How hard is replacing an XJ V12 series 2 engine by a series 3 HE?

XJ
1

Hello

following my topic on search for donor car VIN for a series 3 engine (here : https://forums.jag-lovers.com/t/reverse-xj12-vin-with-engine-number/433013/26)
which has been solved thanks to active contributors, I am going to the next step which is understanding what implications means the replacement of a Series 2 engine (no more available) by a Series 3 engine (already in the car, which I am considering purchasing)
All mechanical job done , but ECU and electricals not yet switched from S2 to S3: the Series 2 car is still fitted with a D-Jetronic ECU (DAC1936 6CU - AEU1121E) and -no surprise- it will not crank

I am therefore considering replacing this ECU by a more suitable DAC4478 16CU (available as spare) or a DAC6336 (16CU) (which I will have to acquire) which seems the correct one for a NON O2 , 1990 engine (as identified in previous post)

has anyone done it already ?

when looking a the diagrams in respective Manuals (although they alas do not mention which DAC they are based upon), there seem to be significant differences which might be difficult hurdles for me :

1/ on S3 there is a “fuel injector amplifier” (identified N°292 on diagram, connected to terminals 4 5 14 24 on ECU), absent from S2, on which is displayed a bank of “power resistors” (N°313); how different ?

2/ on S3 , there is a “throttle switch” N°310, (linked to terminals 12 14 17 on ECU) which on S2 is named “Throttle potentiometer” , also N°310, connected to terminals 7 19 20 ; same stuff ?

3/ on S3, a “vacuum changeover switch” N°360 and a “supplementary air valve” N°361 are present, not on S2 …

there are surely be other differences, but these already seem a bit frightening for being able to cable correctly a new DAC 6338 ECU onto a S2 harness…how much should I worry ?

here are the above mentioned identified on S3 diagram

S3 diff
S3 diff1920×2256 457 KB

and on S2 diagrams

S2 diff1
S2 diff11920×2560 580 KB

S2 diff2
S2 diff21920×2735 524 KB

1 Like
2

Rolando,

impressive analysis and fully understandable problems you’re running into!

This post is rather meant to offer consolation than advice - I don’t have a clue about 12 cylinder cars. The one - admittedly very general - idea is to rather think about what you really want: the SIII engine will perform flawlessly under a complete and functional SII setup - after all the engine itself is only mechanical. The SIII engine will perform a lot better though with its original, complete and functional setup. The issue is just about where you want to have your SII/III interface.

Unless you’re concerned with originality (or need to be for want of historical registration) I’d try to take as much from the “new” original SIII setup as possible and in your reach. Ideally, you don’t only scavenge the engine, but most of the peripherals and all of the fasteners etc. from the donor car to facilitate the swap. Now I don’t know your options as regards the donor car, but, if the purchase of the engine is over and out and the seller is not co-operative, I’d spend a lot of time with knowledgeable people at the usuas suspects and let them advise you which setup parts you need for your “new” engine.

Good luck

Jochen

75 XJ6L 4.2 auto (UK spec)

3

If it was me doing this work I would just shit-can the factory ECUs and go with an aftermarket system like Megasquirt. You would end up with a much nicer running car that would probably be more economical on gas as well.

4

If Megasquirt is too much of a science project, consider a plug and play Mobeck kit. It ain’t cheap, but it is the shortest path to a running car.

2 Likes
5

As fitted to the Series III cars the HE engine had many sub-variations, with many different peripherals, according to market, model year, and type of emission system being used. Much of this is not documented in the manuals such as you have…as they are compilations (and condensations) of several different master service manuals

Your tasks will be simplified IF you desire (or are legally allowed in your area) to use employ just the bare necessities to make it run well. For this you’ll need air and coolant temp sensors and circuits, the potentiometer circuit, ‘trigger’ from ignition system to ECU, injector ballast, main relay circuit, fuel pump circuit.

The first order of business, IMHO, is to determine if your existing ECU connector is physically compatible with the correct ECU that you’ll be needing. If it fits then the pins and/or wires can be re-tasked as needed. It’ll be a brain-teaser but should be doable, I’d say.

If the existing connector won’t attach to the correct ECU then, IMO, you might as well find a complete, correct, F.I. harness and spend a day installing it.

