BMW Speedometer & Tachometer Intermittent Operation – E23, E24, E28, E30, E32, E34
This answer is applicable for many BMWs in addition to the one listed below.
1987 325es (E30 chassis)
The tach and speedo work most of the time, but sometimes work sporadically.
Here is a write-up on the initial steps in tracking down a speedometer and/or tachometer problem.
Applies to: E23, E24, E28, E30, E32, E34 with speed sensor units mounted in the differential housing cover.
1) Speedometer – The speed sensor may be faulty. The sensor is mounted in the differential cover. Remove the harness plug and remove the sensor. Clean the sensor and measure the resistance (should be 0.0 Ohms). If there is any resistance higher than 0.0 (zero Ohms), replace the sensor. If the resistance reads correct, reinstall the sensor.
2) The wiring at the connector plug for the speed sensor (in the differential housing) may be faulty and corroded …. pull the boot back from the plug housing and inspect the wires and the pins.
3) The actual speedometer and/or tachometer head may be faulty. It may have a loose connection within it or a cracked circuit board.
4) The speedometer or tachometer head may be loose on the main instrument circuit board (do the gauges change or move if you smack the top of the dash, above the gauges?). Remove the instrument cluster and check the mounting of the speedometer/tachometer to the circuit board. See the attached issues of our Fast Times newsletter for assistance in removing the instrument cluster, on E30 3-series models (see the articles on replacing the PC board and replacing the odometer gears for info on the removal of the instrument cluster).
5) The plug that carries the impulse signal into the instrument cluster may have loose contacts. The plug for the speedometer signal is located on the driver’s side of the back of the instrument cluster (on the E30 chassis). If you remove the lower dash trim panel (the one above the driver’s knees), you can reach up and touch the plug. It is in a vertical position and has up to 26 wires in it. If you wiggle the plug, while driving, and the speedo comes & goes, you have loose contacts in the plug connection.
6) You may have a problem with the service interval PC board and it’s NiCad batteries. If your oil change and service interval lights can not be reset (or will not stay reset for an appropriate time period), the PC board is likely faulty. This can affect the various gauges, including the speedometer and tachometer. If the PC board is faulty, I would suggest replacing it (after checking the other areas noted above). See the attached Fast Times newsletter article on replacing the service interval PC board.
In your specific case, I would suspect that you need to replace the service interval PC-board, if your service reminder lights can not be reset or do not stay reset.