Car Mechanics (UK)
Electronic Diagnostics: Jaguar X300 3.2
Tracing and fixing faults in electronic engine management systems
For our 300th edition of Electronic Diagnostics we’re investigating the Jaguar X300. This was a pioneering model in its time, so Kim Henson and Edward Haggar take a close look beneath the surface.
The X300 was the first Jaguar to result from Ford’s involvement and was a development of the XJ40 series (itself evolved from the XJ models produced from 1968 onwards). Much attention was paid to ensuring reliability and quality, as well as to its dynamic abilities. The X300
was widely praised for its excellent performance, handling, ride comfort, fuel consumption and interior ambience, and was hailed as one of the best luxury saloons of its time.
Under the bonnet was a straightsix engine (AJ16) offered in 3.2- and 4.0-litre versions, both with electronic engine management. Although not the subject of this article, a V12 version was
also sold, along with the supercharged six-cylinder XJR. The X300 was produced until 1997, then replaced by V8-powered variants (X308) on a similarly-styled theme.
The X300 bristled with electronic control systems and inter-related wiring, fuseboxes and relays. An indication of the sheer complexity of the vehicle is that the official Jaguar Vehicle Service Manual runs to nearly 1500 pages! If you own an X300, it is worth obtaining one of these, which is packed with useful information, as are Jaguar’s X300 handbooks.
For this feature we are investigating a 1994 X300 3.2 Sport, not supercharged. Our guide to the diagnostic aspects of this engine and its Sagem-lucas GEMS 6 management system is Edward Haggar.
The straight-six motors are renowned for being bulletproof and, given regular oil and filter changes, are capable of clocking up 250,000 miles or more. The automatic transmissions and final drive units are similarly strong and longlasting, provided attention is paid to lubrication, in particular ensuring that the transmission fluid and final drive oil are clean and of the correct specification.
If the vehicle is new to you, check its service history. If in doubt, carry out a full service including all filters and engine oil. Always check the ignition coils, removing and inspecting them for signs of blisters/electrical tracking.
A basic method of checking for coil operation is to remove each spark plug in turn, reattaching its coil/high-tension connector and cranking the engine to check for a spark at the electrodes. Take care, though, to not touch the plug, coil or leads while doing this, at the risk of electric shock.
As these cars age, problems can occur with the multitude of electrical connections, in particular the ECU and its plug/socket assemblies. Moisture entering these assemblies can result in running difficulties, including a very high or fluctuating idle speed. Eventually this will lead to an engine that won’t crank (see Fault 1). The reason for the fault may not be immediately obvious. Indeed, many X300s have been scrapped due to being unable to identify the cause.
Other electrical ailments create havoc, too, so we advise carrying out a visual inspection of all electrical wiring, connectors, fuses and relay boxes around the vehicle. It is worth noting that specific information on fusebox location, fuse numbers, rating in amps and circuits protected, is given within the Roadside Emergency Service section of the Jaguar Vehicle Service Manual.
Not shown here is the crankshaft sensor, mounted at the front of the engine. This can fail completely and the engine won’t run, but when it starts to break down, it can behave erratically, giving inconsistent running and a reluctance for the engine to rev freely. An initial check is to observe the tachometer needle as the engine is cranked on the starter motor, which should move to 200rpm if the crankshaft sensor is operating (although this test is not 100% foolproof ). A simple resistance check on the sensor will reveal a failing within the windings.
Closer to the top of the engine is the camshaft position sensor (also not shown here). If this sensor is failing then the engine may suffer from extended cranking before it starts and runs normally. Check that the sensor is correctly positioned and that the electrical connection is good. Fitting a new sensor is not difficult, but it needs to be correctly set up. Note that, on the X300, some relays are switched via the earth side of the circuit.
During your diagnostic work it may be necessary to dismantle individual relays to test them. In each case, check all relay connections for signs of corrosion.
MOISTURE INGRESS IN THE ECU
As X300s have aged, this fault has become more prevalent, manifesting itself with the engine refusing to crank. Before you waste time testing the battery
and searching for a break in the circuitry between the battery and the starter motor, take a close look at the ECU and its connections.
The ECU is located within a small cavity to the right of the right-hand front footwell, and is normally hidden behind a cover panel. Rainwater can enter from above as a result of a leaking windscreen seal or sunroof. As the ECU’S electrical connections are on its top, the water can soak through into the ECU itself.
