EXCELLENT write-up, Kirby!
I never really delved into the V12 wiring harness issues, and maybe you could make a kind of 'permatopic," on this new site, listing all the links from the old site that pertain to V12 harness issues/repairs.
Hi all!
Thanks a lot for your support.
Finally I found an ECU the same as mine (3062 code) that assured me it was ok.
I mounted it, and checked the injectors and it was perfect! it doesnāt always ground on the injectors!
However, my ECU when I turned the key on the fuel pump for a short period, to fill the circuit. However, this new one does not activate the fuel pump so I think this ECU is also defective.
I am talking to the owner of the ECU to return it, could someone confirm me that it is correct that when I give the ignition the fuel pump turns on for a short period of time?
Thanks in advance.
So, if I understood correctly, the ECU does not run the pump with just the ignition on but runs it when you hit the starter and the engine woks? What happens if the engine stalls, the pump keeps running? Maybe you should check that.
Normally all 6CU ECUs should work the same and should turn the pump On for 2sec and then stop if the engine is not running.
My DAC3586 6CU (Cat version) that I had before and the DAC6335 16CU that I have now do that.
Itās bizarre because the ECU uses the signal from the ignition amplifier for not to turn the pump Off and when the pump circuit fails itās either On all the time or simply doesnāt turn On at allā¦
The idea behind this is not to prime the lines but a safety issue, to shut off the pump in case of a major problem/accident and fireā¦
Hi Aristides,
I think I have not explained myself well.
My new ecu, never makes the fuel pump work, neither with the ignition on or anything else, so the car does not start because it does not get fuel.
However my previous ecu did run the fuel pump, it was to confirm that the new ecu is faulty Because if one makes the pump work and the other doesnāt, it is not supposed to be any other part that is wrong.
thanks in advance.
Yes, with key-on (engine off) you should hear the fuel pump whirr up for a few seconds pressurizing the fuel rail. For the 6CU and 16CU the ignition pulse sustains keeping the pump on. The +12V signal from the starter relay (eliminated in later ECUs) may play a role in restarting the pump during cranking. The injectors, however, would not pulse if the ignition pulse signal wasnāt reaching the ECU, so clearly that should be letting the pump run.
The ECU runs the pump by providing a pathway to ground for the control side of the Fuel Pump Relay.
~Paul K.
OKā¦ now I see the problem there. The ECU is definitely defective.
Just for troubleshooting though do this before you return it:
Run the pump manually, i.e. bypass the fuel pump relay or connect it directly to the battery, and see if, and how, the engine runs with the new ECU.
Itās very possible that only the fuel pump circuit is broken and otherwise the ECU works.
If both banks do work, it will confirm that your old ECU is the culprit.
Good luck.
Dear all,
Thanks for your support. Iām searching a new Ecu and I find a v12 ecu but is not the reference DAC3062. The reference is DAC4478, do you know if is compatible?
Best regards and thanks in advance.
Yes itās compatible and itās a 16CU which is better.
V12 ECU
6CU Part Numbers and Applications
DAC1926 6CU 1980 pre-HE OPUS 9.0:1 O2 Sensors
DAC1936 6CU 1980 pre-HE OPUS 10.0:1 NO
DAC2596 6CU 1980 HE CEI 12.5:1 NO
DAC2597 6CU 1980 HE CEI 11.5:1 O2 Sensors
DAC3062 6CU 1981 HE CEI 12.5:1 NO
DAC3586 6CU HE CEI O2 Sensors
16CU Part Numbers and Applications
DAC4118 16CU 1985 HE CEI 11.5:1 O2 Sensors
DAC4119 16CU 1985 HE CEI 11.5:1 NO
DAC4478 16CU 1985 HE CEI 12.5:1 NO
DAC4585 16CU 1986 HE CEI 11.5:1 O2 Sensors
DAC4586 16CU 1986 HE CEI 12.5:1 NO
DAC6335 16CU 1989 HE CEI 11.5:1 O2 Sensors
DAC6336 16CU 1989 HE CEI 12.5:1 NO
DAC6337 16CU 1989 HE Marelli 11.5:1 O2 Sensors
DAC6338 16CU 1989 HE Marelli 11.5:1 NO
Thank you very much Arisitdes, then I will buy it. Why do you say it is better? Is the performance of the car better than with the other one?
Thanks
I think the 16CU is generally much more dependable than the 6CU. Just think of how much PCs changed from 1979 to 1986.
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The 6CU suffers from cold, cracked solder joints which can cause lots of issues. I replaced mine on my '85 with a compatible 16CU after suffering severe intermittent issues with the 6CU. I reflow a lot of solder joints in the 6CU, and it started working correctly, but I never trusted it. The 16CU does not seem to have those issues.
