Rear Light Gremlins

[ElReyes]

Hi all.

Weird one. Mr left rear brake light doesn't work - bulb is okay.

When I press the pedal, the rear fog light lamp on the instrument cluster illuminates very faintly...???

Also, my instrument cluster backlight (speedo and tacho) is generally very faint despite knob on full and bulbs seemingly okay?

Brake light is the main issue.

[scottybeemer]

sounds like earth fault somewhere

[ElReyes]

Eurgh. Tracing that will be fun.

Speedo is dead - diff sensor could be causing both issues?

[Brianmoooore]

It's an earth fault, and will most likely be in the bulb holder assembly itself. BMW used at least three different designs of holder assembly, and the one that looks the 'best' is the one that has these sort of problems. The version that has the wiring elements in the form of round steel bars, all exposed, is the most reliable, in spite of looking like something 'aftermarket'.
If it isn't that, the earthing point for the rear lights and speedo sensor is at the back of the seat pan, LH side, but I doubt if the issues are related.
Cluster bulbs mat have just blackened with age, unless, like the cluster that came on my neon ragtop, someone has actually removed the plastic light guides.

[ElReyes]

Many thanks. Will have a fiddle.

If it isn't that, the earthing point for the rear lights and speedo sensor is at the back of the seat pan, LH side.

Seat pan?

[ElReyes]

[ElReyes]

So I checked out the wiring bundle under the seat pan in the LH corner and it all looks in order. I should add, it only happens when driving. If I switch all light variations with engine off / IGN on its seems to work fine.

Can that help diagnose?

What should I be looking for / testing in that light cluster / earth point at the seat base?

Thanks in advance.

[ElReyes]

Bump.

[Brianmoooore]

It's an earth fault, and will most likely be in the bulb holder assembly itself.

Just try a replacement.