Cerbera fly-by-wire throttle conversion (experimental)
Status — experimental. Bench rig, not in the car. No driving yet.
The Cerbera’s original throttle cable runs over the brake servo, around the steering column, and through the bulkhead at an awkward angle. It’s a notorious source of sticky throttles and is largely responsible for the “feels rubbery” complaint a lot of owners have. The plan: replace the cable half of the run with wires, a controller, and a servo — without disturbing the existing ITB linkage at all.
Architecture
Section titled “Architecture” cabin engine bay+---------------+ 6 wires +---------------------------+| Audi DBW | ─── 2 sensors + V+G ──▶ | Bosch DV-E5 throttle body || pedal | | (motor + position pot) |+---------------+ +---------------------------+ ▲ ▲ │ │ shaft → lever │ CABIN │ → Bowden cable │ +-----------------+ │ → ITB linkage └────▶ | Pico controller | PWM + DIR + /SLP │ | + LM393 cross │ ─────────────────▶ │ | check + safety │ │ +-----------------+ │ │ +───────────────+ │ └────────────│ Pololu G2 H │──┘ │ bridge driver │ +───────────────+Sometimes called “cable-output DBW” or “Bowden DBW” in motorsport. Used by some race teams when they want the side-effects of drive-by-wire (cruise, throttle map, anti-stall) without converting every ITB to its own DBW motor.
What we keep, what we change
Section titled “What we keep, what we change”| Component | Stock | This build |
|---|---|---|
| ITBs (intake throttle bodies) | Stay exactly as fitted | Untouched |
| Linkage between ITBs | Stays — keeps butterflies in sync | Untouched |
| Throttle cable from pedal | Removed | Replaced with wires + servo |
| Pedal | Mechanical lever, sprung | Donor DBW pedal with dual Hall position sensors |
| Throttle servo | n/a | Donor electronic throttle body, shaft drives a lever via short Bowden |
| Mechanical return spring on linkage | Already there | Strengthened — it’s the master failsafe |
Why a “donor DBW throttle body” rather than a custom servo
Section titled “Why a “donor DBW throttle body” rather than a custom servo”Cruise-control actuators look tempting (they already have a Bowden cable output) but they’re designed for slow, gentle adjustments to track road speed. Best-case full-sweep time is around 250 ms — feels rubbery as a throttle. A modern factory Bosch electronic throttle body (DV-E5 family) hits 80–120 ms full sweep and is purpose-built to fight a return spring and atmospheric pressure all day. It’s also rated for engine-bay heat because it lives there from the factory.
We don’t dismantle the throttle body — the motor, gearbox and position sensors are all already inside it, bearings aligned and lubricated. We just bolt a lever to the external end of the throttle shaft and run a short Bowden cable from the lever to the ITB linkage. The butterfly inside the housing carries on spinning uselessly. It doesn’t care.
Parts list
Section titled “Parts list”| # | Part | What it does |
|---|---|---|
| 1 | DBW pedal assembly (donor: Audi A4 B7 2.0 TDI, part 8E2721523J) | Dual Hall sensors give pedal position with built-in cross-check |
| 1 | Electronic throttle body (donor: Vauxhall Z18XER, Bosch DV-E5 family) | Source of motor + planetary gearbox + 2-track position pot. Shaft modified with a lever for cable output |
| 1 | Raspberry Pi Pico (RP2040) | Brain. Dual core: control loop on one, safety + USB tuning on the other |
| 1 | LM393 dual analog comparator | Hardware-only pedal cross-check. Disables the motor driver instantly if the two pedal sensors disagree — no firmware in the path |
| 1 | Pololu G2 18v17 motor driver | H-bridge. Takes the Pico’s 3.3 V logic signals, switches 12 V at up to 17 A through the throttle body’s motor |
| 1 | 12 V → 5 V automotive DC-DC | Powers Pico + LM393 + sensors |
| 1 | TVS diode | Clamps load-dump spikes on the 12 V rail |
| 1 | Strong return spring | On the ITB linkage. Default state = closed. Master failsafe |
| ~ | Pigtails, connectors, automotive wire, sealed enclosure | The mundane but important stuff |
Total bill of materials: ~£125 including the donor pedal and ETB from UK eBay breakers.
Safety architecture
Section titled “Safety architecture”The throttle is the one thing where “controller crashed” must equal “engine returns to idle.” Four independent hardware layers, none relying on the firmware being correct:
- Mechanical return spring on the ITB linkage. Default state = closed.
- Motor driver enable line held high by the LM393 + Pico. Anything pulls it low → driver goes high-impedance → motor coasts → spring wins.
- LM393 dual comparator wired directly between the two pedal sensors. If they disagree by more than a small threshold, comparator output drops the enable line. No software in the path. Cannot be crashed.
- RP2040 hardware watchdog at 100 ms. Firmware hangs → chip resets → enable line floats → spring closes throttle.
Plus a “wait politely” firmware behaviour: if the throttle is sat open
when the operator tries to arm, the firmware refuses with waiting for throttle to be closed instead of latching a fault. Has to start from a
known-good state.
Side benefits
Section titled “Side benefits”Once the controller is in the loop, these things become essentially free (software, no extra hardware):
- Cruise control — Pico already knows pedal position, can hold a steady throttle independent of the foot.
- Programmable throttle map — make the first 20 % of pedal travel softer for traffic, then linear/aggressive at the top. Configurable from a laptop.
- Anti-stall — blip the throttle if RPM drops below a threshold.
- Rev limiter — throttle won’t open past X% if RPM is above Y.
- Pit-lane limiter (if you ever take it on a track).
These are the actual reasons race teams do this on cars that originally had cable throttles.
Status
Section titled “Status”| Design | Complete (architecture, safety, BOM, pinouts) |
| Pico firmware | v0.2 running on bench rig — reads sensors, calculates PID, streams telemetry over USB |
| Tuner (laptop config tool) | Live — four needle gauges, draggable throttle map editor, calibration capture |
| Bench rig | In progress — pedal and ETB on the kitchen table, motor driver still to arrive |
| Bench-rig validation | Pending — 10-point failsafe test plan, all must pass |
| Install in car | Way off. Not until every bench failure mode demonstrably idles the throttle |
This is being built in public — the full build log, schematics, code and test plan live on the mattgreenworks build page.
Watch the build
Section titled “Watch the build”Filmed across the mattgreenworks YouTube channel as part of the Cerbera restoration arc.
Compiled from the in-progress build notes. Things will change as bench testing throws up real-world quirks. Don’t copy this design for a road car without doing your own safety analysis end-to-end.