Fuel Pump Wont Stop Priming, CEL not working PLEASE HELP!!

[exjays]

Hi all, I have been chasing this issue for a few weeks. I have searched all forums high and low for pointers, alot of similar issues but nothing I can find gettinge me any closer to a fix, hence joining and posting here!

96 XJ Limited 4.0 AW4 (RHD, Australia)

WITH KEY IN RUN POSITION:
- FUEL PUMP WILL NOT STOP RUNNING/PRIMING
- GEN AND CHECK ENGINE LIGHT WILL NOT CYCLE/SELF TEST
- CRUISE CONTROL LIGHT ON SOLID

Because the pump wont finish priming, the GEN light and CHECK ENGINE light do not illuminate.
(These normally come on solid when priming for a few seconds, then go out when priming finished)
- or maybe it is because these dont come on that the pump wont stop priming? your classic chicken or the egg scenario here

Because it will not produce the GEN/CEL light cycle, it will not fire if cranked (If cranked there is fuel & no spark)

- Crank and cam sensor replaced initally. Although I now realise this was not the problem as these were not in play yet but they were cheap so what the heck.

- 5v reference circuit works

- Also believe that sensor ground is present after some testing with a multimeter

- hotwired ignition switch with same results so assuming this is not the problem either.

- Pump & ASD relay changed, blessed, jumped, made love to and massaged but it still wants a divorce

- No visibly damaged or burnt wiring.

I have discovered I can sometimes, but not reliably, force it to cycle the GEN & CEL lights on if the voltage is very low, i.e stuffed battery hooked up to it, or a big drain on system getting it under 9v. (this will click the asd and pump relay rapidly but after a few seconds GEN and CEL will go soild and the relays stop clicking and the cruise control light will go out. I can then remove the drain (again not reliably) and it will fire immediately and it runs/revs/drives so the pump and pressure is good as is all the other sensors and the pcm are all capable of running it perfectly. If then shut off, it will fire again normally if restarted within 5 minutes or so lol, I just cant make any sense of it. It also produces no codes when checked after running.

I can see by a wiring diagram (I think from a ZJ) that the cruise control is implicated in the sensor ground circuit but I cannot get the light to go away even by removing every fuse and relay there is also (I cannot find a fuse/relay diagram that is accurate for a 96 online and car didnt come with an owners manual so this isnt helping)

I assume there is some kind of grounding issue, something stopping the computer from turning the pump off or producing the idiot light test but at this point I am stumped on what to try next so I bow to the jeep gods, if any of you are listening. Cheers from down under!

 

[Agreen]

If you've verified +12v and ground (you guys call it earth?) to the ECU, and you have +5v ref coming from it, then you probably have a bad ECU.

 

[Unclewolverine]

I have a 95 rhd so u feel your pain on the no fuse box diagram. Ive searched high and low. It just doesn't exist! Double check the ground on the dipstick bolt, but most likely fried ecu.

 

[8Mud]

Never seen this before. If I was doing the troubleshooting I'd try unplugging sensors one at a time, paying special attention to the Crank position sensor, the Cam position sensor and the Speed sensor. A crude but often fruitful method of finding the source of some issues.

Ideally with low voltage sensor circuits the ground for the sesors should be as close to zero resistance as possible. A quick test I've used in the past is to put my meter on volts and test the ground circuit in various places to the battery negative. It is called standing voltage, kind of like a kink in a garden hose. And seeing as the majority of the sensor grounds use the same ground path with numerous splices it would IMO be a prime candidate for problems.

Maybe you have two separate circuits bleeding into each other. If I remember correctly the Oxygen sensor ground is the same circuit as many other sensors. A couple of prime places for wires to melt down and cause voltage crossover and odd ground paths are the rear O2 sensor wire cooking on the exhaust pipe, the front O2 sensor wire cooking on the exhaust manifold either below or above, where the harness goes up the front of the motor and ties into the fuel injector harness is a classic spot for cooking as is below when the O2 sensor wire holder comes loose and the front O2 sensor wire flops on the exhaust.

Just a couple of ideas.

Please let us know when you find a solution, like I said I've ever seen this one veofre, though I have seen some similiar things.

 

[Tim_MN]

ECM/PCU failure is very highly unlikely, and only should be considered after a thorough and in-depth diagnostics and troubles shooting of the more common and more logical causes for the described symptoms.

A logical assumption with no facts or data yet provided, would be as suggested already, a short circuit due to damaged wires. Start with a visual inspection of all the engine sensor wires including the O2's. Look for chafed wire insulation near sharp metal edges, especially the valve cover, and look for melted insulation near the exhaust. Also look carefully at the fuel pump wiring. The start unplugging things.

