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The Lucas db10 Relay

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The Lucas DB10 Relay

The Lucas DB10 relay was fitted to a considerable number of British cars in the 50s through to the 70s, and it’s there to allow the brake lights to function also as turn signal lights.

When I was rebuilding my Lotus Europa I found that mine wasn’t working, so I took it apart to see if I could fix it. I couldn’t, so looked for a new one – they were over $90. For a simple relay function that seemed ridiculous.

I had meanwhile bought a bunch of Bosch-type relays so that I could fit relays to Head Lamps, the fuel pump I was intending to install, and a few other places that Lotus overlooked. The Bosch relay is about the best known automotive relay there is, and there are several descriptions to be found on the Internet, a very good one being at . It’s also cheap – you can buy today (February 2011) a pack of 5 for under $10. I quickly figured out that 4 of these relays, suitably wired and mounted, would do exactly the same job as the DB10 and for a lot less money.

So I built the device shown below.

It has 4 relays, mounted with a single nut and bolt to a short length of angle, which serves as a common point to secure the ground wires and provide a hole through which another bolt secures the assembly in the same place where the DB10 fitted originally. To give an idea of size, the rectangular shape formed by the 4 relays and their connectors is roughly 5” x 3”.

  • Each relay serves one light; front indicator or rear brake / indicator.

  • The 2 White wires on the left are connected together, as are the 2 on the right. They are connected to the wires coming from the left and right outputs of the Turn indicator switch.

  • The 4 Black wires are connected to Ground.

  • The 4 Yellow wires are all connected together and to the output from the Turn and Hazard flasher units.

  • The 2 Red wires are connected together and to the wire coming from the Brake Switch. (Note there are no Red wires on the relays for the front lights.)

  • The 4 Blue wires are connected to their respective front indicator or rear brake / indicator.

Here’s another diagram to explain it all:

Connection Summary

Wire Colour and Relay

DB10 connection number

Connected To

Relay Terminal Number

Yellow (all)


Flasher Unit

87 (all 4)

Blue (Right Front)


Right Front indicator


Blue (Right Rear)


Right Brake / Indicator


White (both Right)


Indicator Switch, Right


Red (both Rear)


Brake Switch

87a, rear relays only

Blue (Left Front)


Left Front Indicator


Blue (Left Rear)


Left Rear Brake/ Indicator


White (both Left)


Indicator Switch, Left


Black (all)

Not Numbered



How does it work?

In the static condition with no current applied, Red 87a, Brake Switch is connected to both Blue 30, brake / indicator terminals. So when the Brake Switch is activated, current flows to both rear brake / indicator lights.

The relays are operated by power being switched to White 85, so when the Turn indicator switch is operated, then on the selected side only, the Red 87a connection to Blue 30 is broken, and both front and rear Yellow 87 to Blue 30 connections are made., completing the circuit through the Turn flasher unit, and the appropriate front and rear lights flash.
The operation of the Hazard Switch bypasses the Turn flasher unit, connects Hazard flasher unit instead, and connects both the left and right outputs from the Turn indicator switch. All four relays are activated, the Hazard flashing circuit is complete, and all four lights flash.

Hazard warning – Europa TC

Some may wonder about the warning in the Europa Twin Cam Owner Guide that tells you not to operate the Turn indicator switch when the Hazard system is being used. This is because Lotus, or maybe Lucas, or someone anyway, so arranged things that the Ignition is turned On if you do. Very strange, and easily overcome by a couple of diodes, one in each of the wires coming from the Turn Indicator switch before the point where they connect to the Hazard switch. These will prevent current from flowing “backwards” into the ignition circuit and maybe burning out your coil or wrecking your electronic ignition!

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