X300 – Engine electrical, ECU and ignition

5 – Engine Electrical, Ignition and ECU ( )

This is of the conventional 12V negative earth type. The battery is located under the floor of the boot alongside the spare wheel.

Low battery voltage can cause incorrect operation of the engine ECU, resulting in difficulty starting the engine. Modern batteries are also prone to rapid failure so always suspect this before more complex items.

Various electrical systems continue to function for a few minutes after the ignition has been switched off and the car locked, so checking for unexpected current drain, wait for 5 minutes or so after shutdown with all doors closed. Typical drain is around 80mA.

The ignition system on the AJ16 differs from it AJ6 predecessor in having an ignition module fitted to the top of each spark plug. This is a significant reliability boost in that a central coil, distributor cap and HT leads have been eliminated and it is hard for moisture to enter the system and allow the spark to leak away.


5.1 – Spark Plug Choice ( )

The subject of spark plugs seems to cause as much debate as tyre brands, but in general it seems the AJ16 engine is as happy on the factory standard Champion RC9YCC as any other more exotic types. Some engines appear to have problems with more expensive plugs, with rough idle and misfires particularly when starting from hot.

To improve idle quality, Jaguar recommended a change to the slightly hotter RC12YCC plugs. These were fitted to XJR models in the factory from ’97.


5.2 – Spark Plug Replacement ( )

Remove the cap on the engine top cover by undoing the three torx head bolts.

Working methodically along the engine, undo each epoxy cased ignition coil and gentle pull upward to free the assembly from the plug.

Use a conventional plug socket and extension bar to undo and remove the plug. Unless the engine has recently been started and not warmed up, the plugs should be a light brown/grey colour. Whiteness can indicate a lean mixture or water in the cylinder from a blowing head gasket. Dark and sooty suggests a rich mixture, but is normal when the engine has just been started and stopped when cold.

Gap the new plugs to 0.035″ / 0.09mm and fit by tightening until they feel tight, but don’t overtighten. Reattach the ignition modules and gently rebolt them in place. Refit the engine cover top cap.


5.9 – Troubleshooting ( Jag_Fan,  )

Following the advice of Mark Stephenson – X300 Admin – I have split my guide into 5 pieces in order to be able to post the complete guide. This is part 1 of 5.

I would like to point out one thing first: You can call me an expert when it comes to electronics H/W, F/W or S/W design but I am NOT an expert when it comes to combustion engines. So, what I am telling you here – as far as things other than electronics are concerned – I just learned in the past one or two weeks.

We will start with some tests to figure out what the possible cause for you engine dying or hanging at some 1,500 rpm might be.


Borrow a starter battery from a friend’s car which is fully charged and not too old. It doesn’t matter if the amp-hour rating doesn’t match the 72 Amp-hours of the X300-battery. Even 45 amp-hours will do. Replace your ”old” battery with this borrowed battery and check if the problem is still there. If you don’t observe ”engine dying” or ”hanging at 1,500 rpm” any longer, get yourself a new battery and you’re done.


Now, here is what you should do to figure out if your problem is caused by too much (hanging) or not enough (dying) combustion air
bypassing. This can – but must not be necessarily caused by a weak battery

Once you observe the correct idle speed of some 800 rpm with the gear shifter in ”D” position with the car stopped and with the
engine already warmed up, pull the parking brake and leave the shifter in ”D”. Get out of the car and pull the plug which connects that four wire connector to the ISCV. This way you will have a constant amount of combustion air passing by no matter what the ECM does. Now, drive the car for a couple of miles and check whether the problem is still there. You will of course get an MIL indication in the dashboard telling you that there is something wrong with the engine. This is due to the fact that the ECM has a self test system onboard which also checks feedback concerning the ISCV. You can turn the engine off, start it again, and you can drive the car like this for a couple of days if you wish. You will observe some ”unusual” behavior when releasing the gas pedal at higher speeds but that’s about it. And of course you will have a slightly accelerated idle speed with the gear shifter in the ”N” position at some 1,000 rpm.

If the problem is still there you have a mechanical problem which has nothing to do with the ECM control mechanisms.

If it is gone you either have an electrical control problem (not necessarily the ECM itself – please refer to the other tests) or the plunger of your ISCV is stuck.


