Can anyone tell me the location of the ECU on a 91 XJS 4.0ltr

 

On 94 4.0 it is under the passenger foot-well.
Most of the V12s I've seen are located in the boot between the right buttress support and the outer wing.

 

Thanks will try the footwell.

Colin

 

Hi Colin,

I'm sure equiprx may be right. The ecu position changed during the life of facelift XJSs. For some years, it was positioned on the outside wall of the footwell (effectively the inside inner wing) behind the carpeted panel.. If it's not where equiprx mentioned, then see if you can see a bulge in the carpeted sidewall carpet.

Cheers

Paul

 

Hi Colin

Before you do anything, Check the Fuel Pump Cut Out to see if it has 'Tripped' which is the little Plastic Box by the Passenger Side Door Jamb which has a Hole in the Top with a Button that needs to be pushed down to Reset it

Either use your finger, or the Blunt end of a Pencil or something like that

Next Check your Immobilizer which 'May' work by a Button on your Key Fob

If none of that works, undo the Cover of the Fuse Box by the Passenger Side Knee Panel, where just above those Fuses are 2 X Blue Relays, where a very common problem is that the Blades of those Two Big Blue Relays sometimes don't make good Contact in their Holders

So unplug and replug them and jiggle them around or even try swapping them over, or try Two Similar Relays of a different make with Thicker Blades as if they don't make good contact in their Holders, then She won't start

Location of Blue Relays that can cause starting problems

Location of the ECU: This is on the Passenger Side right down by the Footwell and underneath the Carpet where it is so well hidden from View that its a total PIA to even try and find it

One of the major failings with the location of this, is that Water can drip down inside the Trim Panel get into the Plug of the ECU and then Corrode the Pins

Location of the ECU on a 4.0L XJS

One more thing! I believe you said that you replaced the CPS Crank Sensor which is difficult to get to at the best of times, that despite your best efforts may still not be working like it should, so here are a couple of things that you can try

As the CPS is an Inductive Pulse Sensor that generates its own field

While Spinning the Engine over, keep an eye on the Tacho and see if the Needle Moves, not a definitive Test but a good indication

Though if you do want to do a definitive Test, unplug the Crank Sensor and put the Probes of your DVI Meter on the Pins of the Plug of the CPS but since this is also a PIA push a couple of small pieces of Plastic Tubing over the Pins, so the Probes of your DVI Meter has got something to push into

So you don't have to hold the Probes, then Set your Meter to Ohms and Crank the Engine (keeping your fingers well out of the way of the Belts and Pulleys etc)

You should get a reading of 400 to 1200 Ohms and if you change your Meter to Volts you should get about 2 Volts

If the CPS looks a bit Suspect then you can do a bit of a different Test on the CPS when its out of the Car, so get back if you want to do that and I'll show you how

Good Luck

Alex

 

All, Thanks for your advice and help. Will try and do the relays first as advised already done the trip fr the fuel pump and bypassed it but no joy. I have feedinto the boot relay but loks like its not getting to the ecu or its faulty.Will dive into the footwell and look at that connection. I am hoping that I havent bought a duff CPS.

Thanks

Colin

 

All,
Big thanks for the help and advice. Found the ECU and just replaced the main 30a relay next to it Bingo!! Fired first time and running sweet as a nut just got to put neverything back together agin now!!

Thanks

Colin confused but happy