Motronic DME/Pulse Sensor

Can anyone tell me where I might find technical information on the tachometer, Pulse sensor, DME, and on board computer as it relates to diagnosing why I am getting engine speed signal to the DME and the on board computer, but loosing it somewhere on the way to the Tachometer and the analog fuel economy gauge in the Instrument Cluster. The over the counter manuals are vague on this subject.
Re; 88 325i convertable.