Injector Problems with Jaguar XJS V12 TWR!

Hi all,

I am Josep from Spain and I am the new owner of a Jaguar XJS HE v12 TWR w Lucas.
I bought the car a month ago, after 10 years totally stopped.
After changing all the filters and oils I have managed to start the car.
The car starts correctly but a lot of petrol comes out of the exhaust lines.
I have been reading the forum for a few days now, making checks that have been discussed in the past:

  • Changed the plugs
  • Put 9v on each injector listening to the noise.
  • Communicate the injection ramp sensor to see if it was the error.
  • Check if there is no fuel in the vacuum hoses

After doing these checks, I have seen that with the ignition open and the engine stopped, I have 12v in all the injectors of the Bank B, on the other hand, in the bank A there is no current.
The reason why petrol comes out of the exhaust I understand is because the injectors on bank B are always open.
Could it be that I have a short circuit in bank B, because if the injectors are at 9v, I don’t think it is normal to have current with the contact open, and even less so at 12v.
If so, where can I start? I have disassembled the a/c compressor to see the wires better but I don’t see anything strange.

Thanks in advance for your help, a great forum.


First, you should check the coolant temperature sensor for the EFI system. If like a regular Digital P, it’ll be in the left side thermostat housing, pointing straight up, and has the same type connector as the injectors. Also, check the connector; sometimes it’s the connector itself causing trouble.

However, being a TWR, you might not have a regular Digital P EFI system. It’d be helpful for diagnosis if you could report on your EFI system, possibly by just snapping a pic of the ECU in the trunk, preferably showing part numbers.

The circuits for the injectors are pretty simple. There is 12V applied to all injectors, easy to check with ignition on. The injectors are fired by completing the circuit to ground through the ECU in the trunk. There is a resistor pack near the right side headlight bucket that limits the current through the injectors (lest they burn slap up!). Note that the wiring groups the injectors in four groups of three. Within the ECU those groups are merged into two groups of six, each group being one entire bank.

It should, therefore, be pretty simple to check the wiring and the resistor pack for continuity. Here’s the bad news: If the wiring and resistor pack are OK, chances are there’s a problem with your ECU. Repairing or replacing with the regular Digital P unit would be problematic enough, but if TWR used something special it’ll be even more challenging to fix. There are (or were) a couple of TWR owners on here, hopefully one or another can help you.

Hi Kirbert, Thanks very much for your help.

I have checked the temperature sensor. The sensor has a resistance of 2.22 Omhnium at about 20 degrees so I think it’s correct.
In relation to the connector I have seen it OK. It is true that a few centimetres above it has insulating tape as if it had been repaired, so tomorrow I will remove the tape and check that everything is OK.
How can I check the continuity of the temperature sensor wires?

Regarding the EFI system I think it’s the common one for the 5.3 but anyway I’ll try to attach some pictures.

My wires going to the injectors look pretty damaged at least the sheathing, I’m pulling out where the tape is to see if there are any shorts.
I have removed the resistor you mentioned and opened it up and apparently it looks ok.

I’m going to check the wires of the line to see if there are any broken wires as they don’t appear to be in good condition.

Thanks in advance




Coolant sensor wiring repair

Yep, looks like plain ol’ Digital P.

Thanks Kirbert.Thank you very much for the article, very great I’m going to need to build one. Is there by any chance any jaguar specialist that sells them already with the cables prepared for the high temperatures?
It seems to be my problem, I must have some wires broken or making contact with ground.

I have a doubt about the correct operation:

  • With the ignition on, should there be 12v at the injectors? or should there only be 12 volts with the car started?

Right now I have:
Line A. With the ignition on, there is no 12v on the injector connector pins.

Line B. With the contact open, I have 12v on all the injector connector pins.

I understand that the problem is only on Line B?

Best regards.

I think your problem is on Bank A. You’re supposed to have 12V at all injectors with the ignition on.

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To chime in here, I have a few diagrams that might help.

In answer to your question, all injectors should receive +12V from the “Main Relay” on the pink-black wire side with ignition on. The ECU opens, then HOLDS the injectors open via grounding the control wire side.

If there’s a suspected short in the control wire side (negative) causing injectors to hold-open, then the thing to do is to disconnect the battery, disconnect the computer, and disconnect the engine EFI harness at the right wing near the power resistor. Also pull the power resistor. THEN check for any continuity on the control wires to ground at BOTH the injector harness plug-in, AND at the power resistor plug (2 different pathways - On VS Hold). If there are none, then the ECU (or power resistor - unlikely) are at fault. Otherwise you’re tracking down a rub-through, or errant fastener driven through the sheet metal into the main harness (yes the latter happened to a guy who installed a 5 speed… made a group of 3 injectors dump fuel).

