Hi All

I am currently playing with a 1991 XJS facelift that has been stood about a year. Fuel pump isn't kicking in on ignition actuation after being stood twelve months. I put a fresh battery on and after checking for fuel at the engine fuel line it decided to work. Ran it and warmed it up and other than being a little lumpy it ran well. I got some fresh fuel in and ran it some more, all well. Stopped the car for an hour or so then fuel pump was dead and wouldn't start again. What's interesting is how many variations there are on relay locations from model to model, searching has proved a minefield. I have in tank pump and under the RH rear wheel arch there are three relays, as they may have been swapped around and messed with previously im unsure which is which but it appears the relay locator blocks being different colours indicates different uses but again these seem to vary. I have one metal relay in yellow block, one blue relay in black block and one blue relay in blue block. I am trying to figure which ( if any) is main relay and which is fuel pump relay? Anyone know which is which?

Thanks

 

Brenjo,

I can tell you what each relay does. You haven't said if you've got a 5.3 or a 4.0 car? I'm guessing you've got a 4.0 Coupe?

If you've got 3 relays in the colours and bases you've mentioned. I believe they will be:
- Metal Relay in Yellow base = Fuel Pump Relay
- Light Blue Relay in Blue Base = Reverse Lights Relay
- Light Blue Relay in Blue Vase = Transmission Power Relay (for Coupe)

As regards, whether your pump is dead, you could just remove teh pump relay, bridge the relay holder terminals and turn on the ignition and listen.

For wiring diagrams, relay locations and colours etc, Jagrepair.com is your friend.

Cheers

​​​​​​​Paul

 

Thank you Paul

Correct it is a 4.0 coupe 1991. Do you know where the main relay is situated on this car? I need to check tomorrow if the fuel pump is working (it did for a while today) and if its getting live feed from the main relay to the fuel pump relay but I can not pin down where the main relay is. So many relays!!!

Ive found a few other relays in left hand under cubby box (right hand drive car). Two are the same relay as the fuel pump relay I did swap a couple over with no joy.

Brenjo

 

Brenjo,

There are a few known anomalies on certain years, but if you go to jagrepair.com, go to the XJS electrical page then select the 1993 XJS Electrical, not the 1992, then that's pretty close for your 4.0 car.

In the first few pages, you'll find the Relay locations and colours, That indicate the EMS Power relay is on the front A-post. Fig 25 & Data will show you the wiring diagram page where the Fuel Pump Relay etc is shown.

If you remove the Fuel Pump Relay, and bridge terminals 30 & 87 on the holder, that's not a bad way to test the circuit and confirm if it's the relay at fault,

However, note that on the later 4.0 and 6.0 cars, the metal Fuel Pump Relay is what is known as a "Twin 87 relay" even though one of the 87 terminals is not used in the holder. I suspect it's the same on your car. So make sure you bridge to the correct 87 terminal not the blank one. If you do need a replacement relay, we have found a much cheaper one in the Bosch family that is working fine in my car.

Hope that all helps,

Paul

 

I would bet that the pump is dying, not a relay. But download the proper electrical diagram and swap out the fuel pump relay with any other relay. Its a cheap and easy test.

 

C

 

Sorry that was terrible typing!

Took top off tank as pump was not working. Looks like electrical connector is fried as the pump works if you connect directly to it. There are two pumps in the tank that someone has spliced together on the wires! I believe the 4.0 should have only one?

Thinking running two has overloaded that connector and burnt it out.

Not sure which bits to replace tbh

 

4.0 has one pump. I'm not sure I understand most of your post.

My 96 still has the harness for a second pump, leaving me to believe that the harness for the v12 and 6 are the same, but yours is a little older, so I wouldn't be quick to assume that was the case back then. My car shared a harness with the 6.0 v12, yours was still with the 5.3 v12.

 

Mine has two pumps in it and someone has spliced the wires together so both pump will be running! Not too sure why there are two in there on a 4.0 ltr

 

Brenjo,

Sounds strange. Does it look as if the 2 pumps are original fitment? Has the tank possibly been changed for a later V12 twin pump set-up?

At least it seems that you can get it sorted, although I'm not sure whether I'd run it with 2 pumps?

Paul

 

They are standard fitment pumps, I re wired just one and left the other, all back in and now I can hear the pump as it should but no fuel getting to fuel rail wondering if fuel is bypassing past the non running pump and it will need to be blanked.

 

Brenjo,

I'm just guessing that there would be a non-return valve on the feed from each pump in order to stop the other pump just pushing it back down the other inlet pipe?

Still not sure how you've got 2 pumps in that tank if it hasn't been fitted from a 6 litre? Anyway, if you scroll down to page 21 of this doc which explains the 6 litre twin pumps, it may give you some ideas?
94 XJS Model Year Update S83.pdf

Hope that helps,

Paul

 

Think it is a 6 str tank…swapped to other pump and seems to be working but I have a leak on the fuel line now! grrrrr

 

Here's something else to consider.

I think that second fuel pump pushes fuel at a different rate, so I don't think they are interchangeable. Its supposed to kick in at higher throttles to make up for the amount of fuel the 6.0 would require at higher throttle. I'd hate to think that someone simply swapped tanks, and kept the original pumps...yikes what a waste of an opportunity to literally place two perfectly good pumps in there, one to back up the other!

Where is your fuel leak?

 

Good point…
They both look identical and original spec but im wondering now if I wired up no 2 pump as it was running but didn't appear to pump any fuel through. I swapped the wires over to the other pump today and it appears to work as it should. I had a leak on fuel line from tank to filter, must have got disturbed/damaged when I took the tank out…managed to get it repaired at local hydraulic hose company and now fitted and finally running. Bit of a misfire/lumpy idle but cleared at higher rpm so not fuel I don't think, feels like a bad plug maybe, the car has been stood for twelve months so a set of plugs won't hurt.

Cheers

 

Brenjo,

I believe, but could be wrong, that the two pumps in the 6 litre are the same, with the second pump just cutting in at 2840rpm to provide additional fuel flow for higher revs and load.

As the Bosch pump is a gerotor-type pump, I think it will run backwards if polarity is reversed and therefore create no output fuel pressure, preventing the car from starting. Perhaps that's what happened?

At least you've got it started now, Well done!

Paul

 

Think your spot on..I did check polarity and 100% had it right but must be just knackered I guess, running now anyway, next job is she starts an idles perfect for about 20 seconds and then idle gets lumpy, was thinking just sticky injector from being stood but as it is smooth as silk when it first starts Im not sure...