All wires on your 37YO car engine bay can be brittle and need TLC when handling.
Generally the injector harness itself is the only one needing a total rework.
Fix the harness, double check it is correct, plug in the original ECU and crank it.
You may be lucky and it fires up.
If not check for one bank on, one off again.
I assume this happens only with ignition on. Is that true ? ( need answer ).
If so, good chance that is an ECU fault. Maybe ECU was original fault, or maybe it was O.K. but bad harness cooked some transistors.
If the one on, one off fault still there it is possible the harness is O.K., the ECU is O.K. but there is a
wiring open circuit or short to ground between the 8 pin injection connector and the ECU connector.
That would be rare but it can be checked - simple but a bit tedious.
If harness and other wiring checks out and you still have one on, one off fault then the ECU is most likely the problem. You could try a second hand ECU and hope that fires the engine. Bear in mind that apart from the injector wiring there could be other wiring, connector or sensor faults that affect the ECU.
Simple check for your ECU fault is to try it in another XJ-S and see if it works. You gotta have friends with a similar V12 for that.
I think there was an outfit in CA that could test and repair them - anybody ?
I live in Brighton UK & am a retired physicist. I worked in electronics
for high voltage power supplies used in mass spectrometers & X-rays.
When my ECU failed in 2000, I built a test rig & repaired it.
Kirbert
(Author of the Book, former owner of an '83 XJ-S H.E.)
65
Itās a blast! Even SWMBO enjoys it, and she doesnāt drive! She comes along every week to marshall, which means stand at a corner and put cars back on that have come off the track. We even enjoy the drive there and back, about an hour each way along rural roads.
For those of you who enjoyed slot cars in the 1960ās, Iāll point out that perhaps the biggest change is in the controllers:
DiFalco is one of several companies making controllers, and they sell for $200-$300 or more. Ball bearing triggers, replaceable resistor networks to customize response, adjustments for brake and sensitivity, you name it. Options include a voltage display.
We have one guy with a controller he special ordered from Romania for over $600. Five knobs and three switches. Iām not sure heās yet figured out what all those knobs and switches do.
Nice one Jim. I still work in my own electronics factory although well past retiring age. Last time the idea of a test jig cropped up on the forum I think we got stuck because the connector used by Lucas was no longer available as a new item.
So did you find a matching connector easily ? The other source of connectors would be from wreckerās yards.
Naturally once you have a connector you can make a test jig to simulate the operation of the ECU in a car. From there it is a small step to fix an ECU if you are experienced with electronic troubleshooting.
Most of the components in a Lucas ECU will still be available or easily substituted.
The items that might be problematic are the processor I.C. and memory I.C. They are such old technology they may not be easy to find new. That might also apply to the vacuum sensor.
On the other hand they can be plundered from old ECUs and in most cases still function.
To keep XJ-Ss alive it would be handy if there was a test jig available to hire in all the countries of interest. Then owners can check if their ECU is toasted. From that point on anybody with general electronic repair experience could fix them although if there is a circuit diagram available that will speed things up a lot.
One of the most frustrating things for a DIY person, or a workshop, is figuring out what to do if an engine fault points to the ECU being the culprit. The only sure way for now it to borrow a similar ECU known to be 100% correct.
Over to you JIm. What do you reckon ?
Importantly, is there much interest from this forum ? For this to work we need a stash of old ECUs to harvest connectors and possibly unobtainable components.
I got the connector from Simply Performance, who were breaking Jaguars
at the time. The ECUs use quality components (e.g. 125 C tantalum
capacitors) & are well made. Iām told the vacuum sensors can fail as
they have a diaphragm connected to a coil. You are right about the
processor I.C. and memory ROM being probably impossible to find. I
suggest no-one throws an ECU away as they could be a source of parts.
In my case a 1000pF capacitor had failed, an extremely rare failure as
my company used 10,000 of them a year without a failure!
A test rig would be good but it needs an oscilloscope to check the
injectors are firing & a signal generator to feed in the ignition
pulses. My car has the vacuum sensor, so my test rig only checks that
type of ECU. It would probably also check the early vacuum V12 as that
is just another transistor output stage. More circuitry to test the
later mass flow airflow meter would have to be added, I donāt imagine
that would be too difficult. I suspect the ECU for the AJ16 4.0L are
completely different as is the 6L V12.
Roger Bywater is of course the real expert & I understand he has
recovered & servicing ECUs again although he must be even older than me
(Iām 75). Someone on this forum said he had tried to get a relative
involved but it hadnāt worked. Can anyone add to this, it would be a
great shame if all of Rogerās experience was lost?
Ok. I rebuilt the injector harness and installed into the car and lo and behold! Same symptoms as before. 6cu holds the B side injectors on and the 16cu does nothing. What direction should I go now?
Post the year of your vehicle and what your 6 cu ECU # is. Maybe you can find a loaner from a J/L member to test on your engine, or maybe you can send yours to a J/L member to be a guinea pig and test on a different car. I think the 16 cu is dead. SD Faircloth
I have a 1985 v12. I donāt have the 6cu near me but I can post that info next time Iām at the car. Anyone around Atlanta, GA willing to lend a hand?
I am going to assume this engine is equipped with cat converters. Specifically Adams 1985 V12.
If so, and if I read the referenced Bywater info correctly, these two ECUs were used up until 1985 on a cat converter vehicleā¦DAC2597 and DAC3586. .( I read the info as up until but not including 1985) ??
Then the 16CU for Lucas with catsā¦beginning and including the year 1985ā¦DAC4118, DAC4585, and DAC6335.
On Rogers referenced page, there are other ECUs referenced for non-cat vehicles and Marelli vehicles, which I have not included.
I think the vehicle needs a 16cu. What say others ?
One last comment, Adam says the 16 cu he has ādoes nothing.ā Meaning it does not pulse injectors on either bank. So maybe itās the wrong part number 16cu, or maybe as Richard Dowling indicated, the poor condition of the injection harness fried the 16cu that Adam tried out.
85s came with 6CUs; mine did. 16cu will work IF itās for the Lucas ignition system. There are various types of 16CUs, for Lucas or Marelli, with or without catalysts and O2 sensors. I retrofitted my 85 with the proper 16cu.
Adam,
You need be careful asking for a loan of an ECU to try out.
If you still have shorts in your wiring to +12V that will cook any transistors in the ECU.
The wiring for the injectors in the engine bay has wires that carry +12V.
No! Here is what he says:
" 16CU - MICROPROCESSOR CONTROLLER.
This is pin compatible and interchangeable with equivalent HE type 6CUs, at least on cars with Lucas ignition, as all the external components remain the same, however internally the 16CU is completely different, being a true computer system with a microprocessor and programmed memory."
As I said, I replaced the original 6cu in my 85 with a 16cu. Works great .
I have not seen that the 16cu is not a suitable replacement for a 6cu. I found a suitable 16cu for pretty cheap. Iām debating on buying it but now Iām gun shy since I may have fried 3 ecus unknowingly. After my harness rebuild I feel a bit better. Still, nothing I have done so far has had even a minuscule impact on the fuel injection problem. I admit I am easy to frustrate and this car has pushed all of my buttons. Do I buy yet another ECU? Having mine repaired is not an option as I donāt have the time to wait. My garage I lease is not going to be re leasing to me after March. Iām in a bit of a pickle and am having to now consider the prospect of selling or junking it and that hurts my heart and my pride. I love these cars but times ticking and my mechanical prowess has been tested beyond the limits of my knowledge. I look to you folks to tell me what I need to do next.