V12 fuel pump will not run

Hi, I have a 1982 XJ with the HE Engine (european spec) which has been standing for a few years but now it´s time to start her up. Unfortunately the fuel pump does not start when I turn the key to ignition on. I tried to hotwire the pump and it runs ok then. I also checked the fuel cut of inertia switch and it work OK. I hear the tank change valve click in the trunk when I toogle the switch up/ down. I have crank and the engine start when using starting gas.

The manual is a bit unclear on which is the main relay and which is the pump relay but standing in front of the car looking into the engine compartment i have 3 relay (but 4 sockets), going from left to right the first socket has a Bosch 12V 30 A metallic relay, the second socket has a black plastic 12 V 30 A Bosch relay, the third socket is empty and the 4th socket has a round Lucas reöay. Can anyone help me with which of these are the main and fuel pump relay.

With ignition on and changin places of the 2 Bosch relays, the black plastic one is clicking when inserted in the first socket and not clicking when inserted in the second socket. The metallic relay is not clicking in either of the sockets.

Any thoughts/ ideas on how to continue the troubleshooting.

Br/ Mikael

Hi Mikael,

314 is the Fuel Pump Relay.
41 the Pump.
(299 the cold start relay?)

N Brown (powered, always)
NS Brown/Slate (power to pump)
O Orange
KB Pink/Black

I would jump N and NS to try. That’s what has been done to my car. If the pump starts to run wiring must be okay.
If KB, O don’t turn on: they are both from the ECU. KB runs via injectors.

If you need more help I have no clue as I don’t have the 12, but the wiring diagram; but others will.

The main relay powers the ECU so the socket clicking must be the main relay. I suggest you test the relays on a 12v source and with a test light; and get one as spare in any case.

Edit reason, found a better diagram.
Good luck,

I have a 1990 V12 Vanden Plas, essentially a Daimler Double Six badged for the Canadian market. It should be the same as your 1982 XJ12, but I am not certain of that.
Attached is a picture of the bank of relays on the bracket attached to the rear of the radiator top rail. Going from left to right, the red relay circled in blue is the Cooling Fan Relay (DAC1028), the second spot is empty in my car, the third spot has the Fuel Pump Relay (AGU1068) circled in purple, and in the fourth spot is the Main Relay (AGU1070) is circled in red.
The Main Relay is marked with a red diagonal stripe in both my 1990 V12 Vanden Plas and my wife’s 1990 XJ-S convertible. I suspect this was done to eliminate confusion and mixing the relays up, because they are different. Although to the casual observer they may appear the same, they are not. The Main Relay has a diode inside on pin 86 that the Fuel Pump Relay does not.
I hope this helps.


As far as troubleshooting your problem I recommend that you start by making sure that you have the correct Main and Fuel Pump Relays, that they work, they are in the correct location and that they wires are not broken and they are connected properly.
The attached picture shows the electrical details of those 2 relays.


The pump should run for 2 seconds with ign ‘on’, Mikael prior to cranking - but it is not strictly necessary…

…it is more important that the pump runs while cranking, and of course continue running while the engine runs with key ‘on’. You mention start gas to get the engine running - and if it indeed then runs on; are you running on the old petrol stored? The start problem may relate to the lack of combustibility of old gas - or weak sparks requiring start gas to ignite…

Gut also, to start reliably several factors are involved - what do you do with the gas pedal while cranking to start…

The main relay has white (power with ign ‘on’ and ‘crank’), brown (constant power), (2?) pink/black (to injectors, and pump relay) and black (ground). The fuel pump relay has pink/black (from main relay), orange (to ECU), brown (constant power), and brown/slate (to fuel pump)…

The orange going to the ECU is grounded in the ECU for 2 seconds as the ign is turned ‘on’ - and grounded in the ECU while the engine is running. Grounding operates the relay and connects brown to brown/slate - and the pump runs. Jumping fuel relay brown to brown/green as David suggests, will run the pump continuously, even with ign ‘off’. Which is OK for testing and for actually using the car – but not otherwise…

If the pump runs while cranking and running, but do not run briefly with ign ‘on’ is odd - implying an ECU fault. Which in itself is unusual…:slight_smile:

xj6 85 Sov Europe (UK/NZ)

Hi all,

And many thanks for all the good advices!
This morning I jumped pin 30 and 87 on the fuel pump relay and then the pump ran continously. I then cranked the engine but there were no signs of spark. So I might have fried the relays when I swapped them back and forth yesterday. I will get new main and pump relay and more starter gas and try some more.