To make sure that the ISCV plunger is moved correctly remove the ISCV but leave the connector plugged. Now turn the ignition on (do not start the engine) and observe the plunger movement. Turn the ignition off and observe the plunger movement again. If it is moving properly in and out again every time you turn the ignition on or off the plunger and the stepper motor attached to it should be ok.


You should now check the Throttle Position Sensor. It delivers one of several input signals the ECM uses to control the ISCV. There
is a gold plated potentiometer located at the bottom of the throttle body and which is fed by 5 V through the ECM. With the ignition on you should check with an analog voltmeter whether the voltage is moving smoothly up and down while you are pushing the gas pedal smoothly up and down. You can attach the positive probe to pin 12 at P1105 of the ECM with the connector attached to the ECM and the negative probe to ground.

There is also another way. Turn engine and ignition off. Simply use an analog ohm meter and attach your probes first to the wiper and one of the two outer pins of the potentiometer and see if you get smooth movement on your ohm meter while pushing down the gas. Then repeat this procedure by using the other outer pin of the potentiometer.

You can either perform this test directly at the potentiometer with the connector removed. Or you can pull P1105 from the ECM and put
your ohm meter between pins 12 and 11 and then between 12 and 7.

If you observe interruptions or ”jumps” of the ohm meter’s needle you found the cause of your problem. You can try to ”repair” this by using contact spray on the potentiometer. But be careful when taking that part of the throttle body apart. In the workshop manual the say that after renewing this potentiometer the ECM will have to be reprogrammed. This means that some sort of adjustment will have to be performed which is done by changing some parameters in the ECM’s F/W. So you should try not to change the position of the potentiometer.

If the potentiometer is ok as well we can be pretty sure that your problem is caused by faulty output signals sent to the ISCV by the ECM.


You can now try to feed the ECM with a separate battery in order to avoid any supply voltage ripple confusing the ECM. Pull the fuse
of the ECM in the corresponding fuse box (see Vehicle Care Manual of you car). Take a broken fuse or produce one by blowing the fuse
section of a new fuse and remove the upper part of the plastic material so that you are able to access the top of the contact pins.

Solder a wire AWG 14 or better (copper diameter greater or equal to 0.064 inch) to the top of one of the contact pins. Put this ”adapter-fuse” in the place of the original one in such a way, that the soldered wire is at the end that leads to the ECM. (This is the end that doesn’t carry voltage from the car’s battery.)

Connect another AWG 14 wire to chassis ground near the location of the ECM. Connect this ground wire to your borrowed battery’s
negative post and connect the wire from the ”adapter-fuse” to the battery’s positive post.

Keep both wires as short as possible, just that they are long enough to reach the borrowed battery placed in front of the front passenger’s seat. Take the car for a test drive and check if your problems are gone now.

Or you can just disable the generator. In order to do this I recommend you to remove the attached regulator. Now, the battery will not be charged any longer. But with a new and full battery and no lights and fans on you can drive the vehicle for at least one hour. However, you should have a spare battery with you or at least a starter cable – just in case. Anyway, I consider the ”separate battery test” the better option as you do not have any other EMI sources ”polluting” the supply voltage of the ECM.

If the problem is still there you can be sure that high ripple caused by a faulty (generator) regulator or rectifier isn’t the cause of the problem. You will then have to repair the ECM as described later in this document.

If the problem is gone you should now try using a new starter battery. But first remove the separate battery and put the original ECM fuse back in its place.

If with the new battery the problem is coming back you should check the board voltage for high ripple. Please see Test ”Test #6” for


With a new starter battery high ripple can also be caused by a faulty regulator (JLM 11144) or by a faulty diode in the rectifier
assembly (JLM 11147). Part numbers referring to the XJ6.A first simple test is using an oscilloscope for checking the
ripple on your 12 V board voltage. A good point to attach your probes would be the front seat cigar lighter jack. Don’t attach
the probes directly to the battery. With the engine running at 1,500 rpm and all lights and fans off the ripple should not exceed
0.5 V. If it is greater than that you probably have a generator problem, regulator or rectifier that is.

To check whether the rectifier is the cause of the high ripple – one or more of the diodes are not working (open circuit) – you will
again need an oscilloscope. As there should be 6 half sine waves per revolution from the generator you will still be able to observe
a minor ripple which exactly should show those 6 ”peaks” on the board voltage in equidistant intervals. If the intervals are not
equidistant then one or more of the diodes are broken.

