Testing XJ-S Instrument Cluster Components, Especially Speedo and Tach

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?

Regards,

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

Paul,
About 4 years ago I traced the connections in the printed circuit board of
a pre facelift instrument cluster.

I have a pdf file for that. It probably answers some of your questions.
Much easier to follow than the OEM info.
Right now it is 6.00AM and I am going to work.
Let me have your email address and I can send the pdf later.

Richard Dowling, Melbourne, Australia. 1979 coupe + HE V12 + manual; 1989
convertible; 2003 XJ350.

Well, here is the pdf of the instrument cluster.
It has the info for testing the speedo.
It is calibrated 200Hz = 100mph.
You can check with a 60Hz transformer, say 120V to around 5V.
Ground one end of the 5V output to the instrument ground.
Take the other end through a resistor, say 1k to 10k to limit current , and connect to speedo input.

Not sure how easy to read the pdf on the forum site.

XJ-S INSTRUMENT CLUSTER.pdf (19.0 KB)

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Richard,

Thank you very much for this file. This is fantastic work on the XJ-S instrument cluster. I was able to download the .pdf file from the Jag-Lovers website and save it on my hard drive. Then I was able to open the file, look at it, and print it out.

I can see that you did this for your 1979 pre-HE XJ-S and updated it with info from an unknown HE car (probably with Lucas ignition?). The configuration of my wife’s 1990 XJ-S convertible is a further evolution in that some of the warning lights are different and the tachometer input comes from the Marelli ignition. But clearly 90% of this information is very helpful and useful to me.

Regards,

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

I thought I’d check the math on that given that the pre-Marelli transmission mounted speedo transducer is known to put out 8000PPM (pulses per mile.)

So: 100mph / 60 min = 1.6 miles per minute

1.6 miles X 8000 pulses = 12,800 pules per minute.

12800 pules per minute / 60 seconds = 213PPS at 100MPH or about 200Hz.

So if I did that right, all that math squares up pretty well.

The speedo sender eventually moved to a diff mounted VR sensor that went through an interpretation module that I assume calibrated the signal to an 8000PPM 200Hz@100MPH setup. I’m guessing, because I dont’ know if they made any real changes to the Speedo unit.

I can say that the Tacho for the Marelli car is a 6cyl unit as it receives the engine speed pulse train from only the A coil, leaving out 1/2 the pulses (therefore the speedo has to be a 6cyl one in order to represent the RPM correctly), whereas the Lucas Tacho gets ALL the pulses.

Oops… I just figured out I’ve replied to a very old post… sorry… hope the math is interesting though.

~Paul K.