Hello, I'm very confused by a 1995 Jaguar XJ6 vanden plaus that I'm working on as a mobile mechanic and I'd really like to help this person get their car running again. I've worked on jaguars before in fact typically I try to stick to working on unusual cars that other people won't work on. This one really has me stumped though.

Customer complaint is that for a couple of weeks it was guzzling gas but they kept driving it anyway, after that it died on the side of the highway out of nowhere. I scanned it for codes with OBD2 and it didn't have any. I checked for fuel pump and it was working. I tried to access live data but it says this car doesn't have live data. So I went to back downtown shade tree mechanic techniques and just unplugged sensors until it ran. Not my proudest moment. It doesn't help that I can't get the correct wiring harness information from alldata It only has the engine management diagram for a V8 supercharged vehicle.

With the mass air flow coolant temperature and intake temperature sensors unplugged it will run, idle and if you're easy on the throttle it will rev up. If you plug any single one of them in it will not start anymore. I replaced those three items. Obviously it did not fix it or I wouldn't be here. I started checking the ground at the ECT and IAT sensors and I got 18 ohms to ground with key on, and zero ohms with key off. This seems okay to me. I checked the 5 volt reference from the ECU the engine coolant temperature sensor with key on disconnected from the sensor I got the full 5 volts and connected I got 2.6 volts.

It doesn't matter which sensor I unplug or plug in, It only runs with all three unplugged. I'm not sure if there's a specific part of the computer that affects all three of those sensors.

I'm not sure what could have failed that would cause it to guzzle fuel and then not start anymore unless all of the sensors providing temperature information and air flow information to the ECU are unplugged. Hopefully someone else has had this happen and can help me.

Thank you all very much.

 

I would also like to mention that I keep thinking there's something wrong with the coolant temperature sensor, or related sensing circuits in the ECU because the cooling fan runs anytime the key is turned on even with the new sensors and with all the codes cleared from running it with them unplugged.

 

Yes on the ECT sensor to go into closed efficient loop , 165 C abouts

See page 78 as your original can be calibrated against a chart

this doc covers the earlier AJ6 , your AJ16 , and the V - 12

jagrepair.com/images/Training Guides/801S - 2000.pdf


Battery on my computer low so........................

 

Computer battery charged so back

The cooling fans had a TSB change so if done it will be in low mode with ignition key rotation , the coolant fans has a separate " switch " to control by coolant Temperature , the engine regulation ( closed loop ) is a solid-state thermistor 2 wire sensor atop the thermostat housing , the instrument cluster is a dummy gauge as the traditional temperature " bulb " 1 wire device next to the thermistor atop the thermostat housing

Your findings ;

" checking the ground at the ECT and IAT sensors and I got 18 ohms to ground with key on, and zero ohms with key off. This seems okay to me. I checked the 5 volt reference from the ECU the engine coolant temperature sensor with key on disconnected from the sensor I got the full 5 volts and connected I got 2.6 volts. "

Those numbers are way off , the valve cover is not a good test ground
at all , the cover is electrically isolated by plastic top hat bushings on the 10 or so mount bolts

The ignition switch is a ground path and if needed replacing no new key needed as the switch removes from key barrel

Never install O - ring on thermostat , even if comes with one , cracks thermostat housing casting

There is 1 fuse to heat the O2 sensors so they will read correctly , fuse # 14 / 10 amp red , right engine bay fuse box as point 33 circle double hash




The AJ16 engine will pick up OBD 2 and display live data through ELM - 327 software but can be hand held device dependent

To know MAF is alive , 1.2 volts DC on the middle wire ( Green / Pink ) at idle , this will be 4 grams / sec

To know TPS is alive 0.60 volts DC at the idle stop on the middle wire ( Green / Yellow ) , this will be 13 % open from a zero reference both sensors still connected

A good CKPS reads 1300 ohms as a basic meter reading

The supercharged engine regulate almost same and same wire colors as normally aspirated




In the starting sequence the fuel pump will run for 3 seconds only on key rotation then off , the pump will come back on for the duration of drive by the CKPS seeing engine rotation , safety design

The CKSP can fail on this model without giving a CEL P0335 code , and you may see 300 rpm tach on starter but no fuel pump enable

Ask questions , the unplugging of devises doesn't give you trouble shooting information but reverts the ECU to operate in less than optimal data maps to get you home , this reverting can happen during driving with a time dampener in both directions as the device comes back into expected range so can be confusing , this later in ECU developments on later cars was referred to as limp mode

Your whole car 1996 wiring guide : external lights control modules ( 2 ) on 95 vs. 1 on 96 :

That link will not pick up on this wifi hotspot

 

There are 2 power sources for the ECU used at all times , small relay # 9 in the corner of the right engine bay fuse box ( labeled Ignition Positive relay ) , and large right # 5 ECU Controlled relay . This relay is controlled by the ECU and brings high current power back into ECU and includes you fuel injector power " sitting " and 1 / 2 your engine sensors

Both these relays do fail hard and intermittently , place your finger on them and feel for click closed , swapping option is the small relay left engine bay fuse box ( relay only controls car horns ) and the large headlight ; fog ; or A / C clutch

The large relays can be in the wrong mounting position easily and you can keep these and the fuel pump relay commanded on by jumpering socket 3 to 5




There is the thing of the crash / inertia switch being fickle and tripping dropping out your fuel relay . It can be reset or remove connector and jumper the 2 white wires with a paper clip

The secret fuse to control the fuel pump relay requires the small relay in the corner of the fuse box stay closed , same left engine bay fuse box relay swap