Jay - not sure if leaving the real gauge disconnected will impact your efforts but as Norri mentioned as few posts back - this mod was developed by our own forum member WhiteXKR - great guy who would be the best person to ask. Send him a PM - he's pretty quick to respond

 

Seeing this in the 2003 EG... Are there two positive leads to the fuel pump in the 2003 xkr SC?

I don't understand the fueling problem I have...
I have 2 positive wires from the FPM
or Fuel Pump Power Regulator.

 

Anyways,,, with a test light (I ran out sun and will go back with a DMM tomorrow) I get positive,,, from both the R and the YR in the photo above. From the white connector in the harness away from the blue-ish fuel pump housing - kind of over in the RH corner - to the R and YR wires both at the connector on the FPump housing and under Neath on the inside of the tank. Two positive lines and I don't get it...

Any ideas of what I'm looking at here.

Double checking,,,, which is the fuel pump relay and fuse in the BOOT fuse box?

 

Trunk Fuse #7 20amps

 

Got it... Thank you, John...!

That's the one I been checking and amazingly, it's good. I am completely stumped by this fueling wiring problem... Somehow, it seems I have 2 power wires going to the doggone fuel pump... Can't figure it.

And the relay, the one I'm checking, is the first in that right angle down from the top cross of the T? Is that the one?

 

No fuel pump relay in the 2003, just the module.

John, I see F5 (30A) for 2003? Please can you sanity-check me?
However, if there's voltage at the pump I imagine the fuse is intact.

Jay, I'd expect to see the same (or very nearly the same) voltage on both sides of the fuel pump once the line pressure is correct, or in a fault state. The ECM drives the fuel pump via the FPM (pin BT18-3), which returns a control signal on BT18-4. You can't properly check either of these with a multimeter (they're PWM).

Any luck with the OBD reader yet, as there are some codes P0191,2 & 3 that may help the diagnosis, although substituting the FPM if you can will be the quickest way of eliminating it.

 

OMG this thing... I am haunted by it and have a hard time sleeping thinking about what's what. I am CERTAINLY on a steep learning curve here... Answers are so welcomed, Michael, cuz I clearly don't have them. What's PWM?

So, thankfully, I have had a FPM borrowed - and a new one on the way. The OE was looking crispy on one of the corners if if you remember. Not sure if it was fried or not. Anywho,,, are you saying that the fuel pump in this system will take some of the power being sent by the FPM? Returning some to the module? I am, I'm getting power, test light, on both leads going to it... Or, going in and coming out?

Can you bench test the CORRECT fuel pump?

And, lol, NO RELAY, lol? I'm insane. I've been reading here, and in some places I've understood the relay position to be that one just before that line of relays that creates the top of the T in the boot fuse box relay layout. But then, in the 2003 EG I saw none, lol... And I kinda pretended I didn't see it... Also, some places say there's a fuse at 7, others not. Believe me, I could be misreading things. Sometimes tooo much information can be a curse.

So,,, being the "smart" guy that I am, I ordered what looks like the pumps for the pre2003 versions (thinking I got a good deal, which I did, but I guess I'll wait to use them on my 2002xk8) and with an old plug I had, plugged it in thinking I had a positive and negative terminals... Yeah,,, it sings on a bench test,,, does nothing plugged into the car...!

Does the CORRECT fuel pump have essentially 2TWO positives on top? Still confused here.

EDIT ADD - Sorry, no... I "misplaced" the damn obd (I'll find it) in the mess of bits and pieces while working on the thing, lol... I better back up and slow my *** down!

 

So power flooooows thru the 2003 fuel pump, making both sides read as positive?

 

This is what I mean about fuel pump info. This is Welsh... This is wrong, right? It can't be the same fuel pump thru all those model years... 96 - 06

 

Try this:
Imagine the FPM is a light dimmer, with the ECM turning the knob, and the fuel pump is the lamp.

The ECM monitors the fuel delivery; when more fuel is demanded, it turns the dimmer up and the lamp gets brighter i.e.the pump works harder. Conversely, when less is required, the ECM turns the dimmer down, and can go almost to zero.

The FPM sends a signal back to the ECM every second saying 'all ok'. However, the FPM can vary this signal to say 'I'm not getting the pump drive instruction from you' or 'the fuel pump isnt working'. I imagine the ECM will set codes if it receives a 'something's broken' message.

Now to the pump. It needs power across the two connections to make it run. We can supply that power in two ways:

1) Connect one of the terminals permanently to ground. When we want the pump to run, we connect the other terminal to 12V by a switch of some kind.

2) Connect one of the terminals permanently to 12V (B+). When we want the pump to run, we connect the other terminal to ground by a switch of some kind.

Most of the XK's systems use option 2 - switched ground.

