CTS PN: DBC12183
ATS PN: DBC5051 x2 (A & B bank, one for the Ignition, another for the EFI ECU.) The one for EFI ECU is on the B bank air cleaner air intake.

Just b/c it wasn't explicitly spelled out in all the prior troubleshooting: The Ignition and EFI systems on all XJS's are separate. The EFI system is ALWAYS downstream of, and dependent upon the ignition system working, which is why you always start with making sure the ignition is 100% functional. The EFI system REQUIRES the RPM pulse from the Marelli ECU (a separate pin that combines both banks signals to a 12 cyl sig) to pin 18 of the EFI ECU.

The Marelli ECU controls the engine as 2 separate 6 cyls with an A side and B side output - each to two amps that control 2 coils (basics) - thus the 2 tier distributer. The curse there is that sometimes the rotor can fail allowing spark to travel to the center shaft instead of out to a tier (reliably) causing 1/2 the engine (a bank) to not run - or run badly - allowing excess fuel to go into the CAT, eventually causing it to melt down or a fire.

Note too that the RPM signal you see on the Tacho is taken from ONLY the A bank coil. (Technically it's a 6cyl tacho so that it shows the RPM of the V12 properly).

As you already know, the primary INPUTS to the Marelli ignition ECU are the CPS and rear RPM sensor (same sensor PN usually). Secondary inputs are a MAP sensor internal to the ECU, temp sensors & idle switches etc. Nothing that prevents ignition spark but affects the timing & timing map.

When the key turns on, the main relay is activated via ignition power. It transfers battery power to the injectors, and to the EFI ECU. The EFI ECU upon wakeup, grounds the Fuel Pump Relay (FPR) for a few moments pressurizing the fuel rail. Absent an RPM signal, it stops grounding the FPRelay, and won't turn it back on until it sees an RPM signal (earlier ECUs also used to look at a +12V signal from the starter -but not 6.0L ECUs).

The ECU won't command the injectors (by providing a ground pathway) w/o the RPM signal OR - (and this is a way to test that the EFI ECU can command the injectors) - the Wide Open Throttle switch is activated on the capstan. The EFI ECU will provide a single extra injector burst even w/ the car not running. A great way to (hear) them working as a part of diagnosis.

The primary inputs to the EFI ECU are the RPM signal and the CTS resistance. That's it. Absence of either will cause the car to not run. A bad - over-resistance from the CTS will cause massive over-fueling as the ECU will think the engine is off-the-chart cold. (One of the typical failure points).

The next most important input to the EFI ECU is the map signal that comes from the crossover balance pipe to the MAP sensor inside the ECU - determines load. If open to atmosphere the ECU would think the engine is at full load (open throttles & atmospheric pressure) and would increase fueling. Beyond that the ATS provides a bit of fuel trim, the TPS provides an enrichment throttle movement delta, and also alerts the ECU as to when the throttle is at idle stop, And finally the O2 sensors provide trim when warmed up.

Mechanically - if there is excess air entering at idle (when the ECU sees the throttle on it's stop) but the RPM rises above around 1200 b/c - usually a stuck open Idle Air Valve (or other air leak), the ECU will think you're coasting over a hill with foot off the gas, and will cut the fuel until the RPM reaches around 700. In the garage this manifest as idle hunting... bouncing up and down over and over and over again.

Hopefully those basics will help you when mentally thinking about the system & troubleshooting.


~PK