DRL · daytime running lights · 5-pin relay

DRL wiring diagram: daytime running lights with a relay

Daytime running lights should be on whenever the car is driven and go out when the headlights come on. One 5-pin relay does both: the DRLs hang on its normally closed contact, and the headlight switch pulls it in to turn them off.

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The circuit

Battery + → 5 A fuse → ignition-switched + (or D+ for “engine running only”) → relay 30.
Relay 87a (normally closed) → both DRLs; DRL − → ground.
Relay coil 86 → the low-beam feed; 85 → ground. Headlights on → relay pulls in → 87a opens → DRLs off.
Two LED DRLs at about 8 W each draw 1.3 A: 0.5 mm² wire and a 5 A fuse are plenty.

Rules and choices

UNECE Regulation 48, used in Europe and Ukraine, requires DRLs to switch off automatically when the headlamps are on.
Feeding from D+ instead of ignition keeps them off while the engine is stopped with the ignition on.
Many DRL kits have this logic in a small module; the wiring to it is the same: +, ignition signal, headlight signal, ground.

See it switch in the simulator

Run the live demo: ignition-switched DRLs through the 87a contact of a 5-pin relay, and the low beams on their own switch.
Switch the ignition on: the DRLs light.
Switch the headlights on: the relay pulls in, the DRLs go out and the low beams light.

Questions

Can DRLs be dimmed instead of switched off?

Some kits dim them to work as position lights with the headlights. That needs a module; a plain relay only switches them on or off.

Can I use the fog lights as DRLs?

Not legally in most countries: fog lights are not approved as daytime running lights.

Why 87a and not 87?

87a is closed while the coil is off, so the DRLs are on by default and the headlights switch them off. With 87 you would need the opposite signal.

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