More K-Bike Speedo Failure Diagnostics

By: Robert Bini
April 2003

It was my turn lately to have the speedo on my 1990 K75 pack up at 67000 miles. I followed the advice given previously on this site by scraping the connecting pins, twisting them slightly and reassembling with petroleum jelly, but to no avail.

However, the thought of the vast sums of money necessary to have BMW sort it out induced me to persevere and a bit of patience and the use of a resistance meter solved the problem.

Here is the procedure.

1) Check the sensor in the rear transmission and the continuity of the wiring between sensor and instrument panel:
  1. Disconnect the plug that connects to the back of the instrument panel.
  2. As you look at the two rows of female pins on the plug, connect a resistance meter between pins 2 and 3 starting from the bottom of the row on the left.
  3. A reading of approximately 280 ohms will confirm that all is well.

Should there be no continuity, note that the wiring has also a connector situated next to the rear brake fluid reservoir.

2) Check continuity within the instrument panel
  1. Open the instrument panel by undoing the 9 screws.
  2. Lay the panel glass face down. The male connections to the above plug nearest to you.
  3. The matching pins are now 2 and 3 from the bottom of the row of pins on the right. They are connected by the ribbon to the small oblong printed circuit board immediately to their right. This board is the speedo signal amplifier.
  4. Note the connecting plug between the ribbon and this board, it has 4 contacts.
  5. The first two contacts on the left of this plug are directly connected by the ribbon to pins 2 and 3.
  6. Check for continuity by connecting the meter to the FEMALE contacts of the socket, just visible on the underside, which, feed the sensor signal to the amplifier chip on this board.

In my case there was no continuity between male and female contacts of the first terminal on the left. I was surprised at how much scraping, reconnecting and retesting was necessary before continuity was re-established, even though there was little evidence of oxidation.

A further connection can be traced from the fourth pin of this plug to pin 3 of the 3 pins plug right on top of the speedo itself. I take this to be the amplified output signal fed to the meter.

Clearly, the above procedure only checks the path of the un-amplified signal, but as this consists of a very small current, it is the most likely occurrence of a break in continuity.

 

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