Another Cranks, won't start problem - I'm at a loss

1999 XJ 4.0, 5-speed
145000 miles

So here's the story, my Jeep started stumbling about three weeks ago and acted like it wasn't getting fuel. This only happened once for about 30 seconds and then was fine for two weeks of driving. It had never had a fuel pump so I decided to replace it as a preventative measure last weekend as we're taking the Jeep on a road trip a week from today (hopefully). I also replaced the upper and lower radiator hoses, thermostat and water pump yesterday. Not sure why that would be relevant but want to make sure I post all info. Jeep ran fine all day yesterday.

Anyway, I was doing some rewiring of the front speakers today (battery was disconnected) Got the Heep back together and almost fired then just kept cranking. Now it won't start, just cranks.

Here's where I'm at, testing the usual suspects:

- Not getting spark at coil wire
- Swapped the ASD relay with the blower relay, no luck
- Tested the coil - tests fine
- Tested the CPS - test fine

Does the 4.0 have an ignition control module or does the CPU serve that function? My Haynes manual isn't really clear and couldn't find much from searching. I know when I had a S10 Blazer I blew out the ICM from keeping the ignition in the "on" position too long. That is the only thing I did today as I was testing the stereo so that came to mind.

I'm at a loss, is there anything I'm missing? Can a CPS or coil test okay and still be faulty?

Any help is greatly appreciated! Trying to get this thing back on the road.

For 1996-2001 Jeeps, the OBD-II engine computer (ECU/PCM) re-boots, and the idle settings are deleted, when you: (1) let the battery run down by leaving the lights or radio on, (2) disconnect the battery for an extended period of time, (3) or when the battery goes bad and needs replacing.

Your symptoms are: (1) having difficulties starting the engine without depressing the gas pedal, and (2) the engine will have a low idle and probably stall unless you keep your foot on the gas pedal.

The ECU will relearn the idle settings after a short period of time and the engine will start and idle normally.

Keep your foot on the gas pedal and pump it if needed.

The ICM was eliminated in 91+ and the PCM provides that function. Test the primary lead to the coil for 12 volts--getting it?

Does the fuel pump "prime" for 3 seconds when you turn the key to the ON position?

Tim_MN said:

Your symptoms are: (1) having difficulties starting the engine without depressing the gas pedal, and (2) the engine will have a low idle and probably stall unless you keep your foot on the gas pedal.

The ECU will relearn the idle settings after a short period of time and the engine will start and idle normally.

Keep your foot on the gas pedal and pump it if needed.

Click to expand...

It really isn't even acting like it's going to start at this point and the gas pedal seems to have zero affect. Wouldn't I still be getting spark if this was the case???

joe_peters said:

The ICM was eliminated in 91+ and the PCM provides that function. Test the primary lead to the coil for 12 volts--getting it?

Does the fuel pump "prime" for 3 seconds when you turn the key to the ON position?

Click to expand...

The fuel pump does prime when the igntion is turned to ON.

I assume the grey wire going to the coil plug must be the primary because I tested the green and it seems to be the ground. Is this correct? I am not getting 12V at the grey wire with the ignition in the ON position. I am also not getting 12V between both wires going into the coil with key turned to ON (I would assume this would be the same test)

Ok, you need to solve for power to the coil. That power comes from the ASD relay, with a ground from the PCM providing the trigger to the relay. Now, not 100 percent on the late models but I believe that the fuel pump relay passes the 12 volts to the ASD, which then passes it to the coil. If someone knows better, please post up.

I would work back from the coil until you can get the 12 volts for it.

Good luck.

If the pump primes that eliminates the crank sensor from the equation. My two cents...

99_XJ_ said:

If the pump primes that eliminates the crank sensor from the equation. My two cents...

Click to expand...

Interesting. I wonder if the pump priming eliminates anything else?

Is there any possibility I loosened something under the dash? I know it's a long shot but it started until I took out the stereo and rewired the speakers. I also was routing wire behind the glove box and near the passenger fuse block.

Maybe it's just a coincidence.

