I have a 1995 XJS 2+2 with the AJ16 ignition system which has been giving me starting problems.
When I turn the ignition key, sometimes the tach drops below zero. Sometimes it tries to start but while the starter is turning the engine, the tach looks like it's hitting around 1000RPM and then suddenly it just drops to zero. If turn the ignition key to off and wait a couple of minutes, it might start or give me the same problem.
If I turn the ignition key to ACC and wait a few minutes, it might start or give me the same problem.
I sent the ECU (the unit inside the passenger kick panel) to a repair shop and they said they found nothing wrong with the unit.
This has been going on for about two years.
I love the car and I don't want to have to euthanize it.
Thanks

 

Sounds like you might be experiencing a failing crankshaft position sensor, classic symptoms.

Very easy to change.

 

I agree with Allan. What you are describing is what I was experiencing with a failing crankshaft sensor. The tach will read when the crankshaft sensor is failing. Takes about 15 minutes to replace. Have you replaced this yet?