Back again with more on the 1986 jaguar xjs... it seems my fuel issue lies in the relays i think.... did all the things that Doug suggested.. did get the car to finally run, and then die out.... so I spent hours looking for problem.... I decided to start engine and listen for fuel pump it ran to pressurize but would shut off once car would start and run.. so car would run for about 10 seconds then die because fuel pump was turning off.. you could hear relay click before pump would stop...so i decided with key on to pull relay and plug back in.. I did that 2 times in a row. car started and stayed running.. this seems to work when it dies...is a relay or something else..? car runs great.................but for how long...

 

Yep,

The fuel pump relay earth circuit is controleld by the ECU, via the "orange" wire out of the relay base. Some early ECU's, mainly the 6CU units were noted for fuel pump control circuit issues, and that was sorted with the 16CU units somewhere in 1986/7.

I would try another relay, a simple 5 pin 30amp relay with a standard pin layout (NO 87a terminal) will do. I have had a few just go legs up with age.

Also try running a temp earth wire spliced into that "orange wire" and connected to earth, and see what happens, BUT, remember, with that orange wire earthed the fuel pump will run constantly whenever the ignition is ON. OK for diagnostic purposes, NOT for all time use, safety etc.

 

Hey Grant thanks for advice... car will run fine for awhile 30 minutes is about as long as I have driven since fuel delivery issue... Sometimes I will drive it a bit and shut it off and it wont restart. pump runs initially as it should but shuts off as it shouldn't, You can hear the relay click before it shuts off...should there be 2 relays that are the same or should they be different? I have replaced the ECU with another, I would switch the ECU to the original and car will not start and no pump sound. with the new ECU it will start as it should but with with the issues stated above.. any ideas?

 

OK, 2 relays in that area.

The one in the RED socket is called the "main relay" and originally had a red stripe across it top face. This was someones idea to identify that this relay was "different". It IS different, it has a blocking diode inside to prevent ECU damage, in case someone is dumb enough to fit the battery **** about, and actually turn ON the ignition, Fair dinkum, that person should not be allowed anywhere near a car, let alone own one.

The one in the black socket is the fuel pump relay, and it does get a hiding, mainly from the fuel pump itself. I had my fuel pump drawing 20+ amps, and that was just before it died. the new pump draws 6 amps constant.

The main relay "powers up" the fuel pump relay, and the ECU controls the earth circuit via a timer, and then the infamous "shielded wire" keeps it all running.


Reading the posts again, I rerckon you have half sorted it with the ECU change, and then back again to create a problem, good one.

The fuel pump relay may be getting HOT, mine did, and that is a sign that both the relay and the fuel pump are in need of replacement.

The main relay is not normally an issue, but I changed both mine for standard relays. I will NOT be putting my battery in wrong so I see no need for a diode relay. MY car, MY choice.

When you replace those relays, make real sure that the terminals INSIDE the sockets do not push out as you push the relay in, that is a mongrel to track down, so be warned.

Just for fun, check those 2 large spade connectors on the battery cable, about 6" from the terminal. They can be dislodged, and you MUST hold your mouth at the correct anglr when re-inserting them, as it is so easy to slide the female spade down the side of its male spade, and whilst it makes contact, the load will cause a failure. Those 2 connectors supply 12v to the ECU, and the fuel pump relay.

After all that, I am still betting on a fuel pump relay having a hissy moment.

 

Grant thanks... I have had the car in the garage over the winter and now i'm back at it. i have replace both relays, but not the pump, ive checked the connections the the battery (ecu) they look secure.. I have noticed that on cooler days or in the morning before the sun heats up the car it will not stay running the relay will click and engine will stop always after 10 seconds. once the car is warm, to hot from being in the sun it will fire right up and stay running, how long I'm not too sure.. at least 20 minutes or so have not run it much longer then that.. could it be a vacuum leak, and if so what sensor is telling the relay to turn off... It is kind of a warm day today 78degrees the car is in the shade and is cool I will check to see if it runs and for how long....thanks Doug

 

Doug,

A few things sort of bounce around the brain space.

I would locate the Orange wire coming out of the Black socket that the fuel pump relay is plugged into. Splice a wire into that Orange wire and EARTH it, battery -ve is close by. That will remove the ECU from pump relay control, and the pump will/should run at all times the ign is ON, so care is needed for safety reasons.

Vac leak I doubt at this stage of the party.

The Coolant Temp Sensor (CTS), located on the rear section of the LH thermostat housing is another start/running issue with the V12. The sensors go flaky, but the wiring is the main issue.

The infamous "shielded wire" from the ign Amp is another item that will cut the engine. I have had this issue with EVERY V12 I have been involved with, including mine. I ran mine with that Orange wire earthed for some months until I rewired the whole engine bay+, many years ago, and deleted that stupid wire in the process.

The module INSIDE the ign amp is also a suspect thing at some stage. Echlin (made in USA), part number TP45B is what I always use.

SOOOOO, having said all that, I would do the Orange wire first, that just eliminates so many items from the fuel supply system, and see what happens. I am still trying to get my head around the "sun" bit of it, as I dont think the Jag has solar panels, HAHAHA.

 

my 1986 xjs v12 runs for 2 seconds and shuts off any help?

 

Most likely your fuel pump circuit isn't being powered-up beyond the '2 second prime' phase.

Grant's postings above spell it all out. See post #2 above about grounding the orange wire at the fuel pump relay.

Cheers
DD

 

fuel pump relay is in the trunk? on a 1986 xjs v12?

 

Yes, right side, above the ECU. Look in the area of the RH trunk hinge. The relays are behind a protective cover

Cheers
DD

 

Reviving the thread to thank everyone who contributed information on fuel pump relays. I have a 1996 XJS 4.0 convertible that suddenly wouldn't start at all. After quite a bit of hand-wringing I picked up a new relay from Rock Auto and it started right up. Hooray!

New Bosch relays are brown plastic and have a part number of 0332019151. These supersede the silver metal ones.

 

HOW TO DRAIN THE FUEL TANK?
An old cheap used electric fuel pump and a hose on the in and a hose on the out