XJS V12 Tachometer not working

XJ-S
1

The tach on my ‘88 XJS just quit without warning. Engine runs fine. Checked for loose wires and connections around the ignition amp. All good.

Is the amplifier likely bad?

2

DavidDQ,
If the engine is running OK, but your tachometer is not working then the problem is not your ignition amplifier. It is either due to a bad connection at the instrument cluster or possibly a failed tachometer gauge. There are lots of posts in the archives about a variety of instrument problems that were resolved by removing the instrument cluster, cleaning up the connector pins and reinstalling it. In addition those who do this often add an additional ground wire between the instrument cluster and the car. This is also covered in Kirby Palm’s excellent book “Experience in a Book” available on Jag-Lovers.

BTW, I resolved problems with the “lazy” speedometer and tachometer in my wife’s 1990 XJ-S convertible by removing the instrument cluster, cleaning the contacts, and adding a new ground wire. Now those two gauges read properly.

Paul

3

Not exactly true. There is a white blob inside the ignition amp with two resistors in it, and one of those resistors failing would cause the tach to stop working. You could actually just bypass that resistor with another resistor connected to the - terminal of the coil to see if the tach starts working.

4

Thank you for your suggestions. I have done a bit more troubleshooting and here is what I have learned:

  1. ignition amp tach lead measures 10K resistance.and ignition amp tach lead reads 9 volts DC with ignition on. This would suggest the ignition is probably OK -correct?
  2. the tach lead coming out from the gauge measures about 40k resistance suggesting to me the circuit is at least not open.
    I have delayed removing the instrument cluster as I fear the high mortality rate of brittle British plastic trim. Now it appears I must dig into that. If the gauge is bad, any suggestions where To it repaired or replaced?

Thanks again everyone for your pointers.

5

David,
Your 1988 XJ-S has the Lucas ignition system. My wife’s 1990 XJ-S convertible has the Marelli ignition system and so I won’t be able to confirm those values, others will have to chime in there.

However you should be able to easily find a replacement tachometer on eBay for less than $100. Just note the numbers at the bottom on the instrument face and search eBay for “XJS tachometer” with that part number. I believe that the Jaguar tachometer for your 1988 XJ-S with Lucas ignition is DAC1750. You can also search for that.

Paul

6

I believe that is correct, yes. I can’t help you about the tach itself, although 40K resistance sure seems high.

7

I repaired the coupe tacho over 20 years ago.
Both tacho and electronic speedo use a similar circuit to convert a train of pulses into a few mA of current to drive the pointer.
They use a generic “555” IC in the circuit plus one or two small capacitors and resistors.
Easy to fix with a bit of electronic knowledge, otherwise $100 for 2nd hand one reasonable.

Worth just putting your hand up behind the dash and fiddling with the 2 plugs connecting the instrument printed circuit. The plugs rely on copper contacts on the printed circuit board and are a source of trouble.