TX_XJ

After a few hundred miles of driving this weekend, my 1996 XJ 4.0 died while cruising down the freeway.

It wouldn’t restart and when I checked the fuel rail, there was no pressure. Just a bit of a gurgle. I do not hear the fuel pump when the key is in run, and still no pressure at the rail.

I checked the power wires going into the pump. They read 1.2v when the key was in run.

When the key is in run, I get 6.6v across the fuel pump relay triggers and the relay output measured in the wiring harness just outside the PDC is 1.2v.

Is 6v enough to trigger the 12v relay? If not, what could be causing this?

In the PDC, I bridged pins 30 and 87 of the fuel pump relay and now measure 12v in the wires coming out of the PDC. I also measured this same 12v in the wiring connector at the pump. Truck still would not start, and I cannot hear the fuel pump running. Still no fuel pressure.

So it seems like two issues... the relay isn’t triggering properly and the fuel pump isn’t working...

Any ideas how these two things could be connected?

dave1123

It's possible you have a ballast resistor on your 96 for the fuel pump and if so it may be burnt out. I believe it's mounted on the driver's front fender just behind the computer. It's about a 4 inch long square ceramic box with a wire on each end.

cruiser54

Here's a pic.

TX_XJ

Thanks guys. I looked and no sign of a resistor like pictured above.

I assume the pump is dead because when I bridge the pins in the relay socket, I get 12v to the pump and it doesn’t run.

but that still leaves me with the issue of why I am only getting 1.2 volts to the pump when the relay is installed. When the pump should be off, I get 0v between the activation pins for the relay. When the pump should he on, I am getting 6.6v between them.

I assume that this isn’t enough to properly trigger the relay, which is why it is only outputting 1.2v to the pump?

cruiser54

Is that with or without the pump plugged in that you get the low voltage?
Swap a relay in from another location.

TX_XJ

I believe the 1.2 vs is the same whether the pump is plugged in or not.

I have swapped all the relays around with known functioning ones (starter relay) and even picked up a new one to use.

The fuel pump wiring matches that from the 97 manual (thank you, btw).

I have traced the wiring from the pump to the back of the PDC and mechanically it all seems good.

cruiser54

check for voltage along that electrical course. At connectors.

TX_XJ

I opened up the wiring harness right outside the PDC, and the wire that handles the fuel pump measures the same at the output of the PDC as it does at the fuel pump. I believe this should rule out any electrical faults between the fuel pump relay and the fuel pump itself.

cruiser54

Yes. so, how many volts at the feed wire for the relay?

TX_XJ

There is a full 12v at the relay feed. Perfect continuity between the feed and the positive battery terminal.

The relay triggers (not sure in correct term), show voltage when the pump should be on, and no voltage when the pump should be off. The only thing that seems weird to me about the whole system, is that the relay triggers are only showing 6.6v when the relay should be activated.

cruiser54

Check the GROUND to the relay receptacle.

TX_XJ

Ah, good call! One of the relay triggers wires should be going to ground, I believe. The other would be going to the ECU? I’ll check that out this evening!

The ground for the pump itself looks good. I am assuming the pump is dead. Replacing the pump looks easy enough, really struggling to get the wiring side figured out as well.

I really appreciate your assistance!

TX_XJ

Alright, so I think I have a plan.

for the fuel pump relay activation wires, one side runs through fuse 11 and the ignition switch to the battery.

I believe this should be getting 12v, whenever the ignition is on. Easy enough to check.

the other side runs to the PCM, which provides ground to activate the relay. I should be able to check continuity between the wire coming out of the PCM and a known good ground.

this should at least help me narrow down where the issue is.

and this is all in addition to replacing the fuel pump. It definitely seems like it is dead.

TX_XJ

Edit: Wew, I am dumb and just driving myself crazy. All good, just a dead fuel pump !

I had someone else crank the truck and I got a full 12v across the relay trigger circuit. It is kicking on for a second to prime. I guess because it is not starting, the truck stops giving power to the fuel pump. So a truck a dead fuel pump will not be showing voltage to the dead fuel pump when the key is in run (except for the short priming period). I am stoked!

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Alright, getting stumped...

1) continuity between fuel pump relay (pin 87) and fuel pump - confirmed
2) continuity between fuel pump relay (pin 30) and battery positive - confirmed, 12v
3) continuity between fuel pump relay (Pin 85) and PCM Plug (pin C19) - confirmed
- connections between PCM pins and harness plugs look clean with no damage or corrosion.
4) 12v signal to fuel pump relay (pin 86) when key is in run - confirmed

If I jump pins pins 87 and 30 of the fuel pump relay socket in the PDC, I get 12 volts going to the fuel pump. The fuel pump does not kick on. It appears to be dead. This is not a big deal. Right now I am more concerned with the wiring issue. But.. whatever happened seems to have taken the fuel pump out with it (or just wonderful timing).

When the key is in run, I measure 6.6v between pins 85 and 86 of the fuel pump relay socket. I believe the relay needs 12v to activate. I did see a spike in this voltage to around 10v, when the starter is being cranked (my battery is getting a bit flat from all the diagnostics).

When the relay is installed and the key is in run, I measure 1.2 volts going to the fuel pump.

When the key is in run, the PDC should be providing ground to the fuel pump relay, while the ignition circuit provides 12v. The 12v supply is good, but the ground supply from the PCM is suspect. When probing the PCM input in the PDC relay socket, I see no continuity to ground. I do see 6.6v when measuring against the battery positive.

I seem to have found the issue, but have no idea why I am not getting a good ground from the PCM. I have jeep cables installed and the main engine grounds should be good. After studying the FSM, I see no indication of where the PDC is getting the ground for the fuel pump relay.

Or am I overanalyzing this whole thing and I just need to replace the fuel pump? Still, even with the fuel pump being bad, I would think that the truck should still be feeding it 12v power.