Owning a genuine Jeep Service Manual is very strongly recommended. Go to www.pacificcoastmanuals.com

 

[exjays]

Thanks for the responses guys yes I have verified 5v ref a number of times. As I understand it if its good its good and means no sensors are shorted (although I have unplugged everything both all at once and individually anyway including tranny and 02 (defintely not melted) I have even pulled the aftermarket head unit haha. I will do this all another time though just to be sure and report back, will also try the standing voltage test as suggested and pay a bit more attention to metal edges the harness is laying over.

I think the main problem is the fact that it wont cycle the check engine light in the run position, as until this happens it wont do anything in the start position which is when the others come into play. Thats my theory atleast.

Does anyone know what exactly the check engine light at this stage means, does it mean it has tested coms with certain specific sensors, evap or other systems and the pcm or just the pcm or something else?

Is there a cruise control test happening at this stage or is this a red-herring? Does anyone else have the steering-wheel cruise control light flicker on or stay on when turning the key? (I cant remember what is behavior was when the jeep was acting normally) How is cruise powered? I cannot find a fuse or relay anywhere that affects it and it doesnt appear on any of the dodgy fuse panel diagrams I have.

Trying to buy that manual now (thanks for the link!) it says "only $7.95" everywhere but is $14.95 when you put it in the cart? lol, I'll shoot them a message.

I'll defintely keep you guys updated, I despise threads that you get 10 pages into and the OP has not concluded.

 

[exjays]

quick update on the FSM from pacific coast manuals for anyone else with a 96 - they've actually just switched the cover photo on a 97 manual by the looks of it so it is not helpful if youre a beginner like me for much of the 96 electrical system as it has PDC and junction box that are completely different. Also for this issue specifically (check engine light not working) it palming the job off to a DRB scan tool and an additional "Diagnostic Procedures manual" so I think thats a dead end for this particular problem.

 

[Wayne Sihler]

Have you pulled any codes, with the on off on.off on method ? My 96 FSM says start there first.

 

[Tim_MN]

The cruise control should not be ON unless you turn it ON. I would look into this more deeply

IDK about the FSM from pacific coast manuals for 1996, all 8 of my XJ's have been 97-01. Even if not 101% correct, the FSM will provide a better troubleshooting guide than the Internet.

As I recall, the FSM explains the boot-up process of the PCM when starting, and the specific systems/circuits it looks at to have the ASD function, prime the fuel pump, and then turn off. It gives diagnostic procedures, and the expected test results.

 

[exjays]

Alright another update, have been looking over the diagrams and trying to understand how the system is working so I can devise a way to test it, which I think I have done.

Keeping in mind the FP will not stop running with the key in run/on position,

As I understand it from the wiring diagram;

- The FP is powered through the relay obviously
- The relay has is energised by a switched ground from the PCM (pin19 grey connector)
- The ground comes into the relay from the PCM when you turn the key,
- That then closes the power circuit and the pump comes on.

Now to test the signal directly from the PCM to rule out if its being interfered with along the way by short or something;

remove battery neg,
remove PCM grey connector
attach neg of test light to pin19
pos to bat+
reconnect battery, turn the key
test light should stay illuminated for 3 seconds only then go out.

Result for me is that the test light didnt go out,
So the PCM is not cutting the ground.
WHY?

Either:
1. PCM is stuffed
2. Something internally in the PCM waiting for another reference in order to start or end the FP ground signal timer (Short of a speaking to a chrysler engineer I have no idea how I would verify that other than to test another vehicle)

So I dont know 100% for sure if this test sufficent to blame the PCM as at fault because I dont know if this is normal behavior in a working Jeep.

Maybe a long shot haha, but are there any kind volunteers out there who will do this test on thier XJ and see if their test lights go out after 3 seconds?

--- I am assuming in all of this that the FP not cycling off is the cause of my CEL or GEN light not coming on with the key, I could be backwards on this (highly likely) and that the GEN or CEL not coming on is the cause of the FP not cycling off but have to start somewhere.



Wayne - no codes possible mate, CEL doesnt work.

Tim - have tried to dig into cruise more but am at a dead end with that at the moment cant locate any relay or fuse that effects it. This is also on the grey pcm connector so it is not present when turning the key with that one unplugged (I have read elsewhere that this is perfectlys safe to do btw) The next step ofcourse would be to cut FP ground at the plug and run a test wire directly to the FP relay so it can be tested with the grey connector plugged into the PCM but at this stage didnt want to cut the harness until I could hopefully verify the behavior on another vehicle.

 

[exjays]

I have a 95 rhd.

Off topic & you've probably been asked this a million times but i'll bite, how tf do you have a RHD in Kansas!?

 

[Wayne Sihler]

If- I,m reading this correctly from the FSM. the PCM operates the CC circuit.>12 volts to the LED in the steering wheel.Circuit also 12V to stop lamp switch.

