It would be helpful to see the internal diagram for the fan relay, but I haven't come across that yet.

The tests I've seen done are from the socket side with the relay removed. But if the relay is removed no amount of jumpers on the socket will get it to pick up. So, tests that jumper the relay contact from the socket side are effective. And tests that put a ground on the socket to complete the lamp circuit are effective. But a test that grounds the pin of the socket that bypasses the temperature sensor would do nothing.

I need to apply voltage to the relay unit to see if its contact picks up to start the fan. But that test would have to be done with the relay in the socket. Or just applying voltage to the relay outside of the socket.

The (+) terminal of the relay appears to be A3, but for that to work the impedance of the coil and the lamp would have to be such that the relay picks up before the lamp lights. I've seen similar in older control circuits but its a bit of crap and small changes in impedance can result in a dim light when there shouldn't be any.

I'm guessing the diagram is drawn in a misleading way. It also is not clear what the (-) terminal is. If I had to guess, pin E is on the (-) side and feeds the relay through a diode, and all the outputs are isolated from each other through diodes. That could mean I have a bad diode or a bad relay.

Anyone have the actual internal wiring diagram?

Hi
had my ignition switch go bad on me too, i replaced it with a generic one that fits on Japanese quad bikes ect. it has on key can't be withdrawn and off key can be withdrawn positions- simple right 4 wires coming out of it, Red, black, green and black with white trace.

after much head scratching, wire diagram's and google, came up with this:

Green wire and black/white wires on switch- can't be used, cut back and insulate accordingly

attached red to red (battery positive)

on the bikes side:
add a diode onto the grey wire
add a diode onto the grey/blue wire

attach black to green and join the other ends of the diodes together and join them to the black wire of the switch so solid black wire from switch joins to green wire of bike and the two diodes.

stripe of diode to be on the wire side where joining diode to the bike's wires

solder the two diode other ends to the black wire with the green wire to the black wire.
insulate the diodes carefully from each other before taping the whole lot up

I did this and I have reliable switch, have the rear taillight and park light working when bike on, but for obvious reasons lost the ability to leave the park light circuit on with key withdrawn as per the OE switch- but I'm not bothered by that.

Reason for having to use diodes is to stop the back feed of power to the rear taillight and instrument cluster when switching off remaining on.

I used 10A10 MIC diodes, a little heavy duty for the job, but it works, try perhaps try an 8 amp one not as much current drop going to the park light circuit, but its miniscule and is like splitting hairs. I used these as i had a few of them left over when i did my alternator charge light back feed mod.

As part of the first round of repairs I replaced the I replaced the temperature sending unit (new) and the fan relay (used from Euromotoelectrics).

Following the guidance in the link, all the tests were good, other than grounding terminal E. That should have bypassed the TSU and started the fan, but it did not.

The temperature sensing unit possibly is kaput.
Try this test.

Good evening Motobrick'ers! I can't believe it's been almost ten years since my last post here. Ya'll were instrumental when we rebuilt the K bike, and I wanted to share a recently released article on the rebuild and an update on the boy. Hope everyone is well. Jay

https://bmwmoa.org/bmw-owners-news-november-2026/