Intermittent Speedometer Function
 

1984 318i -- as the title says, the speedometer only works when it feels like it. I can go weeks on end with nothing, and then one day it works like it was never broken -- and it'll continue working for a week or two -- then, for no apparent reason, off it goes again.

I've pulled and tested the sending unit from the differential -- appears to be functioning as it should. Pulled the instrument cluster out looking for anything out of the ordinary. All connections seem intact -- no obvious failures -- cleaned everything up with contact cleaner.

Anyone have any ideas as to where I should concentrate? Is there a test I can perform or common weak link in the system? Or, does anyone have a wiring diagram so I can work through? Needless to say, the problem is becoming rather annoying... This can't be that hard, but I could use some direction.

Any help would be appreciated.

Thanks!

Hello - take a look at the link below. I was able to find some wiring diagrams that may help you out. I'm also including a link to the Bentley manual for your E30 - it's an excellent resource if you work on your car.

E30 (1983 to 1992 3 Series) Wiring Diagrams

https://www.pelicanparts.com/BMW/catalog/shopcart/BOOK/POR_BOOK_bkbnBW_pg6.htm

Many thanks, Dmitry! It's something for me to sink my teeth into this rainy afternoon! Will report back once I have resolved.

The sensor for your speedometer is in the back of the differential. It can be found in the upper right side of the rear of the differential. There are two wires that connect to the sensor. I might suggest that you check that sensor and those wires. It might pay to pull and clean the sensor. At some point, you may have to trace the wires from your sensor to the dash in order to rule out a short in the harness. Let's hope it doesn't come to that.

In addition to my E30, I also own a BMW K75 that has a similar speedo sensor. The speedo on my K75 developed problems and it turned out to be in the speedo sensor in the rear drive. If you have performed any work on your rear suspension that required lowering the differential (like swapping in new rear springs), the connection to the speedo sensor can be damaged if the speedo sensor wires aren't disconnected first.