on the early 928's they used the k-jetronic fuel system and a distributor so there is no crank speed sensor as on the late models
if you can tell me your issue I'll try to help further
if you look at this wiring digram
you'll see that the speedo gets its speed from an 'impulse generator'
this is located on the transaxle / differential carrier
however if the cruise control works, the sensor and wiring to the Central Electric Panel are OK.If both the speedometer and odometer are dead, but the cruise control works, the most likely problem is a faulty connection on the instrument panel foil or connectors.
The circuit starts at the speedometer, and a trace on the flexible printed circuit board runs from there to Terminal 1L. A brown/red wire runs from 1L to H1. H1 is the lower left connector on the 8th plug from the left end.The circuit runs behind the Central Electric Panel from H1 to T1. Count from Plug A on the left end up to T, skipping I and skipping the two short plugs in the middle. Again, 1 is the lower left terminal. There should be a brown/red wire in the terminal. The circuit runs from there to the sensor, changing color to solid brown at the sensor connector. The sensor is a simple magnetic switch, operated by a ring of magnets mounted on the diff carrier
The circuit goes thru the switch, and returns on a brown/red wire to Terminal T2 on the Central Electric Panel. T2 is next to T1. The circuit runs behind the panel from T2 to H2, and a brown wire runs from H2 to the instrument panel ground on Terminal 11R on the printed circuit board, then to Terminal 7R, and then on a brown wire to ground at MP 3.When the sender switch is momentarily made (contacts close) by the passing of a magnet on the ring, the circuit from the speedometer is completed to ground, and both the odo and the speedometer receive a pulse.You can start troubleshooting the speedometer/odo sending unit circuit by checking for voltage at H1 with the ignition switch on. If there is no voltage here, the problem is probably a faulty connection on the flexible printed circuit board. Could be at the speedometer or at the edge connector. If there is voltage here, move to H2 and check for pulsing voltage when the rear wheels turn. If there is voltage at H1, but no pulsing voltage at H2 when the wheels rotate, the problem may be a faulty connector or broken wire. Start by cleaning the connector in the spare tire well.
glad to help
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