My thinking here is that the FPM supplies ignition-switched B+ on the Red wire, and will vary the voltage on the Yellow/Red wire to run the pump - providing the 'switched ground'. For whatever reason, the FPM isn't doing the latter so you get 12V on both terminals and the pump doesn't run.

It's a tad more complex than that, but I hope the story isn't too cheesy...

 

Thank you so much Michael!

In that senerio,,, is the dimmer the FPM? From the diagram above (back a few posts) and from when I took it apart to check internals and clean (see photo below), there was a big metal brace in the device that seemed to take ground directly from the device (board) to the exterior box (two torx screws holding the outer box to this brace),,, which was then grounded by bolting to the body of the car.... Is the grounding handled separately (to car body) and the device is a variable and self modulating "rheostat" for lack of a better term???Controlled by the ECU...?

Lol,,, I think I just repeated what you wrote, lol... Told ya,,, nutts!!!!

Does that mean DC power in on one terminal,,, DC power OUT on the other? Sorry, I'm still confused I guess.

There was NOTHING I could do to get the OE fuel pump running out of the car (on the with bench) the usual method of connecting the positive and "negative" leads to a battery. It seemed to just LOCK the pump. I was taking it slow, tapping the leads from the battery to the pump leads fearing to burn the "locked up" pump by holding them together tooo long. For some reason, it fired and ran for a sec but I could never reproduce that response a second time... I gave up on it looking for another way to temporarily rig a pump.

What is that ground point BT2AL, in the schematic below??? A ground point to the body of the vehicle?

 

Michael, re: fuse. I bet I was using a 1998 diagram. Thanks for the correction.

Jay, looked at the 2003 electrical schematic. Yes it shows current is flows through the pump through red wire and red yellow wire. Both are connected to the fuel pump module.

Wish it showed direction (+ / -) but it doesn't.

BTW, to Michael's point- the 2003 advanced electrical manual doesn't seem to show fuse layouts by fuse box. Knowing Michael, there's no need to 2x anyway.

John

 

Hey John. Thank you...

Take a look at the post just above your last... See if that explanation seems plausible...? The ground (somehow) is part of the housing,,, ground to body? Is that possible?

In that the current passes thru the motor and the ground is found outside of the positive loop, outside of the motor and assembly,,, and achieved out side by the FPM by grounding to the chaise? I don't know **** about electrics (not as much as I need to know) and can't go test the theory today. Does this theory make any sense?

 

The aluminium piece is a heat sink for the power semiconductors underneath. Heat dissipation rather than electrical grounding.
BT2AL is the FPM ground.

I need to go and do some reading.

 

Could it be both? PCB ground and sink? When messing around with it it seemed to me (I remember thinking) why did they make the connection of the aluminum cover to this plate so robust... It seemed like overkill. Plus,,, I was pissed because I didn't have a torx 8 bit, lol. Had to wrestle with it 😅😂🤣

Could this be the answer?

The grounding issue doesn't HAVE to be handled immediately at the appliance, does it?

 

Thinking out loud here...
Is it that the FPM controls the speed of the pump by slowing and speeding current flow by making changes in the ground? Varying degrees of resistance? Everything else seems switched by grounding. Why not this?

​​​​​​

 

Unlikely.
The heatsink is secured firmly to the metal cover to help further with heat dissipation - makes the heatsink larger.

There are further DTCs in the workshop manual:


and the related pinpoint test are there too. You can get the full manual from the stickies (it's large)

Again, if you can get the OBD reader functioning then the car may be able to help you.

 

Thanks Michael... I will make it a point to chase down that obd... I half-assed-edly looked around for it yesterday.

Q: is there a pump, aftermarket, that folks have found works for the 2003+ xk8/xkr??? All I find are pumps that look like the older xk8/xkr double pump set up. With clear positive and negative posts. I think the fuel pump appliance for the new system must have a pump that will accept varying current.

​​​​​​For some reason, I have a feeling things are the way they are supposed to be. That I had a frozen pump - of course, it didn't work.... Removed the pump, worked on it (probably destroyed it - I'll have to see) and when I couldn't get the OE pump working on the bench (not know what I was doing), I gave up....I then Mickey moused a pump (that doesn't work on the positive positive varying voltage/current type system). I have a feeling if I plug in the proper pump made for the 2003,,, it'll fire up (not literally, lol)... I need a working pump...

I hate that I am at work right now and can't get dirty.

 

Jay, great question. I was uncomfortable with the idea of using a gas tank as a ground, that plus I cant think of any reason for 2 hot leads into the fuel pump led me to my conclusion. Folks will assuredly correct me if I'm wrong.

Gotta ru . John

 

Here's some info from another thread,
https://www.jaguarforums.com/forum/x...nation-127565/