Br/ Mikael


Does the pump run if you ground the Orange wire with the ignition in “run”? If so it’s a common fault with the 16CU ECUs. I believe that Roger Bywater at AJ6 Engineering can repair your ECU if this is the case.

83 XJS Carmel
Lithgow AUS

I doubt you fried the relays but I‘d swap in new ones to eliminate any possibility there. And as frank said, don’t jump the connections for everyday driving, obviously.

You can still test the relays of course by switching them outside the car, with test lamps everywhere!

The ignition is a separate subsystem, Mikael - the only part used by both is the ign amplifier…

Even if you remove both relays the ignition will be unaffected - however, the ign amplifier may react to some misconnections, but swapping the relays in itself should be harmless to it…

xj6 85 Sov Europe (UK/NZ)

Hi Again, Here comes some updates on my troubleshooting. Please excuse me some of the things I do seems strange but I´m not to familiar with this. Anyway, since last I got a new pump relay and installed it and cranked the Engine with no start, Retried with starter gas and then it sputtered a bit then died. The main relay is on backorder so it will take a few day´s Before I get it. Also measured the pins and here´s what I got.

  1. Pin 30 alway´s has 12 V independent on ignotion on or off.
  2. With the ignition on I measured between ground and pin 85. No voltage.
  3. With the ignition on I measured between pin 30 and pin 85 and has 12 V there.
  4. With the ignition on I measured between pin 86 and ground. No voltage.

I did the mandatory engineering novice check and loosened the main relay and shaked a Little bit and there is something loose in there.

Overall I am note sure what conclusions to draw as I dont quite understand the function of the relay between pin 85 and 86 and I didn´t want to jump that part and risk damaging something.

What i get is that ignition seems to work as it sputters when I use the starter gas. Also the pump works when I jump between pin 30 and pin 87 so that part is OK.

I will continue when I get the new main relay.

Is there something else I should check until then?

Many thanks/ Mikael

I added indicator LED lights in all my relays, parallel to the coil and with a 800Ohm resistance, makes troubleshooting so much easier.




Yes indeed Mikael, if your car sputtered with starting fuel, i.e. you have spark, and if your fuel pump works when you jump the fuel pump relay your car should have started.
Maybe you have two separate issues ?
Does the pump deliver fuel?
Are your Fuel Pressure regulators working and not leaking ?

Maybe not energizing the fuel pump is not the only ECU’s problem …?
Check the ECU’s plug for corrosion.


‘85’ to ‘86’ is the relay coil on both the main and fuel relays, Mikael, switching the relay - and should definitely(!) not be jumped!

You can ohm between 85 and 86 - it should read very high resistance, confirming coil continuity. ‘1’ means coil break and a non-functioning relay. Power to the ‘main relay’ coil, white wire, comes from the inertia switch - and should show voltage with ign ‘on’. Relay should operate if 12V is applied to ‘86’…

While searching for the fault; connect a spare spark plug, triple gapped, to any plug lead - it should spark while cranking, or there is an ignition issue. Also, as Aristides imply; if the fuel pump is running and the spark is OK - the engine should catch and run. However; if fuel pressure is insufficient - the engine will not start. So a fuel pressure test is required to confirm or reject conclusively if this is a factor.

There may be other reasons for a non-start - but it is useless to pursue them if the fuel pressure is the cause…

xj6 85 Sov Europe (UK/NZ)

:-):grinning: Hi All,

I got the new main switch today and as soon as I got home I installed it, turned the ignition on Voila the fuel pump runs. Strangely enough it did not stop after a few seconds. So I let it pump for about a minut the circulate som fresh fuel (I filled about 20 liters before the start trials) and get us much old stale fuel out of the rail. Did crank it and started nicely and runs. After some 5 minutes the idle goes down from 1200 rpm to the normal 800. at idle it runs a bit unstable but at higher rpms it runs smooth (or at least it felt that way. But I guess there were some 10 liters of stale fuel in there before I added fresh fuel so I guess I will need to continue to run and fill here up fresh fuel to cure that.

