92 XJS V12 Intermittent stalling

Recently my car has died after 25 to 30 minutes driving. Symptoms are as follows: The tachometer will start jumping erratically with a loss of power. The tach will drop to zero but the engine will be still running but very poorly. It feels like it is running on six cylinders only. After sitting for a few hours the car starts and runs fine until the above repeats itself. I feel that the tachometer feed may be a clue as to what is causing the problem. My question Is: Where does the tach feed come from?
Or any other clues would be helpful.

Tach feed comes from one ignition amp. You might try swapping the amps, which might just switch the misfire to the other bank while the tach works perfectly.

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1992 has the facelift cluster and Marelli ignition. Same as mine 1995, for which I happen to have the wiring diagram.

Intermittent stalling and the tach needle bouncing is a telltale sign of A-bank ignition coil failure. The coils (yes, I would recommend to replace them both) are not expensive and reasonably easy to replace.

Hi Steve, Thanks for the good info and diagram. I just happened to have 2 new coils that I bought a couple of years ago. They have been installed and now awaiting a road test.
Thanks again, Nordy


I had the exact same issue and suspected coils, amplifier etc as posted. At the end, it was an occasional short at the TPS connector (harness side). The leads coming out of the connector lost their insulation (go figure why) and some strands shorted, probably triggered by vibration.
Easy look-up …


Hope the car is working now, but here’s the diagram specific to the problem. If the engine were not running on after the tacho drops to zero I’d suspect a cranks sensor, but since it seems to be running on 6cyls, I agree w/ the the thinking that it’s the amp or the coil (as the A side is the singular output to the tacho).

It def needs to be fixed quickly so that raw fuel doesn’t get dumped into one bank’s catalytic converter which can cause a fire or a meltdown.

The amp charges the coil and controls dwell (charging time). When signaled by the ignition ECU it cuts the negative power to the coil causing a field collapse and a spark. This cyclic open circuit of the coil is what the tacho makes a count of.

Paul Kobres