I know this is probably really common electrical engineering knowledge, but I am not an electrical engineer so I am quite lost on this.

I have a 2004 BMW and the fog lights work something like this: They start off, and when I press the fog light button, it is a momentary switch that then turns them on. They then turn off when I press the switch again, or high beams come on, or the lights are turned off.

Here is a really crude diagram I found online. (Ignore rear fog lights as I have none)

enter image description here

Also then here is a link to the actual relay which also has a quite sort of diagram.

OK, now on to my question or a couple of questions. Am I right to assume that this is some sort of latching relay? Essentially, when 12 V is applied between pins 85 and 86 it energizes, closes the switch from 30 to 87, and completes its circuit. But since the fog light switch is only momentary, it essentially latches the relay between the 30 and 87 pins until 12 V is again applied between pins 85 and 86 OR power to pin 30 disappears (this would be why when you turn the headlights off or high beams on the fog lights turn off).

A next question I have is, is there a Bosch-style relay like this that is a changeover relay? To explain, instead of either switching on and off the switch from 30 to 87, switch between either 30 and 87 or 30 and 87a? I am having a hard time finding a relay that does what I am looking for. It is possible it does not exist. In that case is there a combination of relays or wiring that I could do to achieve this effect?


1 Answer 1


The alternating action is typically inside the push button switch itself, and the relay merely acts as a power switch.

Alternating relays do exist but they are not often found in automobiles.

  • \$\begingroup\$ Ok but then how in the push button switch does it reset itself without being pressed? \$\endgroup\$ Commented Apr 22, 2018 at 19:23
  • \$\begingroup\$ Your one may be a module rather than what is shown in the schematic, containing some electronics. \$\endgroup\$ Commented Apr 22, 2018 at 19:25
  • \$\begingroup\$ After some additional research I think you are correct, I found this diagram members.roadfly.com/kcroncarter/bmwetm135.jpg So answer me this, if I simply put a changeover relay in place of the current relay, and hooked up the foglights to 87a essentially they would be on from the get go, and when the switch is closed, they would turn off. Is that correct thinking? \$\endgroup\$ Commented Apr 23, 2018 at 6:11
  • \$\begingroup\$ Sounds right, if '87a' is the N.C. contact of the relay. The fuse should be retained. \$\endgroup\$ Commented Apr 23, 2018 at 11:19

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