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Rush SR Service Manual
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    Gear Control Unit (GCU)

    The Gear Control Unit sits between your paddles and the gearbox. It reads the paddles, fires the air valves that turn the shift drum, keeps the compressor topped up, and — on current cars — watches the engine and refuses a shift that would over-rev it.

    Every Rush SR has a GCU. Which one you have depends on when the car was built, and the two are different enough that most procedures only apply to one of them. Start here.

    Which GCU Is in My Car?

    Section titled “Which GCU Is in My Car?”

    Open the rear clamshell and look at the box. The GCU Gen 3 is built on a far smaller board than the Legacy one, so once you know what to look for the two are hard to confuse.

    GCU Gen 3Legacy GCU
    ConnectorsOne, 23-pinTwo
    Status lightsOne multicolor LEDSeparate red, yellow, and green LEDs
    USBUSB-C, on a piggyback breakout in the harnessUSB-B on the box, the square printer-cable type
    Firmware files.bin.hex
    Firmware version5.x3.x

    The surest check is the version. Connect the GCU Flasher and it prints the firmware version as soon as it connects.

    What Changed Between Them

    Section titled “What Changed Between Them”

    The short version: the Legacy GCU opens a valve on a timer and assumes the gear went in. The GCU Gen 3 reads the gear sensor, confirms the gear actually went in, and refuses the shift that would hurt the motor.

    GCU Gen 3Legacy GCU
    Fitted toCurrent buildsEarlier builds, both Gen1 and Gen2
    Processor32-bit, 120 MHz, 192 KB RAM8-bit, 16 MHz, 8 KB RAM
    ECU linkNative CAN — reads RPM, gear, and throttle liveNone — the shift signal to the ECU is one-way
    Shift confirmationClosed loop off the gear sensor: confirms in under 10 ms and retries a missed shift on its ownOpen loop, timed
    Money-shift protectionWorks out the RPM the downshift would land on and refuses itNone
    Logging2 MB onboard, one file per session, GPS wall-clock timestamps, crash and reset recordsNone
    Settings40 live settings through the flasher, changeable any time2 stored values, and only before the first shift of a power cycle
    Only on this unitAuto-downshift, brake-light control, wheel-speed emulation, dash telemetry, watchdog and safe mode, car/moto neutral modes, progressive neutral finder—

    Where to Go Next

    Section titled “Where to Go Next”