I had an interesting conversation with an AI and i wanted to share the suggestions it made.
finally this will let me avoid starting something with megasquirt which is probably interesting but i don't want to race or get more power.
So here is the result, maybe less expensive than playing with programmable efi.

🔌 Lucas 13CU → 15CU ECU Conversion Guide

For Jaguar XJS 3.6 (AJ6 engine, 1988) upgrading to
Lucas 15CU from XJ40/XJS 4.0 (DBC6085, DBC6266, etc.)


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📘 Pinout Mapping

Function 13CU Pin 15CU Pin Notes

+12V Battery feed 1 1 Same
ECU main ground 2 2 Same
Sensor ground 3 3 Same
Coolant temperature sensor (CTS) 4 4 Same
Intake air temperature sensor (ATS) 5 5 Same
Throttle position sensor (TPS) 6 6 Same
Engine speed signal (VR crank sensor) 7 7 Same
Injector bank A 8 8 Same (but use high-impedance injectors)
Injector bank B 9 9 Same
Fuel pump relay control 10 10 Same
Main relay control 11 11 Same
Idle air control valve 12 12 Same
Shield ground (sensor cable) 13 13 Same
Ignition trigger to amplifier 14 14 Same
Oxygen sensor input — 15 Optional (unused on early 3.6)
Diagnostic output / check engine — 16 Optional
A/C idle-up input — 17 Optional
Fuel temperature sensor — 18 Not used on 3.6
MAF signal ground 19 19 Same
MAF signal output 20 20 Must come from Lucas hot-wire MAF
+12V to MAF 21 21 Same
Tachometer output 22 22 Same
Secondary crank signal 23 23 Same
Knock sensor input — 24 Not used on 3.6
Secondary ECU ground 25 25 Same



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🧰 What You Need to Do

1. Injectors

Replace your low-impedance (2–4 Ω) injectors with high-impedance (~14 Ω) ones from the AJ6 4.0 L or BMW M30 engines.

EV1 connectors are compatible.



2. Airflow Meter

Replace the Bosch flap-type AFM with a Lucas/Hitachi hot-wire MAF.

Adapt three wires from your existing harness:

+12 V (from AFM power)

Ground

Signal (to ECU pin 20)




3. Crank Sensor

Keep your existing inductive VR sensor. Both ECUs expect a sine-wave signal, not a Hall square wave.



4. Idle Air Control Valve

The 2-wire PWM idle valve from your 3.6 works perfectly with the 15CU.



5. Throttle Position Sensor (TPS)

Both use a standard 0–5 V potentiometer. No rewiring needed.



6. Optional Oxygen Sensor

You can add one to pin 15 if you want closed-loop operation.





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⚙️ Suggested Parts

Component Recommended Source Notes

ECU Lucas 15CU, DBC6085 / DBC6266 From XJ40 or late XJS 4.0
Injectors Bosch EV1 14 Ω, 220–240 cc/min From BMW M30 3.5 L engines
MAF Sensor Lucas/Hitachi hot-wire From 4.0 L AJ6
Fuel Pressure Regulator 3.0 bar Original OK, or 3.2 bar for E85 use



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💡 Extra Notes

The Lucas 15CU runs ~8–10 % richer by default than the 13CU, which already helps with ethanol-blended fuels.

With injectors + MAF swap, it can handle up to 70–80 % E85 without any piggyback controller.

The TPS, IAC, and crank sensor wiring are 100 % compatible.

Grounding quality is critical: check continuity between ECU pins 2, 3, 25 and engine block.



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✅ Summary

Once these adjustments are done:

Your AJ6 3.6 will run smoothly on the 15CU ECU.

You’ll gain better cold starts, cleaner fueling, and future flexibility for E85 or tuning.

No need for aftermarket boxes or external kits — it’s a near plug-and-play OEM upgrade.

I could add as warning that fuel hoses must be replaced with ethanol compliant hoses. The stock were not intended for this use, replace as well the fuel filter and fuel pump while you are there.

 

Please don't. It is absolutely not a trustworthy source.

 

Yes, that's why a check by people who know what they speak about is interesting but it can be used as a start framework. From what i read, it's not completely weird. It suggests an ecu able to drive high impedance injectors and move from old maf to one which works accordingly with the 4.0 ecu.
What could not work with this setup ?