1988 JCB 3CX Grey Cab Electronic Tachometer Debug

[Soulman]

Hi

On my JCB 1988 3CX the Hour meter, tachometer, fuel gauge and temp gauge are not working. I have decided to take on this as my next project to fix. On the hour meter/tachometer side, as i understand it, the alternator has a send terminal (W terminal) that delivers an A/C voltage to the tachometer in the instrument cluster. I noticed that on my machine, there was no wire connected to the W terminal on the alternator and i spotted a loose screened cable not connected to any terminal in the loom feeding the alternator. I believe this screened cable is the tachometer cable. I checked the output voltage of the W terminal using a voltmeter and it read 7 Volts approx. I also did a resistance check between the screened cable and chassis ground and it is giving a very close to short circuit reading so i didn't connect the screened cable to the W terminal until i figure what is causing the potential short circuit.

It seems according to the wiring circuit diagram i have, that the alternator terminal is connected to the tachometer directly. Does anyone know if that is actually the case? btw item 33 in wiring diagram is the alternator and item 31 is the speedometer

i had a look at the back of the instrument cluster but i cant see straight off, a cable similar to the screened cable down at the alternator. see images attached.

[Soulman]

so an update. I got the hours meter working. I see it changing after a little while running. The issue was that the outside braid and inner red core wire of the tachometer cable were shorted together, i separated them and then i got 8vac up at the console connection. However the tachometer RPM counter is still not working. Is the Tachometer RPM counter controlled from same feed from the alternator and also road speed ?? Perhaps there is another voltage feed needed for the RPM counter ?? .. see picts for alternator connection and hour meter updates. Since the Tachometer and the Road speed dial are combined, it must be a common signal? there are no physical cables attached to the console as you might note in my first post where i show the connection to the rear of the console.