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'02 Hazard Circuit Primer

Written by Chris Blumenthal Friday, 30 September 2005 '02 Hazard Circuit Primer

By Chris Blumenthal

The hazard switch circuit is probably the most complicated circuit in the 02. Here is how it works, in a nutshell:

The Relay

The flasher relay contains two solenoids/flasher circuits. One drives the turn signal and hazard flasher bulbs, and one drives the flasher indicator bulb in the instrument panel. The flasher relay has four terminals:

31, brown wire ground

49a green/yellow wire load (to turn signal bulbs)

49 green/violet wire power (provides +12V to relay, and ultimately to the lights)

C black/white wire blinker light in instrument panel.

(Please note, this is based on a '73, but should provide general guidance on all 02 hazard switch problems)

All of these wires except for terminal C run to the hazard flasher switch, where there are corresponding terminals (marked the same). The flasher relay is powered (on terminal 49) from the hazard flasher switch (+12V originates in a switched circuit from the ignition switch through the fuse block).

Operation

There are two scenarios of operation for the flashers:

1. Turn signals. Ignoring the wiper function, the turn signal switch is not the source of power for the turn signal circuit, and acts only to complete the circuit from the flasher relay to the turn signal bulbs to the flasher relay. The switch completes the circuit and provides the load from the bulbs/ground to the relay (on terminal 49a). In summary, +12V flows through the hazard flasher switch to the relay; when you toggle the switch left or right, power flows through the switch down the respective right or left turn signal circuit to ground. It is the presentation of the load at the relay (by toggling the switch left or right) that makes the relay function (opening and closing intermittently, causing the turn signal bulbs to flash).

2. Hazard flashers. In this scenario, when you push (or pull, as the case may be) the hazard flasher switch, essentially the same thing happens as described above, except that the hazard switch routes the load from both sides of the turn signal circuit at the same time, thereby causing all four corners to flash.

Conclusion

The bottom line is that the flasher relay is the crux of this whole system and obviously neither the hazard flashers or turn signals will work without a functioning relay. However, there are other reasons why either the hazard flashers or turn signals might not work. This is intended only to give an overview of the system and how it works. Hope this helps and good luck!

If you have any questions, feel free to post them to the Message Board!

COPYRIGHT 2002, BMW 2002 FAQ, Rob Shisler and Steve Kupper. All Rights Reserved.

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