2003 XKR Fueling Electrical Riddle

Hello all. I’ll try to describe this the best I can, with my limited understanding.

This is a 2003 XKR SC 4.2 Convertible

In the photo below, what’s depicted is the fueling system from the 2003 electrical guide, specifically referring to the Fuel Pump Module or Fuel Pump Pressure Control Module and Fuel Pump. The module is located on the “C” pilar on the RH side. Bottom corner of the fuel tank… It’s working.

From the schematic, it seems to show that there are two outputs supplying voltage to both terminals of the fuel pump with no ground. As of now, the fuel pump is inoperable.

What is to be understood from this? How is the appliance grounded and what is the path of ground, control or variable operation of the pump?

I am not blowing fuses. The FPM is the relay. There is no relay. The pump is connected but not spinning and WILL spin on a +/- connection at bench test. With test light, reading voltage at both the YR and R wires at all connectors right up to the pump at the SAME time.

I am completely stumped and open to all and any advise, direction, suggestion and any questions if I can be of any use to add additional info or clarify.

Please advise

What is to be understood from this? How is the appliance grounded and what is the path of ground, control or variable operation of the pump?

Pulse Width Modulation.
The fuel system is a ‘returnless-system’ and the pump pressure varies with the demand for fuel.

If you have an oscilloscope or know someone with one, you could check to see that you are getting the correct waveform at the pump. If not, I think I would see if a new pump solves the problem.

Thank you, Bob.

I am familiar (and becoming more so) with the concept… Still, I would think that one of the leads would need to be grounded, no?

Writing to both Eric and yourself Bob, I have a pump that I tested using conventional +/- voltage, using a battery away from the car - bench. The pump fired right up. Plug it into the car, turn the key to the ii position, check voltage, positive at both leads/terminals and following connection back, same thing… Pump NOT running.

I am trying to solve this problem. Granted, I could go clip a line and ground it to the car, but that’s not the design, how I found it or how it’s supposed to operate. Right?

Off the top of my head I don’t know anyone that has that, but if I do, did or could, what is the correct wave form?

The old pump is dead… I have been “testing” with an old XK8 pump from the dual pump system… Will this work OR are the pumps needed different? Does it have to be a pump that works specifically with Pulse Width Modulation? Might be a stupid question, but I’m learning here.

You probably already know this but pulse width modulation is usually a square waveform with variable duty cycle. As such, it doesn’t need to be grounded because the two connections to the pump may be in opposite phase. Trouble is, we don’t know what is happening because, as you know, the fuelling is under the control of the ECM and we don’t know what the ECM is sending.

That’s why I suggested scoping the lines. I’m getting muddled up with all the messages at the moment but haven’t you also got a working XK8? If so, you could compare outputs of the pump control modules to see if they are similar.

I didn’t see your last message before sending mine but I think that the principle of the fuel pressure control by this means is common to the system and will be the same for XJ and S Type fitted with AJ2x engines. Bob can probably confirm this.

My xk8 is a 2002 and uses the 2 pump system without Modulation, I believe…

Eric! Thank you for this. This is the clearest explanation I’ve gotten about the problems I’m having with my fueling.

I think I (sometimes I wonder) have been making great progress with this flooded XKR… She cranks today with spark, just not fueling. I really believe that if I can get some pressure in the rail she will start right up. I just don’t know what is causing the fueling issue. It’s been frustrating… I am tempted to, for now, just ground out the negative lead, see if I can get the pump running, and just letting her idle a while so I can begin to see whats what and what will happen, if things come more normally on line, once running…

I just ordered the Walbro 255LPH pump (I refuse to pay 700bucks for the plastic bucket). Be here for Saturday when I can get out and get dirty again… These days waiting are the worst, lol

Thank you so much for your help. You too, Bob…

Honestly… I am completely afraid to connect the new pump and just BURN IT because I have 2 positive leads going into it…

If, as you say, you have the 2 fuel pumps then you need to look at figure 04.4 of this manual, http://www.jagrepair.com/images/AutoRepairPhotos/jagxk2002.pdf

Check the fuel injection relay for correct operation and also the inertia switch hasn’t tripped.

I have an xkr with one pump… For testing, I’m using a pump from a 2 pump set up LIKE the one in my 2002 XK8… The one from the xkr was toast…

So… I’ve been reading about PWM… It seems that in one way or another it is controlled either by power or a positive lead, turned off and on by intermittent grounding OR always grounded and what is intermittent is the powering…

Either way, it seems (and maybe the 2003 PWM system is got its own approach) that grounding is a necessary component going into the appliance or solenoid… Am I getting this wrong?

Many of the things I am reading (and watching) says that the duty cycle(s) are controlled by alternating or intermittent powering and grounding.