What size fuse is used for the fog light power circuit?

A 20 amp inline fuseholder is shown between the battery positive terminal and the relay.

Which relay terminals are used and how are they connected?

A Bosch relay is used: terminal 30 gets fused battery power, terminal 87 feeds the fog/driving lights, terminal 86 goes to ground, and terminal 85 connects to the switched control circuit from the switch. Terminal 87A is shown but not connected.

What kind of switch does the diagram call for?

An SPST on/off switch, with On and Off positions labeled.

Where does the switch circuit get its power?

From the fusebox, labeled 'IGN or Battery Power'.

How is the indicator light wired?

The indicator is fed from the switched side of the circuit (with the switch/relay control wiring) and has its own ground connection, so it lights when the fog lights are switched on.

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Fog/Driving Light Relay Wiring Diagram - Bosch Relay, 20A Fuse, SPST Switch

Fog and driving light wiring diagram with Bosch relay, switch, fuse, and battery

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Wiring diagram for fog/driving lights using a Bosch relay, SPST switch, 20A fuseholder, indicator, fusebox, and battery.

AI caption October 2007 · Photo 52 of 181

This document is a color-coded schematic wiring diagram for adding aftermarket fog or driving lights to a vehicle using a Bosch-style relay. It shows two fog/driving lamps, each independently grounded, fed from relay terminal 87. The relay's terminal 30 receives power from the battery positive terminal through an inline 20-amp fuseholder. Relay terminal 86 is grounded, while terminal 85 is connected to the switched control circuit. The control side runs from the vehicle's fusebox (labeled 'IGN or Battery Power') through an SPST on/off toggle switch, which also feeds a dash indicator lamp (grounded separately) so the driver can see when the lights are on. Terminal 87A of the relay is shown but unused. Power wiring is drawn in red and ground wiring in gray, making the circuit easy to follow. The diagram is credited to Craig Ueltzen, 1999. A reader can learn the correct relay pinout (30, 85, 86, 87, 87A), proper fuse rating (20 amps), where to source switched power, and how to wire an indicator light for a typical auxiliary lighting installation. It is a generic how-to diagram, not specific to any particular vehicle make, model, or year.

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Frequently asked questions

What size fuse is used for the fog light power circuit?: A 20 amp inline fuseholder is shown between the battery positive terminal and the relay.
Which relay terminals are used and how are they connected?: A Bosch relay is used: terminal 30 gets fused battery power, terminal 87 feeds the fog/driving lights, terminal 86 goes to ground, and terminal 85 connects to the switched control circuit from the switch. Terminal 87A is shown but not connected.
What kind of switch does the diagram call for?: An SPST on/off switch, with On and Off positions labeled.
Where does the switch circuit get its power?: From the fusebox, labeled 'IGN or Battery Power'.
How is the indicator light wired?: The indicator is fed from the switched side of the circuit (with the switch/relay control wiring) and has its own ground connection, so it lights when the fog lights are switched on.