Total mystery to me I have xjsc v12
And I am at a total loss to way my fuel pump won’t Prime or run unless I put a direct feed to it which means it runs all the time.
I have
Have the ECU checked out that’s working fine replace the relay and anti-roll cut off in the door
So I am at loss now hopefully somebody might have some other idea to the problem which would most gratefully received
What year is your car?
Inside the driver’s side door jamb is an inertia switch, intended to trip when the car gets in an accident and shut off the fuel flow. Check the plunger on it.
As a general rule, the fuel pump is controlled by a relay, and the relay is controlled by the ignition switch and the ECU. The 12V power to the relay comes from the ignition switch, the ground is through a circuit in the ECU. In the 80’s cars, the fuel pump control circuit in the ECU was known for failing even though the rest of the ECU works fine. Having the ECU fixed is one option, but another is to simply disconnect that wire from the relay coil to the ECU and connect it directly to ground instead. That means the relay – and the pump – will be energized whenever the ignition is on. No more cycling off after two seconds if you just turn the key to on without starting; it’ll just keep running until the battery is dead. Likewise, no more cycling off if the car is upside down in a ditch and on fire; it’ll keep running until the fire engulfs the car. But it can be a useful temp solution.
Which relay was replaced. Not sure what MY your car is nor if all years are the same but the main relay in the boot/trunk (right alongside the pump relay) and the EFI main switching relay both play a part in the operation of the pump relay, if one of those go bad you may not get a switching feed to the pump relay.
Andrew,
Do you have either the Jaguar XJ-S Repair Operations Manual (ROM) or S-57 Electrical Guide so that you can see how the fuel pump electrical system works?
Did this car run properly for you and recently start having this problem, or did you get this car with this problem and you are trying to figure it out? The reason I am asking this is find out if a prior owner or shops did anything to create this problem.
Paul
Hi kirbert. Thank s for your reply. The car a 1987.
I have done the temporary job that you suggested and the car runs .but like you said it can only be a temporary measure as the pump is constantly running when the ignition is on. When I asked them to check the ECU unit out i asked them to check the grounding in it as i read that its a common fault . There are two relays together in the boot of the car I change the one with a blue making on it which is the pump so I can rule that out. Could i try rewire the inertia to the the ignition do think this could be a option
Thanks Paul
I have two manuals with electrical diagrams.
The pump work fine from when I bought the car
So its bit of a mystery I do have it running now though
Thanks baxtor .I have look at the option
Andrew,
In my wife’s 1990 XJ-S convertible there are two different relays in the right forward trunk area that are part of the Electronic Fuel Injection system. Although they look very much alike they are different. The forward one is the Pump Relay and the aft is the Main Relay. They must both work correctly in order for the fuel pump to work properly and they are not interchangeable. This information comes from the S57 Electrical Guide that we have for our 1990 Model Year car.
Relays can fail, as can their connectors and wiring. You said that you changed out one relay. Did it test bad? If I were you I would take another look at these relays to make sure they work properly and that they are the correct but different ones.
Paul
Will do Paul thanks very much
I used that temporary measure for a couple of years. In fact, I would have used it forever except that I wanted to send my ECU to AJ6 Engineering for some mods anyway, so I had the fuel pump circuit fixed while it was there.
Interesting idea. In fact, you might check to see if it’s already wired to the ignition! Trip the thing, see if the fuel pump keeps running.
The purpose of the fuel pump circuit in the ECU is to shut off the pump if the engine isn’t running. Independent of the inertia switch. So in theory either one should stop the pump in the event of a crash. If having just the inertia switch shut off the pump in a crash is good enough for you, make sure that works and fuggetabout the ECU circuit.
The easy way to test the ECU to see if it’s grounding the pump relay or not is to simply remove the pump relay, look at the bottom find the pin labeled “85,” pullout the multi-meter, set it on continuity, shove it one end in the Pin 85 hole, and keep the other touching ground, then have the wife/etc turn the key to “ON.” You should get a buzzing / zero Ohms reading for 3-5 seconds. If not either that circuit in the ECU has gone bad, or there’s a wiring problem.
1987 I’m pretty sure was the first year to get the 16CU - an improved digital version of the 6CU which was prone failure on the FP relay circuit. It’s worth checking to see which you have.
Other things to check while you’re there. Key on or off the relay socket hole corresponding to pin labeled “30” should ALWAYS have +12V power - light up a test light. Socket/hole for pin labeled 86 should get +12V power with key on (that power comes from the main relay).
I can say for near 100% sure that pin 86 will test OK because your car actually starts. There’s no power to the injectors or ECU without the main relay…so…
If you do hard wire the FP relay to ground, it might turn off if the car turned upside down as that would usually trigger the inertia switch killing the Main relay which would kill the FP relay… but you wouldn’t have that extra layer of protection from the ECU as he noted.
~Paul Kobres
Could not help noticing that the 1987 schematic shows the inertia switch provides power to the main relay.However, the 1989 schematic shows the inertia switch GROUNDING the main relay. That could be confusing!
You’re right… it’s an unusual change. I’m assuming it actually occured in real life, but it is possible that the diagram is erroneous. I haven’t looked at the later obsureish errata to see if there were corrections. ATM I’m just going by the diagram.
You bring up a good point, and although it can be frustrating… it’s not a bad idea to be familiar with several years diagrams.
The 1990 diagram goes BACK to showing the inertia switch upstream of the EFI main relay rather than on the ground side.
~Paul Kobres