As the pump did not stop as it should at ignition on, I was wondering if air in the system could cause that? What triggers the pump to shut down when You have the ignition on (pressure or time).

Other than that I´m realy happy now and wanted to thank You for all the tips, ideas and help. This is a great forum!

Many thanks again/ Mikael

And Aristides, Great idea to add those diods to the relay´s. I should do that as well.

Br/ Mikael

Slick for sure,

But LEDs not diodes A bit confusing, at least to me.

But, actually correct. The acronym LED stands for light emitting diode. The common understanding is as a “one way electric valve”.


Congratulations on getting your XJ12 to start and run.

The EFI ECU turns on the fuel pump for a few seconds when the ignition is in the “ON” position. If the fuel pump does not turn on and then off automatically with the ignition in the “ON” position there is either a problem in the ECU, a wiring problem, or perhaps a Prior Owner made some sort of modification.

I have used the fuel pump and the fuel tank selector switch on the dash to drain stale fuel from both tanks before adding fresh fuel to get the engines running again in a car that has sat for a while. I connect a hose to the fuel pump outlet in the spare tire area, or to the main fuel supply line in the engine bay, (whichever is most convenient) and empty the stale fuel into empty gas cans. Of course I do this outdoors and away from buildings.


I assume you mean a new ‘main relay’ - not a new ‘main switch’, Mikael…?

The fuel pump should not run permanently in ign ‘on’ without the engine running. This a safety feature; in case of an accident fuel must not be pumped through possibly burst lines even if ign is left ‘on’.

On the V12 this is controlled by the ECU, which triggers the fuel relay at initial ign ‘on’ - by grounding the orange wire which goes to the ECU from fuel relay ‘85’, for 2 seconds. While the engine is running (or cranking) the ECU permanently grounds orange to run the pump…

The main relay is energised with ign ‘on’ (and in crank) by white/blue to main relay’s ‘86’ via the inertia switch. This is an additional safety feature - with the inertia switch tripped; the main relay disengages. While main relay is operating, it powers fuel relay ‘86’ from main relay ‘87’ - main relay disengaged there is no power to the fuel relay; no pump action…

Both relay get permanent main power on brown to their respective ‘30’. Power to actually run the pumps comes from brown/slate from relay ‘87’ - which is connected to ‘30’ while the relay is operating…

I go in some detail as your ‘continuous pump’ may be caused by misconnections. The pump will run whenever brown/slate is powered for whatever reason - but it should only run when the fuel relay operating. Which requires power on ‘86’ - and ‘85’ grounded (in the ECU). Or fuel relay ‘30’/‘87’ shorted due to relay malfunction.

To test; disconnect orange from relay socket - the pump should stop as the relay should disengage. If pump runs on; the relay is faulty - or misconnected. If the pump stops - it implies that the ECU wrongly maintains permanent ground, or again a relay misconnection…or something deeper…

That the pump runs permanently with ign ‘on’ is a fault - but won’t in itself interfere with normal engine operation…

xj6 85 Sov Europe (UK/NZ)

Thanks Frank, I will do some more checks tomorrow to see if I can understand why the pump is running continously. What confuses me is that I know that the pump used to after ~2 seconds Before and I havent made any Changes to the wiring in many years.

Best regards/ Mikael

It would confuse anybody, Mikael…:slight_smile:

But as you have been doing invasive testing, like swapping relays etc - the ‘fiddle factor’ is ever ready to strike in the oddest of ways…

The gist of my ramblings is that power to the fuel pump can only come from the fuel relay - brown/slate connected to brown inside the relay. It’s the control of that relay that complicates matters - and the ECU is involved by grounding the orange wire…

xj6 85 Sov Europe (UK/NZ)