BMW E28 instrument cluster batteries and erratic cluster behaviour
Why do the E28 speedometer, temperature gauge and service lights act up?
The cluster circuit board has small backup batteries that leak as they age and corrode the board. As a result the solder joints oxidise or come loose. Symptoms are a dead or wobbling gauge and service lights that stay on constantly.
How do you replace the E28 instrument cluster batteries?
1. Remove the two screws at the top edge and tilt the steering wheel. 2. Disconnect the coloured connectors and pull the steering column out long, which lets the cluster come loose. 3. Desolder the batteries with a soldering iron and clean the leak residue off the circuit board. 4. Solder in 1.2 V batteries with a capacity of about 500-600 mAh. Batteries over 1000 mAh should not be used. Do not overheat the batteries, as they can burst.
What else can cause erratic instrument cluster behaviour?
The circuit board traces may be oxidised or the solder joints cold, in which case they are re-soldered. If the speedometer and odometer work only intermittently, also check the speed sensor wire just before the connector. The fault is in the temperature gauge if the needle does not move when the sensor wire is grounded.