Cheers
DD

1 Like
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Thanks John and Steven
to be honest I’ve always been reluctant in :
1/ incorporating more electrocnics in a car which already has enough, potentially leading to additional issues
2/ using “out of the blue” critical components from companies that might stop business overnight
3/ spending on the ECU 30% of the car price …
the main question I always ask myself is “what if I am stuck with a car that does not crank” ? phoning a norwegian guy who might have retired in the meantime does not seem an option to me…

7

Thanks a lot Doug,
alas the answer about compatibility of connectors is no, they are not compatible between a 25 Pin D-Jetronic ECU and a 35 pin Digital-P ECU
so very good advice to imagine purchasing and replacing the complete harness from S2 to S3;
SNG Barratt has 2 injector harnesses for S3 XJ2311 & 2312

injection harness XJ2311
injection harness XJ23111345×502 91.5 KB

injection harness XJ2312
injection harness XJ23121315×631 111 KB

they have this connector at an end
injection harness XJ2312 connector

which I believe should go to this one on the firewall
connecteur tablier

now is this present on the S2 I do not know
and then there is an LH harness going from the firewall to the ECU in the boot DAC7156 (discontinued) ?

LH harness DAC7156
LH harness DAC71561123×552 82.9 KB

or is it the “main harness” DAC2533 ? not manufactured any more

main harnes DAC2533
main harnes DAC25331123×595 90.2 KB

however you mentioned it was a very serious task to change them ?

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thanks for your support Jochen

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Rolando,

Not familiar with the D-Jetronic but know some about the Digital-P.

First, as far as I know, all 6CU and 16CU have the same pin configuration and any of them, as long it’s the right CR and Cat/No Cat type will work.
So if your DAC4478 is indeed working it will do the job fine.
No way of testing it though until everything is in place.

Two completely different things.

The Ignition Amp (GM Module): is what gives the trigger signal at the ECU.

The Power Resistors: the ECU has two states, Open and Hold
Open is the initial firing of the injectors, full voltage
Hold is through the Resistor Pack so the current is limited to protect them.

Don’t really know, but looks very different.

This must be an early configuration, and what I imagine it’s for:
When engine is cold (Oil Temp Switch) the Vacuum Changeover Switch kills the Vacuum Advance to waste heat and help light the Cats faster, and activates the Supplementary Air Valve to compensate the idle.

Later this was done with a timer, 45secs in some markets, 15min in others.

This is what you are aiming for:

Without 0² Sensors:

euro_wiring

With 0² Sensors:

fed_wiring

If you go to all that work, I would strongly suggest that you add O²Sensors (heated).
Big difference on economy and the engine will run much better.
.

Depends on your skills and patience.
Complicated, but at the end of the day it’s just cables, relays and sensors.
A replacement harness might be less complicated but I don’t know how easy it is to find one.

This article explains all the components in detail and if you have questions just ask.

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Yes, this is what you need, and maybe plus the other two.

You might be lucky and find a used one?

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[quote]

…the Series 2 car is still fitted with a D-Jetronic ECU (DAC1936 6CU - AEU1121E) and -no surprise- it will not crank

This may be ‘mechanical’, Rolando; there is a ‘start inhibit switch’ which prevents cranking unless in ‘P’ or ‘N’. The switch, either on the gearbox or in the consol, may fail, disconnect or is maladjusted. Or there is something wrong with the starter/relay - have you tried jumpwiring?

As Aristides says; the resistors are protective devices. The injectors are powered from the main relay; the ECU just grounds the injectors internally - it’s plain 12V, no need for an ‘amplifier’. It may be an euphemism for the ignition amplifier - the ECU trigger injection based on ignition data.

Basically; the connections from the resistors must(!) be to the appropriate connectors on the selected ECU. It’s unfortunate that these are not standardized - but reconnecting at the connector should work.

Careful(!), the output from the two are most likely different - test procedures for the two differs…

[quote]
3/ on S3, a “vacuum changeover switch” N°360 and a “supplementary air valve” N°361 are present, not on S2
[/quote]…

These are related to the ign advance/vacuum set-up - and the special requirements on the HE. To add; there are market differences in the vacuum set-ups…

There are two elements to worry about; an ECU needs specific quality signals from the sensors. If all relevant sensors (like the throttle switch) came with the engine; fair enough - if not, they must be sourced…

The second; having all the correct sensors - they must(!) connected to the specific connector as required by the specific ECU. And with uncertainty (to put it mildly) of ECU identification - it’s 'complicated.