Note connector 6 @ the ballast resistor

Check the non-pink/black connector pins for pathway to ground:

Besides testing the resistance across pins, you want to test the HOLD pins on the harness side for continuity to ground (without the ECU present).

If the ECU ends up being suspect because there’s no continuity to ground w/o it in place on any pins, you can always plug the ECU back in and re-check for continuity to ground on the same pins w/o battery. I suppose it could be checked w/battery & key on if it’s a control fault rather than a dead short.

~Paul K.

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I see you have a DAC3062 ECU so:

No cats, no o2 sensors:

Paul K.

Hi Paul and Kirbert,

Thank you very much for your help.

I will do the checks you suggest and report back.

Best regards and thanks :slight_smile:

Hi all,

Thanks for your explanations.

After your explanations I have checked and I see that I have two problems:

  • Bank A: I didn’t have 12v, now I have power.
  • Bank B: I have 12V but I also have ground when I turn on the ignition, that is, if I remove the injector connector and plug it back in I hear the clik, therefore, I understand that I have a ground problem since I should only receive ground when the engine is started, is this correct?

Thanks in advance.

Ground is the control mechanism to operate the injectors, so yes, the ground side should be open circuit until the ECU recieves the RPM signal from the amp on pin 18 and begins injection by the peak and hold method (IE one set of MOSFETS/Transistors OPEN the injectors with a high current load, and then instantly a second pathway going through the power resistors takes over to limit current flow & keep the injectors open for the required amount of time.

With ignition off, or even battery disconnected, I’d check pins 1 & 5 on the car side/ecu side plug for the injector harness where it plugs. (See diagram above of injector harness layout.) Those should be open circuit. If they’re NOT open circuit unplug the ECU. IF they’re still not open circuit you have a rub through or fastener going into the wiring harness (probably) causing continuous ground.

If on the other hand they’re open circuit as they should be, turn the key on w/ battery in place and check again. They should STILL be open circuit. IF they’re not, (i.e. one or both pins go to ground w/ key on) then I’d suspect the ECU for having a control issue that cause the mosfets to go short circuit upon power up. ECU problem.

~Paul K

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Dear all,

Sorry for the delay in replying, I have not been able to validate it until today. The problem I had where I have ground in one of the banks seems to be with the ECU. Let me explain:

I have one side where all the injectors are continuously open.
I have been unplugging as you told me and in the harness connector I always have ground only with the contact open, so I discard that they are the harness.

I disconnected the resistor and I still had ground with the contact always on in the line of the injectors of bank B, so I thought it was the ECU.

I went back to the ECU and checked the pins that should have ground when the engine is started, to see if there really was a short circuit with the body of the car but it wasn’t because there was no continuity between the ECU connector and the ground.

I am going to try to find a second hand ecu and I will tell you the result.

Best regards,

Ok, so to verify, if you unplugged the injectors, and the ECU, then checked ALL the plug-side pins for the injectors, you get open circuit? (That’s the way it should be). If so, then yes, the ECU would be suspect.

~Paul K.

You mean only with ignition ON?
If so, then yes, the ECU might be at fault.

Yes, this would mean your harness is not shorted to ground.

Hi Josep,
I had the same symptom you have with petrol pouring out of the exhaust. I pulled the fuel rail and turned the key(after I put each injector over a container). One bank on mine was completely stuck open! I have a cheap ultrasonic cleaner so I pulled them one by one, disassembled it, put it in the cleaner then rebuilt it with new parts. I tested each one after rebuilding and the injectors were good as new. You may try pulling the rail to see what is going on with the injectors. Not a bad job.


Hi Paul,

Correct, I disconnected all the injectors, I could see that I had ground with the contact open.
Then I disconnected the resistor, and I still had ground on the injector connectors, so I saw that it was the ECU that was always giving ground to the injectors with the ignition on.
The mechanic checked the pins of the ECU to verify that there was no crossover with the body of the cable going from the ECU to the front of the engine.

Best regards.

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Hi Aristides,

Yes I mean only with ignition ON.


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The problem is that if they are always receiving power regardless of whether they are stuck, they will be injecting fuel all the time.

I did the test of putting power to each injector and I could hear the noise of opening and closing.


The harness tested OK
The injectors tested OK
IIRC, the ballast resistor interferes at the positive side and has nothing to do with the ground.
I think we can now say with 99% certainty that the problem is the ECU.