Hey all, been a bit since I have been on but finally got my XJS engine running but I found when I plug in the O2 sensors my fuel pump would turn on without the fuel pump relay. I traced it back where the ground plug for the heating of the the O2 sensor has 12V, but should be just a ground. When I plug it in it back feeds power through the fuel pump relay and turns on the pump. My question is where is the ground wires that plug into trace back to, so I can figure out how this is accidentally getting power?? The 1990 and 1989 electrical manual states it goes back to just engine ground (but is not specific). The 1992 manual says it goes to the rear of the A bank intake manifold (but I can’t find any ground wires there). I did see the Jaguar S494 and added the additional grounds to each intake manifold, and it made no difference. I assume I accidents put the ground wire on a power lug since I have had the engine completely out.

Little backstory:
I have a 90 XJS that I swapped in a 94 6.0 and 4l80e. Everything on the car is as it would be for the 1990 XJS except the 6.0 fuel rail/injectors and Roger Bywater retuned my original ECU for the 6.0 +TT pipe + SE ecu. Car and engine has been mostly sitting since 2012 but been doing a little here and there until finally a couple weeks ago I was able to fire it up. 6.0 was pulled from a hail damaged XJ12 that ran great when it got it.

-Scott

 

Not sure on your year, but I reckon they be the same.

The fuel pump relay, in the Black socket, has an Orange wire that goes to the ECU, and that is the Earth for that relay via the ECU timer circuit, and then full earth when the coax to Pin18 gets the Ignition activation pulse from the front.

Technically that Pump Relay has NO earth path unless the ECU is active. Unless, someone has by-passed that circuit and simply earthed that Orange wire, as we do to the Lucas cars with fuel pump control issues with the 6CU ECU, not related to your year car at all, just mentioning for the sake of i.t.

That Pump relay is supplied 12V by the Main relay (Red Socket). So, it is really dead unless the Ign is ON, and the ECU is doing the earth path timer thing.

I will keep thinking.

 

Thank you!

it is a 1990 XJ-S Marelli car. Due to the Aj6 engineering TT exhaust I had to extend the O2 wires for the O2 sensors, however I am pretty confident that my wiring connections are good.

If you disconnect the heater plugs from both O2 sensor and have the fuel pump relay installed, the fuel pump operates properly. If you have a O2 sensor heater plug connected and the fuel pump relay pulled the fuel pump runs but fuel pump also makes a loud clicky noise and the engine will run.

I will disconnect the Fuel ECU in trunk and see if the 12V on the O2 ground wire goes away. This would tell me if somehow ECU power is where the 12V is coming from.

I want to find the end of the ground wires for the O2 sensors (where my finger is pointing in the electrical diagram), so I can confirm they are actually attached to chassis ground. I followed both O2 sensors wires in engine bay into the passenger(USA) footwell to the ignition ECU area. So the grounds must not be in the engine bay, I will start hunting in the pass footwell.

I also have a misfire on the group of 1A,2A, and 3A cylinders.( exhaust manifold temp is much less then other 3 manifolds). After pulling plugs, the Cylinder 1A plug was really clean. I confirmed the spark plugs are all firing as they should, by resting the plugs on the block and cranking it over (when trying to get rid of fuel pressure prior to this so I didn’t get a fire is when I found pulling fuel pump relay wasn’t stopping fuel pump). The fuel injectors are all clicking when I manually give them power from my 12V drill battery. I am getting tomorrow some fuel injector lights to see if the harness is giving power to the 1A fuel injector. I originally did a clean of the injectors 5 years ago so I don’t think they are clogged.

-Scott

 

I got it!! Found that I had the ground wire attached to the main 12V post for the starter. The length of wire was perfect length for that post so I just attached it a long time ago. Anyways it is now attached to the back of the A bank intake manifold.

Now on to figuring out what is causing the misfire...

-Scott

 

Well found, SIMPLE cars these V12's as I always say.

I just opened a beer to do some thinking, now I need another to celebrate, thanks Scott.

I suspect a loom issue.

You know the bundling of the Injectors
1,3,5A
2,4,6A
1,3,5B
2,4,6B.

Based on that, and the cylinders with issues, loom gets to #1 on my list.

Many times I have Injectors "click" when powered up, but NO spray, so a LIGHT tap with a VERY SMALL Tack Hammer, and away they go.
Bosch schooling on EFI back in the '70's taught me that one, and its saved me many $$ ever since.