Car Mechanics (UK)

Elec­tronic Di­ag­nos­tics: Jaguar X300 3.2

Trac­ing and fix­ing faults in elec­tronic en­gine man­age­ment sys­tems

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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.

Fault 1


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.

Fault 2


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.


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.

Fault 4


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.

 ??  ?? NOTE: All references in our text and cap­tions to ‘left’ and right’ sides are from the point of view of some­one sit­ting in the car and look­ing ahead.
NOTE: All references in our text and cap­tions to ‘left’ and right’ sides are from the point of view of some­one sit­ting in the car and look­ing ahead.

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