I recently obtained two complete XJ-S instrument cluster assemblies out of junked cars to try to rebuild a spare one with operational instruments for my wife’s 1990 XJ-S convertible (5.3L V12 w/ Marelli ignition) due to some ongoing issues with instruments and lights. Both junked clusters have issues, and I am not sure how good or bad the instruments are. I would like to verify that everything operates before I try to install the rebuilt cluster in the car. One cluster was supposedly from a 1988 V12 (Lucas ignition) and the other from a 1991 V12 w/ Marelli ignition. I can see the differences in the configuration of the Warning Lights, and the part numbers of the Speedometer and Tachometer.
I plan to start with the 1988 cluster because that is the best one and the plastic is in pretty good shape. Most of the plastic bits on the 1991 cluster are broken, scratched, or have cracks that make them unusable. The cluster from the 1991 also has a lot of corrosion including on the surface of the flexiboard and on some of the instruments. As I start to put the best of the two clusters together (I can see that the 1991 has some jumper flexiboards that I need to add to the 1988 flexiboard) I plan to test each of the warning light circuits and instruments with the cluster outside the car before installing it to see if everything works. Testing the light circuits and bulbs should be relatively simple with the information provided in the XJ-S ROM and Electrical Guide that I have on hand, and I believe that testing the 4 barrel gauges (coolant temp, oil pressure, fuel quantity and voltage) should be easily done once I build a tool to vary the voltage (using a dashboard lighting rheostat out of one of my parts cars) to observe their operation as voltage varies. But how can I test the 1991 Marelli speedometer and tachometer outside the car to verify that they at least function? What kind of signal does the Speed Interface Unit send to the speedometer and how can I simulate that? What kind of signal does the Marelli coil send to the tachometer and how can I simulate that?
Paul M. Novak
1990 Series III V12 Vanden Plas
1990 XJ-S Classic Collection convertible
1987 XJ6 Vanden Plas
1984 XJ6 Vanden Plas
1969 E-Type FHC
1957 MK VIII Saloon
Ramona, CA USA