You just have to ensure you get the right 16CU, Lucas or Marelli ignition, with or without cats and O2 sensors, etc. Once you get the correct one, itās plug and play.
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Thanks a lot! I bought ecu reference DAC4478 and its working correct in my Jaguar! Thanks a lot and best regards. 
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Hi guys, my name is Arek, Iām in US New Jersey, Iām kinda new to Jags. I bought my XJS few years ago as a project. The car is 84 XJS HE US spec.
Its Lucas ignition with 2 coils.
Now Iām trying to get it running and you can imagine that i have few problems.
I started another post because i had no power to coil, thank to the members and great diagrams i was able to solve that issue, getting good spark.
Now another problem is that injectors are being open with key on dumping fuelā¦
here is what i did so far:
-all injectors resistance test OK
-I chose 3 random injector, applied power and i hear them click
-power resistor 2PR resistance test OK, there is NO continuity between any pins and body of the resistor
-injector wire harness is brand new ( just got it, old was crap)
-coolant sensor resistance OK at the sensor
- there is NO continuity to ground on any pins on injector harness ( car side going to ECU) w/o ECU present, same with ECU present, battery not connected
- with everything connected i unplugged 2 injectors on opposite banks and Iām getting 12V on one pin and 9.4V on the other pin, weird? do i supposed to get voltage on both pins on the injector pigtail?
- with injector harness disconnected, key on, measuring injector harness pigtail ( car side) Iām getting 12V on pins 2,3,4,8 but also Iām getting 0.7V on pins 1 and 7, 1.6V on pin 6, 2.5V on pin 7
- ECU is DAC2597, i had sent it for repair and got it back , same issues, Iām not sure if they did the repair correct or if they did anything to it, the repair shop stated that they found hardware issue and repaired but i have doubt if they even opened the ECU. there are small screws holding the top cover, 2 of them got aluminum covers on them, like a some kind of seal, i would assume that those covers needs to be removed/damage in order to get to those screws, after repair those covers looked like not being removed.
i donāt want to like a fool talking to the computer repair shop so i would love someone input on this issue because i still think that ECU is at faultā¦ this is a shop that repairs all brands of computersā¦
unless according to everything i stated, did i miss anything that maybe a fault besides ECU
Any input will be greatly appreciated
Thanks in advance
Arek
Not sure, bit I think it means your injectors are grounded permantately.
Measuring voltages on live transistors can give weird results.
For the ground pin you should measure continuity and it should normally show no path to ground with ignition ON and engine OFF.
How can one repair hardware issues on a ECU without taking the cover off is beyond my kenā¦
Maybe they have magic powers?
You should ask for your money back.
If you are 100% sure that your harness, resistor pack and injectors are all ok the only other possible reason your injectors are staying open is the ECU indeed.
Aristides, i had an extra hand today so i did little more testing. There is no path to the ground on any pins on the harness or injector pigtail with key on.
when key is turn on ALL injectors switch on, i just touched both ends of the harness and injectors switch on. what i did i put a voltmeter between hold pins and power pins, when key turned on i get 12V so it meant the the circuit get completed inside the ECU switching all injectors on. I reached out to the computer repair shop and they told me that it maybe intermittent problem or there equipment is faulty or the problem is beyond repair. well, the problem is not intermittent because it switches injectors on every time key is on. I think they are not capable to test or repair Jag ECU.
If you or anyone knows repair shop in US or have compatible good ECU let me know, the one i have is DAC2597
thank you
Is that a 6CU or a 16CU? The DAC part numbers change but if you match 6CU for 6CU and 16CU for 16CU youāre probably good to go with a used unit.
#1 best repair shop is http://aj6engineering.co.uk/ but heās often on vacation, ill, or backed up.
You probably would be better off with a used unit. Best prices are on car-part.com because you arenāt paying specialist Jaguar prices, its just junkyard prices.
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John, according to Aristides it is 6CU
DAC2597 6CU 1980 HE CEI 11.5:1 O2 Sensors
Iām not sure if i want to send ECU across the pond, it will take forever to get it back ā¦ Iām just not sure what would be compatible getting for a junkyard
Check with Jaguar Heaven in Stockton, CA. They have a website but you will need to fill out a part request form, or call them
I have found this one on a car-part thank to John
DAC4585 16CU 1986 HE CEI 11.5:1 O2 Sensors
seller says that unit is testedā¦
the one that sits in the car right now is
DAC2597 6CU 1980 HE CEI 11.5:1 O2 Sensors
I believe that it would be compatible, correct? I just want to verify with you guys if i can change to 16CU
thank you
The general consensus is yes, it will work, and be more reliable. See this thread:
https://www.jaguarforums.com/forum/xjs-x27-32/upgrade-6cu-16cu-172051/