Before disconnecting anything, check what happens when the ignition key is turned. If the 12V battery is fully charged, all the dash warning lamps should light up when the ignition is turned on, then extinguish within a few seconds. After this, the ‘Engine Check’ light in the lower left-hand corner of the instrument panel should re-illuminate to confirm that the ECU is OK. If this lamp doesn’t light, there’s a problem with the ECU.
To gain access, disconnect the battery and then carefully release the cover panel and detach its rear vertical edge from the door aperture seal. Once you have access to the ECU, detach the twin connector socket assemblies from the top of the ECU, then unbolt the ECU from the car. Note: A vital earth cable is attached to one of the securing bolts; remember to reattach this on reassembly.
With the ECU out of the vehicle, examine the connector pins – they should be dry and with no evidence of corrosion. Also inspect the socket assemblies that connect to the ECU, which should also be sound, dry and clean. Electrical contact cleaner spray can be used to deal with minor dirt and corrosion on the connector pins/sockets. However, in severe cases, one or more of the pins may have completely dissolved.
If things are really bad, water may also have run down into the ECU. The ECU’S cover can be unscrewed to check the internal components. If you find corrosion or moisture within, or if one or more of the connector pins has been damaged, you will need to either replace or refurbish the ECU. Prices for this can vary from £20-£200 secondhand.
It is essential to fit a compatible ECU, ideally from a vehicle very close in age to yours and with the same part number. Fitting an earlier or later unit may allow the vehicle to start and run, but could result in running difficulties, such as a markedly high idle speed or a fluctuating idle. The number ‘211’ after the ‘/’ in the part number indicates that the unit is for a car with automatic transmission, whereas ‘212’ denotes a manual gearbox.
Before installing a new ECU, it is essential to cure the moisture ingress problem, either clearing any blocked drainage channels or replacing the windscreen’s rubber seal. If the seal is still sound, drying and cleaning the groove in the seal and applying a butyl rubber/mastic type of sealant can do the trick. When replacing the ECU, the generous application of petroleum jelly or water-resistent silicone grease around the connector block assemblies at the top of the ECU provides a line of defence for the future.
STICKING THROTTLE BODY FLAP
With this problem the car won’t idle at the recommended 800rpm, but instead rises to 1500-2000rpm with no throttle applied. This is potentially dangerous, especially on automatics, because the brakes need constant application to keep the car in check in slow-moving traffic. It also means the vehicle would fail an MOT or roadside emissions test.
Try cleaning the throttle body flap and adjacent areas in the throttle body assembly. If the idle speed is still too high, clean the idle speed control valve attached to the throttle body. If cleaning doesn’t effect a cure, it may be found that it is necessary to renew the idle speed control valve.
Another possible culprit is the throttle position sensor. You can test this using a digital multimeter set to ‘DC volts’ on the signal wire, looking for a gradual increase in the voltage as the throttle moves. If the voltage reading sticks, this is a sign that the internal track is worn; an oscilloscope will show the fault more clearly. If your car’s sensor is worn, you will need a new one or a complete secondhand unit in good condition.
It may also be found that moisture in the connector block assemblies serving the ECU are causing the problem (see also Fault 1). Restoring the management system to factory settings using a suitable diagnostic tool may temporarily reset the idle speed to the correct levels, but the trouble will reoccur as more moisture takes its toll.
Note that corrosion anywhere in the electrical circuitry can cause havoc as the resulting high resistances can affect the signals being relayed to the ECU.
Fault 3 CHOKED FUEL FILTER
Neglected X300s can suffer a multitude of running problems due to lack of maintenance. These include a reluctance of the engine to start, inconsistent firing/hesitation and a feeling that the engine is holding back and not delivering full power. Rough or inconsistent running can result from worn out or incorrect specification spark plugs and/or a choked fuel filter. The engine is sensitive to spark plug resistance and if the coils are having to work harder due to worn plugs extra strain is placed on them. Worn-out spark plugs may manifest themselves in the form of a mild misfire, eventually building to bigger problems.
The fuel filter is often ignored for long periods due to its inaccessible location beneath the rear of the vehicle. Because the original mild steel fuel pipes in the vicinity are prone to damage as the old filter is removed, you should work with care and ideally renew the fuel pipework in this area with longer-lasting copper pipework and brass unions.
BRAKE LAMP or SWITCH FAULTS
If brake lamps are non-operational, the X300 is designed so that the driver will not be able to engage ‘Drive’ or ‘Reverse’ due to the selector interlock mechanism preventing engagement as the brake pedal is pressed. Check the brake lamp bulbs and ensure that the brake lamp switch is operational.