • It must be coded to match the vehicle’s VIN and specific final drive ratios.

2. Clutch Statistics (PIS):

   • Upon installation, you must perform a “Point of Initial Slippage” (PIS) calibration. The TCU needs to “learn” exactly where the friction material of the clutch meets the flywheel.

   • Warning: Failing to perform this calibration can lead to instant clutch overheating or “judder.”

3. Hardware M15705:

   • This hardware designation refers to the Magneti Marelli CFC301 platform. Ensure your original unit matches this hardware code; while Audi part numbers sometimes supersede each other, mixing hardware generations can lead to communication errors on the CAN-bus.

4. Failure Symptoms:

   • If your R8 is stuck in “N” or displays the red “Gearbox Malfunction” icon, common TCU-related causes include internal capacitor failure or corrupted EEPROM data, often caused by jump-starting the vehicle with incorrect voltage.

Compatibility Table

Inventory Tip: This unit is located in the electronics box behind the passenger seat (LHD models). Ensure the pins are free of corrosion, as moisture in the cabin can sometimes seep into the TCU connectors in Spyder models.