As I'm mulling through some wiring diagrams in an effort to 1) get my XJS running right and 2) convert to megasquart EFI, I'm curious about how the stock ECU knows when to fire a certain bank of injectors.

my car is an 85, and I'm fairly certain there is no crank sensor. The ECU gets an RPM signal, which is based on the 12 tooths inside the distributor, but nowhere do I see that it's able to phase the injectors in time with which cylinder is firing, as in, if 1A is ready to spark but injector signal is being sent to B 2,4,6. I understand batch fire is not necessarily important for it to be squirting exactly in time with the firing order, but at lower RPM's it does make a difference - so much so that early EFI Dodge's have a sequence for setting that phasing up. I'm sure others do as well.

Does anyone have any idea behind the logic?

if you watch my video here, you can see not only are my injectors not batch firing firing correctly, but they are sometimes missing a sequence. all most likely due to a bad signal from the distributor.

Thank you!

 

The V12 HE Lucas ignition version, which your car should have, fires the injectors in batches of three. The actual firing of a batch is not related to valve opening, merely to the rev signal from the Lucas amp. The duration of the injection is governed by the TPS signal (throttle position sensor, under the throttle capstan) and by manifold vacuum, the pipe for which goes rearwards to the ECU from a take off spigot on the centre of the inlet manifold crosspipe at the rear of the engine.

 

The “crank sensor “ is the distributor rotor.

 

No, in the HE Lucas Digital P system, it starts at the ignition pickup in the distributor which is fed to the ignition amp sitting on the intake manifold. That amp then sends a signal to the ECU telling it when and how often to fire the batches of injectors. There is no adjustment to that fire signal. Since the injectors are batch fired, injection timing isn't that critical.
The distributor rotor has nothing to do with it on an HE engine. Only in the early, pre-HE D-Jetronic injection systems did the rotor play a part.
Here is an article on the Lucas Digital P injection system.
https://www.jagweb.com/aj6eng/lucas_efi.php
If your injectors are not batch firing correctly, several things could be the cause; wiring connections, injector resistor pack, or the ECU. I have an 85 XJS where the original 6 CU ECU had bad solder joints, and the engine would run fine one minute, and totally misfire the next. The 6CU was very susceptible to cracking solder joints. I reflowed solder joints in the ECU, and it worked, but imI didn't trust it. I replaced it with the later 16CU, bolt in replacement, and it's run fine since.

Jon

 

I understand exactly what you are saying. The OP was asking about how to get that signal into an aftermarket ECU like the MEGASQUIRT. It depends on which Megasquirt you have. If you have the MS2
That gets a little more complex. You can pick it off the distributor but then you’ll have to retain the batch firing. The MS3 pro has enough IO’s to trigger each injector at the correct time which does a lot of nice things, a slight power gain, reduced fuel usage and less deposits On the back side of the intake valve.
There are also enough IO’s. To trigger individual coil on plug. . Eliminating the need for the distributor at all. That not only cleans up the look of the engine dramatically but offers some great tuning options. In either case it’s best to pay the extra and buy it prewired.
The problem then is an actual crank trigger. Solved one of 2 ways. Find the later 6.0 liter engine and use that crank trigger or go aftermarket.
ECUMAXX is more Jaguar friendly but significantly more expensive.
I’ve found some ease in altering inputs. Using the throttle body off the GM Atlas ( Trailblazer or Envoy 4200 ) and wide band sensors. While it does take a little adapting it saves buying the larger aftermarket intakes. Since there are so many in the junkyards. And you don’t have to “teach it to speak British”
I also take the fuel rail so I can use DELCO injectors and it just clips on unlike the Jaguar system that requires a deft touch and I always manage to destroy my fingertips.
Yes grab 2 sets. Typically they sell me those for $15 each.
I also use Chevy Injectors and a Chevy fuel pump. ( I’m a racer using E85 and Jaguar injectors don’t flow enough. )