Fuel pump wiring

[Htchevyii]

Sorry, it's the 89 XJ again, always the XJ 😒. Strangely the XJ has half the miles of the MJ and spent it's life in the high desert VS the coast and yet it always seems to have a new weird electrical problem. The MJ never does, (knock on wood).

 

89 4.0 AT. I've been reading through old posts trying to figure out how the fuel pump is powered at various times. Anybody know how the prime cycle is powered? During the start cycle I believe it's powered by the starter relay and then switches to the PCM?

 

Here's the deal. The pump is a Delphi, about a year old. Grounds cleaned and a few extras added. I did add an additional fuel pump ground yesterday, even though it ohmed  out fine at the FP connector. The ballast resistor was removed before any of this started, or it never had one, I don't remember.

 

Three times over the last year, it has failed to start. Cranks over, but no fuel. It never happens at home and never stays broken, so I'm trying to figure out the system so that I know where to start. 

 

Once I gave it shot of brake cleaner into the intake and it fired up and then was fine. Perhaps  a coincidence, I don't know. The other times, it just went back to normal after a few minutes. 

 

Does anyone know where the power for the prime function comes from? PCM? What isn't making sense is that if the power in start mode comes from the starter relay and the prime mode comes from the PCM that neither one seemed to be working, but once it started on the brake cleaner, the PCM was powering the pump? Or maybe it just "fixed itself" at that moment?

 

When I checked today, there was just under 12 volts to the pump during the prime function. When I removed the FP relay, it still started until you moved the key to the run position, then died like expected. It has never been an issue while driving, always happens during a start. Maybe the pump is just junk and doesn't want to start at times?  Sorry for the long post, lots of information to divulge.

 

 

[ghetdjc320]

Those rotary vane pumps have always been problematic for me. Never got more than a year before they started to fail. That being said, it could also be the 12v+ side. Have you checked out the body harness connector near the parking brake?

[Htchevyii]

That's the only connector that I know of that I have not checked. I'll put it on tonight's to do list. Intermittent problems are the worst.

[Ωhm]

8 hours ago, Htchevyii said:

Anybody know how the prime cycle is powered? During the start cycle I believe it's powered by the starter relay and then switches to the PCM?

 

Three conditions feed (B+) to the fuel pump motor. One (1) using the ballast resistor and two (2) bypassing the ballast resistor. Conditions are as follows:
1. During CRANK. B+ is supplied by the Starter Relay. Ballast resistor is bypassed.
2. During WOT. ECU knows when vehicle is in WOT and activates the 02 Heater Relay. Ballast resistor is bypassed.
3. At all other times, Fuel Pump Relay supplies B+ using the Ballast resistor in series with the fuel pump motor circuit.

[Htchevyii]

1 hour ago, Ωhm said:

 

Three conditions feed (B+) to the fuel pump motor. One (1) using the ballast resistor and two (2) bypassing the ballast resistor. Conditions are as follows:
1. During CRANK. B+ is supplied by the Starter Relay. Ballast resistor is bypassed.
2. During WOT. ECU knows when vehicle is in WOT and activates the 02 Heater Relay. Ballast resistor is bypassed.
3. At all other times, Fuel Pump Relay supplies B+ using the Ballast resistor in series with the fuel pump motor circuit.

Thanks OHM, I was hoping you'd chime in. Any idea where power comes from for the 2 or 3 second prime cycle? 

[Ωhm]

The ECU controls the FP relay at KEY ON and ENGINE RUNNING. This is the source for B+ to Ballast Resistor to Fuel Pump motor. When ECU see NO start signal (KEY ON only) at C201_C3 (Starter (+)) circuit, ECU will time out and shut FP relay OFF (prime cycle).

[Htchevyii]

Thanks. That makes it sound like it has to be a problem with the pump or wiring  since I wasn't getting fuel prime or fuel during cranking.  It seems unlikely that both separate circuits would malfunction at the same time. 

 

What was really throwing me for a loop was the time that  was sprayed brake clean in the intake and then it fired up. It must have just been a coincidence, but had me thinking that perhaps just the starter cranking circuit wasn't working, but I never heard it prime either.

  I have noticed that it only primes twice in a row though. I wonder if there is a delay with the prime cycle or f the vehicle was recently shutoff? Guess I'll play some more tonight.

[Ωhm]

12 hours ago, Htchevyii said:

When I removed the FP relay, it still started until you moved the key to the run position, then died like expected.

 

Swap your relays around with one another. See if different trouble follows the relays.

 

 

26 minutes ago, Htchevyii said:

It seems unlikely that both separate circuits would malfunction at the same time.

 

Splice_A (Engine Harness) is common point for all four (4) circuits (FP motor, Ballast Resistor, O2 relay and Starter relay). One of your Ballast Resistor wires will lead you to this splice, the other wire back to the other side of vehicle.

[Htchevyii]

Thanks!

[Htchevyii]

Thanks for the help OHM, it wasn't easy finding out how the system was supposed to work. In the end, I never could find anything wrong, all connection were good and clean, the ground was good and no amount of wiggling things caused a problem.  I swapped in a US made Walbro pump hoping that the aftermarket pump just wasn't working sometimes. I did add an extra ground at the pump.

In the event that it malfunctions again, I rigged up a plug that plugs into my auxiliary plug for my fridge and plugs into the fuel pump. I tested it and it works, so I won't get stranded if it does happen again.