I recently had an "Engine System Fault" error and thought I would share with the forum how I resolved the underlying issue for a supercharged X350. The Engine System Fault is a particularly troublesome fault situation because it disables several systems (DSC, park brake and throws the car into limp mode where speed is limited to around 10 mph) and does not necessarily throw any engine trouble codes.

The forum posts where this problem is discussed tend to identify two primary causes:

1. low battery; or
2. a problem with the throttle position sensor or its ability to communicate with the ECU

Here are the steps that I followed to identify and resolve my issue, with some pictures at the end.

1. The easiest possible cause to eliminate is the battery. Remove the battery and have it tested. In my case, it came back looking good, so an aging battery was not my problem.
2. The next item that could be at fault is the throttle body sensor, which is integrated into the throttle body, or its ability to communicate with the ECU. Because it could be a communication issue, you should not race to replace the throttle body and should instead do some additional checks.
3. There could be corrosion of the contacts in the connector to the throttle body sensor. These connectors are prone to water damage (as has been documented well on the forum). Remove the connector to the throttle body (the 4-wire one, not the 2-wire one) and inspect it for corrosion on both the male and female side of the connector.
4. If there is no corrosion on the connector, you should then check for continuity between the connector and the ECU plug. Water damage to the ECU plug has been documented, and so the ECU plug can be visually inspected. Additionally, the + and - supply lines have splice points where multiple wires split off from one, and these can fail over time. The ECU connector is underneath the cabin air filter housing; you will need to remove 4 10mm nuts and 1 10mm bolt to remove the cabin filter housing and splash tray, and you will also need to remove the windshield cowling for access. The ECU plug is attached to the firewall using a TS30 bit, which is a 5 prong (not 6) security bit. These can be found on Amazon. A standard T30 torx bit will not work.
5. Grab a copy of the X350 electrical manual, some of these are on the web. Look up the ECU pinout diagram and check for continuity using an Ohm meter from the ECU plug to the throttle position sensor connector. If you find any wires with high or infinite resistance, congratulations, you've probably found your issue. Note: I found that 1/4 lb. safety wire fit the ECU connector holes well, and I then connected the safety wire to my multimeter with alligator clips.
6. If your connectors are free of corrosion and you have continuity across all wires, then you're probably looking at replacing the throttle body. I have seen postings where it was described that only the throttle position sensor was changed, but I have not done that myself and so can't confirm that that approach works.

In my particular case, the car had already had the throttle body connector replaced years back because of water-related corrosion. But upon inspection this time, the connector looked excellent, with fresh copper exposed on the connection points.



So, the next step was to test the wiring harness from the TPS connector to the ECU plug. Here are pictures of the TS30 bit and ECU plug removed.




Using my multimeter, I started checking continuity of the wiring harness. I started with the ground source for the throttle position sensor, which runs from pin 019 on the ECU plug to the TPS connector, it's a black and green wire in the 2005 XJR. It was immediately clear that something was wrong. The multimeter showed infinite resistance until I twisted the connector's pigtail at the solder point and would briefly flash continuity. It was then clear that the joint where the harness was repaired and the new TPS connector attached had failed.

Much to my surprise, the underlying issue turned out that while the prior mechanic had soldered 3 of the 4 wires in place, they had missed the solder for the ground wire. For several years, it had only been held together by shrink tubing. I was easily able to simply pull the wires apart at the un-soldered joint. Here's a picture of the ground wire with the heat shrink tubing removed:



I ended up re-soldering all 4 wires to the connector. While the other 3 wires did have solder, the penetration of the solder was not particularly good and it was clear that soldering wax or flux had not been used.

With the connector repaired, I verified continuity with the multimeter and then went for a test drive. All was well!

There may be other causes of an "Engine System Fault" and I am not a mechanic but given the lack of information on the forums I thought I would post this, and hope it is helpful to someone.

 

Good report. Anybody know the reason to use an odd type of screw head for securing the ECU connector ?
Unless the screw thread is equally odd I would replace it with something sensible.
Although if that screw is a specially made captive type it might be more of a challenge.

 

Thank you for this great report!

 

scooterforever,

Thank you for taking the time to photograph and post your diagnostic process and repair. Your report will be helpful to others for years to come!

The bit required is called a "5-lobe Tamper-Proof Security Torx" bit, so it seems obvious that the Jaguar or harness engineers thought it should be difficult for shadetree mechanics like us to disconnect the ECM without sufficient knowledge of the risks and without taking proper precautions (disconnecting the battery and allowing the keep-alive-memory capacitors to drain down, taking steps to prevent electrostatic discharge, etc.). It is very possible to brick the ECM if we're careless.

Cheers,

Don

 

Yes and no Don.

Yes because as you say there could be harm to the electronics if the plug is removed without following a safe procedure first.

No because with a small amount of effort anybody can buy a suitable bit to remove the screw.
Would that anybody know about a safe procedure ?
Better to have a standard screw and a warning notice "Risk of major damage - refer to safe procedure before removal".
The oddball screw head has little advantage.

 

Without this forum, very few shadetree mechanics would have any idea what type of tool that fastener requires for removal. And during the years the X350 was manufactured, it was not nearly as easy to find and purchase a set of correct bits as it is today. In fact, there are plenty of accounts on this forum and others of owners mangling the head of that screw out of ignorance, so in that regard you could say the security screw has been effective.

Jaguar isn't the only automaker to use 5-lobe Tamper-Proof Security bits to dissuade the uninformed. I've encountered them on Mercedes-Benz, BMW, Porsche and Chrysler vehicles, and not only on ECMs/PCMs, but on other modules and components as well. Tesla uses male 5-lobe screws on some electrical connectors that require a female 5-lobe socket.

Regarding a warning label, that's never a bad idea, but it could apply to many connectors in any modern vehicle, and Jaguar and other automakers assume that the vast majority of their vehicles will be serviced by trained professionals, at least during the warranty period. I don't recall removing many ECMs/PCMs/DMEs/ECUs/Motronics that had labels warning of the risks of damage. There may have been a few that mentioned the risk of damage due to ESD.

Cheers,

Don

 

Perhaps the saving grace is that the percentage of DIY car owners is dwindling.
Partly because people can more readily afford to dump problematic cars and buy a new one, partly because there is less interest in DIY remedies, and worst of all ( in the eyes of we true believers ) you MUST take it to a dealer with all the diagnostic gear to fix some problems.
Therefore why make anything easy for the DIY brigade ?
Got to say I will dive into any of my two XJSs to fix a problem. Much less enthusiasm if my XF ever plays up.

 

Noting the ECU removal risks that Don points out, and the specialist tool bit required to remove the ECU, you can do a partial check on 2 of the 4 wires to the TPS sensor. For the 2005 supercharged cars (may be the same with other X350s, but I only looked at the 05 supercharged wiring diagram), the following wires are common and shared with the following components:

• Black and Green wire --> shared with the BG wire going into the IAT sensor that is integrated into the MAF sensor
• Orange and Yellow wire --> shared with the OY wire going into the MAP sensor behind the throttle body / intake elbow.

So, you can remove the MAP and MAF clips and test for continuity between those points and the BG and OY wires on the TPS connector. It will not tell you if you have a problem at the ECU plug, but if you did have a problem at the ECU plug it would also (theoretically) cause you an issue with MAP and MAF sensors and there might be codes pointing to the MAP and MAF issues.

As for the remaining R and Y wires on the TPS connector, those go straight to the ECU plug and do not appear to be shared with other components (sensor return values I presume).