As the diameter of the generators pulley is smaller than the one at the engine the rpm’s of the generator will be greater than the ones of the engine. I don’t know the exact ratio. Let’s assume the ratio is 3.

This would result in 2,400 rpm at an engine idle speed of 800 rpm or 40 revolutions per second of the generator. With 6 ”pulses” per generator revolution this would result in 240 peaks per second on the board voltage.

You should set the oscilloscope to AC coupling and to 5 msec/unit so you can observe 12 peaks at idle. (Note that this is only correct if the ratio I assumed to be 3 is correct. Otherwise adjust your scope accordingly, so that you see something between 6 and 12 pulses on the screen). It might be a bit tricky to trigger the scope that way! Make sure that you don’t see any jitter. Otherwise you cannot observe if all peaks are equidistant.

If all pulses are equidistant than the rectifier works fine. One missing peak represents one faulty diode.

And this is how the regulator works:

The output voltage of the generator depends on the revolution per minute and the strength of the magnetic field generated by a special coil. In order to keep the voltage at a constant level this magnetic field is controlled by a power transistor which itself is controlled by a regulator which actually is just a comparator. This comparator compares the generated voltage fed to a voltage divider with a fixed voltage generated by a resistor and a zener diode. Depending on the generator voltage the transistor feeds this additional coil with more or less current to keep the generator voltage at a constant level.

The conclusion you can draw from these tests is as follows. If the rectifier is ok AND you have a new starter battery AND you do not
experience the problem with the regulator assembly removed from the battery, you should try renewing the regulator.


There are two ways of ”repairing” this.

a) Real Repair

You will have to open the ECM and – just to make sure – replace all axial and radial electrolytic capacitors. If you don’t want to replace all of them you should at least replace the ones that read 47 µF or above. The capacity should be the same or up to twice the capacity, i.e. you can replace e.g. a 47 µF capacitor with a 68 µF or a 100 µF capacitor. The voltage rating must be the same or higher with no upper limit. Usually 16 V should be enough. Note, that you can also solder new capacitors in parallel to the old ones. But in this case the capacity should be the same as the original one.

Make sure of all safety precautions concerning protection against ESD (Electrostatic Discharge). Don’t use soldering irons without an isolation transformer!

b) Substitute Repair

With this repair method you don’t have to open the ECM. Instead you will have to attach a filter between the ECM and its original 12 V power supply. The problem is that this way you will have to filter the sum of all currents drawn by the ECM. As the ECM is also feeding components like spark plugs for instance, it will have to be a pretty big filter.

What you need is a so called low pass filter similar to a filter used for bass loudspeakers. However the values of the capacitors will have to be much bigger due to the lower cut off frequency needed for the ECM application.

A simple low pass filter consists of a choke (coil) which is in series between 12 V and the ECM and a capacitor between the 12 V supply input of the ECM and ground near the ECM. If you use more of these in series ripple rejection is of course better.

The choke should have 1 mH (milliHenry) and the capacitor should have 100,000 µF (microFarads) and at least 16 V. The bigger the better is applies to both components.


This is what I once experienced with the lamp modules of my old XJ40: The were many bad solder joints which caused broken bulb
messages being displayed by the board computer all the time until I re-soldered everything in those modules.

If you have the time and are able to do this try to re-solder all solder joints on the printed circuit board of the ECM. But there
is of course still the chance of faulty components which you cannot find and/or replace that easily.

But there are also a lot of used ECM’s for sale and if you consider to get yourself such a used ECM make sure that it is the correct
one for your car. From the Workshop Manuals and the Part Lists I learned that there are many different ECM’s. So, make sure the
part number matches your vehicle. In case you don’t have access to that information please let me know and I will check it for you if
you give me the details of your vehicle including VIN.


There are a number of sensors that produce signals monitored by the ECM. And the ECM controls the ISCV based on those signals. The
important and most likely ones I discussed here. But there are also the ”Intake Air Temperature Sensor” and the ”Mass Airflow Meter” that might have an influence concerning the dying or hanging problem. But to my understanding this is rather unlikely. And if you suspect the Intake Air Temperature Sensor just replace it with a fixed resistor for a try.


Please remember that all the tests I tried to describe here are only referring to one single cause of the dying or hanging problem, i.e. the wrong amount of bypass air fed to the engine. Even though this is the most likely one in my opinion there are many other possibilities which I did not and could not cover here.