Not trying to thread jack but I just started having this problem on my 97 yesterday. I was thinking it was fuel related too but I checked the little nipple on the fuel rail and I'm getting fuel/pressure. Not getting a spark though. It will crank in P but if I try to start in N I turn the key and it won't even crank, maybe a NSS and a CPS at the same time what are the odds of that!?

tone said:

maybe a NSS

Click to expand...

What is a NSS?

neutral safety switch

See if it will start in neutral, if it starts in N then that's what it is. If it still does the same thing then maybe the CPS

MtnBkr said:

........(battery was disconnected) Got the Heep back together and almost fired then just kept cranking. Now it won't start, just cranks.

........ Can a CPS or coil test okay and still be faulty?

Click to expand...

The first part of your post points to a re-boot.

Cranking and not not starting is most commonly the CPS, and yes they can test good and still be failed.


If the NSS is dirty or failed, it will not crank at all.
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The most likely cause of it cranks and cranks but won't start up is the Crankshaft Position Sensor (CPS) located on the transmission bell housing. Often this part is also referred to as the CranKshaft Position Sensor (CKP) CPS/CKP failure is very common. The CPS/CKP can stop working with no warning or symptoms and the engine will not run or the engine may randomly stall for no apparent reason.

Crank Position Sensors can have intermittent “thermal failure”. This means that the CPS/CKP fails when the engine gets hot, but works again (and will test as “good”) when it cools back down.

Begin with basic trouble shooting of the start and charge systems. Remove, clean, and firmly reconnect all the wires and cables to the battery, starter, and alternator. Look for corroded or damaged cables or connectors and replace as needed. Do the same for the grounding wires from the starter to engine block, and from the battery and engine to the Jeep's frame/body. Jeeps do not tolerate low voltage or poor grounds and the ECM/ECU may behave oddly until you remedy this.



Symptoms –
- Starter cranks and cranks but engine won't start up and run.
- Fuel gauge and voltage gauge may not work or display properly.
- You sometimes will have No Bus on the odometer after 30-60 seconds.
- A failed CPS/CKP may or may not throw a CEL trouble code.
- No spark at the spark plugs.
- Fuel pump should run and prime for 3-5 seconds.

If the CPS/CKP is failed sometimes the OBD-II trouble code reader cannot make a connection to the ECU/computer or cannot read Check Engine Light/MIL codes because the CPS/CKP has failed. Disconnect the code reader, disconnect the CPS/CKP wire connector, and reconnect the code reader. If the code reader establishes contact with the ECU and scans, your CPS/CKP is failed and needs to be replaced.


Diagnostic steps to confirm the CPS is the cause of your no-start


You should be able to verify a bad cps, by unplugging it, and turning the ignition key to on. If the voltage gauge and/or the fuel gauge now displays correctly and/or the No-Bus is gone, replace the CPS.

Unplugging and reconnecting the CPS sensor where it connect to the main harness near the back of the intake manifold usually resets the ECU and if the jeep fires right up after doing this you can bet that the CPS is faulty and needs to be replaced.

Exchange the fuel pump relay and the ASD relay with one of the other similar ones in the PDC to eliminate the relays as the cause of the no-start. Check the ASD relay fuse.

Confirm that the fuel pump to runs for 3-5 seconds when you turn the ignition key to ON.

Eliminate the NSS as a cause of no start. Wiggle the shift lever at the same time you try to start. Put the transmission in Neutral and do the same. Do the reverse lights come on when the shifter is in Reverse?

Inspect the wires and wire connectors at the O2 sensors on the exhausts pipe. A short circuit from melted insulation or from broken O2 sensor wires can blow a fuse and the ECU/ECM will loose communication. Don’t get tunnel vision and assume the sensor is bad (unless it tests bad with a meter). Damaged wiring or a dirty connector can inhibit the signal from making it to the computer. Inspect/test/clean/repair wiring or connectors as necessary.