The VSS sensor in the trans is also in play too, SO my Guess Is problems with those sensors or wiring to them...or the PCM is bad...This is only a guess .

 

[Unclewolverine]

Off topic & you've probably been asked this a million times but i'll bite, how tf do you have a RHD in Kansas!?

I'm a rural mail carrier, there are companies that would re-import them to sell to mail carriers like me. Mine is from Japan, has Japanese registration stickers all over it and a honda of yokohama dealer sticker.

Thats right, be jealous, I get paid to drive my xj everyday, and equipment allowance for repairs too.

 

[RCP Phx]

I'm a rural mail carrier, there are companies that would re-import them to sell to mail carriers like me. Mine is from Japan, has Japanese registration stickers all over it and a honda of yokohama dealer sticker.

Thats right, be jealous, I get paid to drive my xj everyday, and equipment allowance for repairs too.

They weren't even imported, AZ bought hundreds of them for gov work along with the Forest and Postal service.

 

[exjays]

More investigationing by testing PCM ground outputs, the PCM is NOT sending a ground to the ASD relay. (I thought I tested this as was working before as one of my first steps, but maybe I did it incorrectly) I can feel it click though so.. stumped again

Anyway, I have retested with the multimeter to see if there is 12v coming through to that circuit with the key on (on it is coil, injectors & 02 heater). Found there is not 12v here, dead.

So, rather than look for busted wires immediately I tested to see if the PCM is actually supplying ground to the ASD relay with they key on in order to be getting the 12v out the otherside, and it is not (this pin is C3, grey plug for anyone dealing with this in the future)

My new theory is that the PCM is requiring the ASD sense input (verifying to the comp that the ASD has powered its gear) in order to stop the pump cycling (and maybe also produce the CEL test)


------ From my manual, list of actions when the Key is switched on ------

This is an Open Loop mode. When the fuel system
is activated by the ignition switch, the following
actions occur:

• The powertrain control module (PCM) pre-posi-
tions the idle air control (IAC) motor.

• The PCM determines atmospheric air pressure
from the MAP sensor input to determine basic fuel
strategy.

• The PCM monitors the engine coolant tempera-
ture sensor input. The PCM modifies fuel strategy
based on this input.

• Intake manifold air temperature sensor input is
monitored.

• Throttle position sensor (TPS) is monitored.

• The auto shutdown (ASD) relay is energized by
the PCM for approximately three seconds.

• The fuel pump is energized through the fuel
pump relay by the PCM. The fuel pump will operate
for approximately three seconds unless the engine is
operating or the starter motor is engaged.

• The O2S sensor heater element is energized via
the ASD relay. The O2S sensor input is not used by
the PCM to calibrate air-fuel ratio during this mode
of operation

I assume that these are not in order as it wouldnt make sense, but it does raise another question "The fuel pump will operate for approximately three seconds unless the engine is operating or the starter motor is engaged" perhaps the computer thinks the starter is engaged when it isnt and that is running the pump and screwing everything else up, or maybe the PCM is just dodgy on ASD ground supply. So thats where I'm at for now, I'm not sure what to do next at the moment. More coffee.

- Wayne cheers mate I hadn't got onto CC stuff yet since just getting this fsm, will follow those up tomorrow but I think I'm at a major snag now with no asd ground from the pcm, it may indeed be bad. I'll tear into it for a look and post photos if it comes to that.

Nice one wolverine it must be a real talking point when people see it around! that repairs allowance sounds nice for some free upgrades, I mean "repairs"

 

[Wayne Sihler]

Local mail delivery was by a fellow in an XJ.He would stop & ask about parts & repairs until he retired.Then a woman took over and drove her XJ also, until thee postal service made them use the white box POS they have now,,,she was not happy. Thing is too small,wont go in the snow and is always breaking down. She tells me she wishes she had her XJ back.

 

[fahadwizz]

I am having the exact same issue.

Did you ever find out what was wrong ?

 

[fahadwizz]

Guys. Ill really appreciate some help here!!!

 

[fahadwizz]

Replaced PCM and the problem was solved. Also prior to the break down my "GEN" light would stay on all the time. No problems with the alternator as it would put out 14.5 volt. Could it indicate a problem with my ECU's internal voltage regulator? I also had a weird problem of burning through ignition coils quite frequently. Burned through a couple of MOPAR coils but eventually had to settle for the cheap ones as none would last more than a month. Eventually shifted to coils used by old Japanese cars. Even used the ones designed to run with a ballast resistor but i'd use it without one and the jeep would run fine. I think this 6v coil lasted the longest. I have however moved to OEM coil after replacing the ECU in an attempt to figure out what messed up the previous PCM.

Just to shoot down the coil problem, I have already replaced CPS, Distributor Cap, plugs, plug wires. Now i am hoping that it was always the PCM with a bad regulator that was frying my coils.