The actual connections may be done with separate wires if required - sourcing, and changing, wiring harnesses poses similar problems of ECU/car identification, of course.

Frank
xj6 85 Sov Europe (UK/NZ)

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hello Aristides and Franck
thanks again for input and documents
the task seems a bit too ambitious for me ; even though I manage to find the right harnesses, ECU, sensors, connectors, and settings, the risk is high I remain with a still steel lump in the frontyard… let’s remember nobody saw this engine running on this car
and I can understand not many people delt with such a modification: why change to an S3 engine if you have a S2 sedan ? just buy a better S3 sedan instead; Only on a coupe S2 XJC would the mod be “relevant” , yet not “stock” original
Moreover, the car being in Italy, I would also run a risk of refusal when registering it in France

I think I am going to pass my turn this time and leave the seller with a difficult car for sale…

but the hunt for a decent XJC is still on !

thanks a lot for your help, I am now much more knowledgeable on V12s ! (although not yet “hands on”)
Roland

13

Yes, Roland; not knowing if the engine is in fact in running order an is far away, is a deterrent.

It’s a pity - it would be interesting to participate in such a worthy cause! The HE V12 requires some extra gadgets for perfect running. Jaguar had to go to some sophistication to meet US emission and still keep the engine running as a V12 should…

Mind you; the fuel requirement is just a matter of connecting the injectors to the proper places in the ECU. But the ECUs require specific inputs, depending on variants, to operate the injectors - the fuel requirements for various V12s are, for all practical purposes, the same…

Frank
xj6 85 Sov Europe (UK/NZ)

14

thanks Frank
Yes I also feel sorry for this car, all the more since the owner who had it already for close to one year at his mechanic’s workshop does not want to invest more in the car, which is otherwise rustless and complete;
a good bargain for a motivated restaurer ready to spend time, in Northern Italy or South Eastern France as I am; if anyone interested, send me an MP and I can settle the contact

I’ve had an XJ6 S1 several years ago, and I miss her, I will only replace her by an XJC though

15

There seem to be many obstacles there, the major being the immatriculation in France, even though I think that the FFVE is not very strict about the engine number As long as the car was registered with a 5.3lt V12 it maybe not that hard. Look at their website info. But it’s still a big maybe.

As for the ECU, instead of paying 1.000ÂŁ for a harness, you could get a stand alone ECU and its sensors, harness and all the accessoirs with the same amount and bring the engine management to the 21st century.

That’s what Harry 's garage did to his XJC, even though he paid an astronomical amount for it.

Good luck with your search and keep us posted.

1 Like
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Which ECU are you thinking about ? I’ve seen a few YouTubes from Harry, he didn’t seem happy with his initial choice ? second attempt was better ?

17

is this Harry’s Garage video you were thinking of Aristides, with “Emerald” ECU ?
Tom Lenthalls the mechanic quoted 10-12 kÂŁ + VAT for its installation !
I didn’t understand the new Distributor stuff…

18

Rolando,

don’t worry! While the video is fun to watch and all of the parts and stuff are beautifully made my mechanic would be close to suicide if he’d have to admit that a system that had been running beautifully before wouldn’t run reliably after he repaired it …

The D jetronic is no rocket science even though experts are few and far away at times. Nevertheless, I seriously doubt that you’d be chasing one defect after the other as the mechanic suggests.

Another source of trouble I only learned about last night from a friend with a BMW K 100 were the injectors that he had bought “new” in the UK leading to catastrophic CO figures. Once he replaced them with good used originals everything was fine again. So maybe in Harry’s case as well, the new injectors may have been a problem …

So find a good mechanic and redo the - either SII D or later SIII L - jetronic correctly. 15k for the engine setup on a 12 cyl is rich people’s blow-your-bonus game.

Good luck

Jochen

75 XJ6L 4.2 auto (UK spec)

1 Like
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quite agree Jochen !
it’s the mechanic’s selling points to frighten customers and make them buy “his” stuff at 12k£…
in any case I always much prefer restoring components - like fuel injectors (kits are available), test them, and re-assemble them rather than buying supposedly “equivalent” modern ones “made in god knows where” by “god knows who”
except if they are provided and guaranteed by Bosch
:wink:

20

What is wrong with your present engine, Rolando…?

The improvement with HE was mainly better fuel economy - but early versions performed well enough to preserve…

Frank
xj6 85 Sov Europe (UK/NZ)