Cerbera lights ECU — schematic reference

This page documents a community-designed lights ECU for the TVR Cerbera. It is a small relay-and-diode board that sits between the car’s original lighting loom and a replacement engine ECU, taking over the lighting switching that the factory ECU used to handle. The design is entirely passive — no microcontroller, no firmware — so it can be diagnosed with a multimeter and repaired with off-the-shelf parts.

The notes below are compiled from a KiCad schematic analysis (lights_ecu.kicad_sch). Treat them as a reference for understanding or rebuilding such a board, not as factory TVR documentation.

What the board does

It handles five switched lighting functions on the Cerbera:

• Side lamps
• Head lamps (with sidelamp follow-on)
• Dip beam
• Front/rear fog lamps and fog warning light
• Main beam (independent LH and RH feeds), gated by ignition

All switching is done with automotive relays. Diodes are used both for coil flyback protection and to implement wired-OR logic so that more than one input signal can energise the same relay coil without back-feeding the other input.

Component summary

Ref	Part	Type	Role
J1	TVR 17-way connector	—	Interface to car loom
K1	Finder 40.52	DPDT 10 A	Main beam (LH + RH together)
K2	Finder 40.52	DPDT 10 A	Head lamp / dip beam logic
K3	Finder 40.52	DPDT 10 A	Side lamps
K4	Omron G2RL-1	SPDT 12 A	Fog lamps + warning light
K5	Omron G2RL-1	SPDT 12 A	Dip beam output
D1–D8	1N4001	DO-41	Flyback + OR-gate steering

Total: 14 components.

Relay pinouts

Relay	Coil +	Coil −	Pole 1 COM / NC / NO	Pole 2 COM / NC / NO
Finder 40.52 (DPDT)	A1	A2	11 / 12 / 14	21 / 22 / 24
Omron G2RL-1 (SPDT)	A1	A2	11 / 12 / 14	—

Typical coil currents at 12 V are around 33–37 mA per relay (≈ 320–360 Ω coils), so with all five energised the total coil draw is roughly 175 mA — comfortably within signal-wire capability.

J1 connector — 17-way loom interface

The connector has an A-side and a B-side. Position A5 is intentionally absent (gap in the TVR connector), which is why the A-side runs A1–A4 then A6–A9.

A-side

Pin	Direction	Function
A1	Output	Rear fog lamps
A2	Output	Fog lamp warning light
A3	Output	Sidelamps feed
A4	Input	Sidelamp signal (from switch)
A6	Input	Dip beam signal
A7	Input	Battery feed — dip beam
A8	Input	Ignition-switched 12 V
A9	Input	Battery feed — main beam

B-side

Pin	Direction	Function
B1	Input	Battery feed — sidelamps
B2	Input	Fog lamp signal (from switch)
B3	Input	Head lamp signal (from switch/stalk)
B4	—	Spare / unassigned
B5	Output	Dip beam drive
B6	Output	Main beam (related output)
B7	—	Spare / unassigned
B8	Output	LH main beam
B9	Output	RH main beam

Power architecture

Three independent 12 V battery feeds come in from the loom, one per lamp group, so a fault in one circuit cannot pull down the others and the original in-loom fusing is preserved.

Supply	Pin	Feeds
Battery (Side)	B1	K3 / K4 contacts
Battery (Dip)	A7	K5 contacts
Battery (Main)	A9	K1 contacts
Ignition	A8	Main beam enable gate only (low current)

A single ground bus runs along the board, joining all coil returns to the car’s chassis ground.

Circuit blocks

Side lamps

The sidelamp signal from the loom (A4) energises K3 via a steering diode. K3’s contacts pass Battery (Side) through to A3. Because K3 is a DPDT, both poles can be used — for example to feed front and rear sidelamps separately, or to drive a tell-tale.

The headlamp signal also feeds into the same coil through its own diode, so turning on the headlamps automatically lights the sidelamps as required for UK road use.

Head lamp / dip beam

The headlamp signal (B3) is OR-gated with the dip beam signal (A6) via diodes onto the coil of K5 (and K2). K5 then switches Battery (Dip) (A7) onto the dip beam drive output at B5. D6 is K5’s flyback diode; D8 isolates the two driving signals from each other.

Fog lamps

The fog lamp signal (B2) drives K4’s coil directly. K4’s contacts feed the rear fog output (A1) and the dashboard fog warning light (A2). D5 is the coil flyback diode.

Main beam

Main beam uses the DPDT K1 so a single coil switches both LH (B8) and RH (B9) outputs at once from the Battery (Main) feed (A9).

Importantly, K1’s coil is gated by ignition: the Main Beam Enable net is derived from the ignition input at A8, so main beam cannot be energised with the ignition off. This avoids accidental main beam on a parked car and keeps behaviour consistent with MOT expectations.

Diode OR-gating pattern

A common pattern on this board is two signal nets driving one relay coil through separate series diodes. Each diode’s cathode joins at the coil’s positive terminal; the coil’s other end goes to ground. Whichever input goes high pulls the coil up; the diode in the other branch blocks back-feed into the inactive switch line.

This is what lets, for example, both the headlamp switch and the dip beam signal independently bring the dip beam on, without either signal appearing on the other’s wire.

Notes for anyone rebuilding this

• The 1N4001 is fine here because the diodes only ever steer relay coil current (tens of mA). Don’t repurpose them to switch lamp current.
• The mylibrary:TVR_17-Way symbol in the source schematic has no footprint assigned — a matching PCB footprint for the chosen Cerbera connector must be added before laying out a board.
• The Finder 40.52 and Omron G2RL-1 are both common, currently-produced parts, which is part of the point — the board is meant to be serviceable.
• Always cross-check pin assignments against your own car’s loom before powering anything up; Cerbera looms vary across model years.

Safety: Lighting is a safety-critical and MOT-tested system. Fuse every supply feed at the value used in the original loom, verify polarity of every relay coil and diode before applying power, and confirm with a meter that main beam genuinely cannot energise with the ignition off.

Compiled from a community schematic analysis of a Cerbera lights ECU — verify against your own car and the original schematic before relying on it.