Early S2 V12 FI ECU

(tony) #1

I have a chance to buy a S2 V12 XJ with “failed ECU” (fuel injected)

prior to me inspecting it, or ringing the seller

(I will ask him how he came to this conclusion, I am fairly handy with electronics,
and a vehicle in this state can sometimes be had for a bargain)

what are the options if the ECU is failed ?

are they available used on Ebay or specialist wreckers?

can they be rebuilt or replaced with ECU remanafactures, as per some companies advertise ?

any other options ?

(Aristides Balanos) #2

option 1: Fleebay and the usual suspects.
Yours must be the 6CU but you can also fit a 16CU. Just make sure it’s the right one for your engine (Cat /no Cat, compression ratio)

option 2: Send it to Roger Baywater at AJ6 Engineering


(Doug Dwyer) #3

They’re probably rather hard to find

When I was searching for a spare HE ECU (rather easy to find) I don’t recall seeing any early ones out there.

I would contact Roger Bywater at AJ6 Engineering. He can probably repair the existing one


(Doug Dwyer) #4

Those are “HE” ECUs.

A Series II car would have the 3CU


(Aristides Balanos) #5

Yes you right…

(tony) #6

thanks for that link, that they dont have Integrated Circuits is probably a bonus if they do need specialist repair

I note the 3CU comes with various part nums, for XJS etc

such as this

would one of them likely be a plug & play for a 76 XJ12 ?

(Doug Dwyer) #7

Non-expert disclosure !

I suspect the variances are minor in most cases. Tweaks for various emissions standards, hi-compression/low compression, and such. The engine would probably run OK with any of them.


Did the Series II cars ever use Lambda/feedback systems? A feedback ECU replacing a non-feedback ECU, or vice-versa, might not work too well


(Robin O'Connor) #8

I can confirm that a feed back/non feed back ECU dosen’t work well from my experience with an old XJ40 I had.
Just my 2c

(Aristides Balanos) #9

I imagine one could just retrofit O² sensors if a feedback ECU is used to a non-feedback engine? A neat upgrade.
On the vice-versa scenario one could negate the O² sensors, but not an optimal solution in my books.


(Frank Andersen) #10

The early models used ‘D’ system fuel injection, Tony - which uses a trigger unit in the distributor…

This set-up is incompatible with any later digital ‘P’ system ECUs - and it is pertinent to clarify with the seller whether it is a ‘D’ or a ‘P’. The former has a trigger unit, an oblong piece fitted below the rotor.

The point here is that the trigger unit failure will imitate an ECU failure - but a replacement trigger unit in working order may be very difficult to source.

While diagnosing a fault is difficult at the best of times; identifying the ‘D’ set-up is simple - and will have some repercussions for further actions.

Generally; ECU faults are uncommon - and the sellers reasoning is very relevant. A failed trigger unit is in a different ball-park from a failed ECU - the latter, in one form or another, is readily available. And sometimes DIY repairable - which may or may not be the case for a trigger unit…

Again; the ‘D’ and ‘P’ ECUs are incompatible - and converting one set-up to the other is unknown territory…

xj6 85 Sov Europe (UK/NZ)

(tony) #11

thanks Frank…do you (or anyone) know when the change took place ?

I have the '78 XJ12 Parts and Service manual, but they dont indicate by VIN or engine number when the change took place, as earlier Jag Manuals do

Can the distributor ignition model be replaced by something else (similar to the Crane for earlier carb models) ?

sounds like it could be a nightmare to diagnose, but if the bloke ends up having to virtually throw it away, would be best to know as much as I can

(Robert Laughton) #12

I have 3 of the 3CU ECU’s, but I’m a long way from you.

Is it possible the seller is mistaken, and it’s the Opus amp that has failed?


(tony) #13

just spoke to the owner, its been at the Jag Specialist over 6mths, had the Opus replaced,
they dont know, could be wiring

he is sort of stuck, had some lowball offers, as he said, if it was going he would want 3 times the price

luckily, it is nearby to me, so I can inspect it, I know the tech well, and I trained & worked as an electronics tech many aeons ago

apparently it is a very early model, and the tech hasnt had these issues before, so it must be a tricky one, the owner was complaining to me, but I told him the guy working on is the best there is, the bill$ are adding up

the car is rust free, mainly in very good condition, fairly low km, from a collector, was a goer

(Frank Andersen) #14

There is a cryptic remark referring to UK/European ‘P’ - and Japan/Australia ‘D’, Tony…

…but whether that was temporary, and when, or if, changed - or what applied to US is not stated…

The main difference is that the trigger unit (‘D’) provides one trigger pulse to the ECU injectors per revolution. The ‘P’ is triggered from the coil neg to the ECU, nominally triggering injectors on each bank once every third pulse…

If it is ‘D’ it hasn’t an OPUS amplifier - so it is still important to find out whether you it’s a ‘D’ or ‘P’ before you start searching for an appropriate ECU…:slight_smile:

In theory, with a ‘D’; if a trigger unit cannot be acquired, it should be possible to trick the ‘D’ ECU by giving it a ‘single pulse’ trigger from some other source. But the ‘D’ ECU will not work with a ‘P’ dist - and vice versa…

And for all we know; a PO may at one stage have ‘done something’ - outsmarting us all…:slight_smile:

xj6 85 Sov Europe (UK/NZ)

(tony) #15

the owner was muttering about this, so thanks for your clarification.

when I go see the mechanic, I shall take notes of what you have said, so we can identify exactly what is what, even though I am familiar with both V12, electronics, and having worked on electronic ignition vehicles, I would be very reliant upon the learning of this esteemed group to get it going

the last non-goer I bought took 20+ hrs of electrical from me to get it roadworthy
(not a V12 Jag though!)