CPS Testing


Crankshaft Position Sensor Connector (CPS/CKP)



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TESTING PROCEDURE 1991 – 2001 4.0L H.O. engines

1. Near the rear of intake manifold, disconnect sensor pigtail harness connector from main wiring harness.
2. Place an ohmmeter across terminals B and C (See Image). Ohmmeter should be set to 1K-to-1OK scale for this test.
3. The meter reading should be open (infinite resistance). Replace sensor if a low resistance is indicated.

TESTING PROCECURE for 1987 – 1990 4.0 L engines

Test # 1 - Get a volt/ohm meter and set it to read 0 - 500 ohms. Unplug the CPS and measure across the CPS connector's A & B leads. Your meter should show a CPS resistance of between 125 – 275 Ohms. . If the CPS is out of that range by much, replace it.

Test # 2 - You'll need a helper for this one. Set the volt/ohm meter to read 0 - 5 AC volts or the closest AC Volts scale your meter has to this range. Measure across the CPS leads for voltage generated as your helper cranks the engine. (The engine can't fire up without the CPS connected but watch for moving parts just the same!) The meter should show .5 - .8 VAC when cranking. (That's between 1/2 and 1 volt AC.) If it's below .5vac, replace it.


The 2000 and 2001 will have the CPS in the same location on the bell housing, but the wire connector may be on the passenger side, not as shown in the diagram below. Simply follow the wire from the sensor to the connector.

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Thanks Tim I'm gonna try that when I get home! :cheers:

Thanks, this is all good info.

The CPS definitely tests fine and doesn't give any of the above symptoms but I might just replace the damn thing anyway.

One other thing, the fuel pump probably only primes for 1 maybe 2 seconds, definitely not 3+. Not sure if that really matters. Where is the fuel pump relay located?

Also, what are the relay's under the dash on the passenger side block? Can't find anything in my Haynes or owners manual.

In the FWIW category, the fuel pump in my '93 has never primed for more than a second, if that. Three seconds is a looong time -- you can move a lot of gas in three seconds.

Well, I replaced the CPS to no avail....

On a side note, the CPS took 15 minutes to swap with the help of someone guiding the socket on from the top.

MtnBkr said:

Well, I replaced the CPS to no avail....

On a side note, the CPS took 15 minutes to swap with the help of someone guiding the socket on from the top.

Click to expand...

Not too surprised about this. The CPS/CKP crank sensor is actually a signal generator, it generates a high or low signal depending on the notches on the flexplate/flywheel, and that allows the ECU/PCM to count the signals and know exactly where the engine's components are at a given time--the ECU/PCM will trigger the coil or ICM/coil at the appropriate times and fire the spark. Now, if there is no power to the ICM/coil or coil primary windings, well, there won't be any spark.

Solve for no voltage from the ASD relay to the coil (or B Latch relay to the ICM/coil).

Good luck.

joe_peters said:

Not too surprised about this. The CPS/CKP crank sensor is actually a signal generator, it generates a high or low signal depending on the notches on the flexplate/flywheel, and that allows the ECU/PCM to count the signals and know exactly where the engine's components are at a given time--the ECU/PCM will trigger the coil or ICM/coil at the appropriate times and fire the spark. Now, if there is no power to the ICM/coil or coil primary windings, well, there won't be any spark.

Solve for no voltage from the ASD relay to the coil (or B Latch relay to the ICM/coil).

Good luck.

Click to expand...

Yeah, I don't like throwing parts at it but I figured it wouldn't hurt since I've never replaced it.

What are the chances it's the ECM? Is there any way to bench test it? Would my next step be to take it to the stealership? I've about reached the limit of my diagnostic skills.

The ignition switch provides the 12 volt supply to the fuel pump relay, and through it to the ASD relay. the ECU/PCM provides the ground for the relays--used to be pin 51, not sure exactly for your year. Unplug the ECU/PCM, turn on the ignition switch, jumper a ground to both the relays and see if you can get 12 volts to the coil.

joe_peters said:

The ignition switch provides the 12 volt supply to the fuel pump relay, and through it to the ASD relay. the ECU/PCM provides the ground for the relays--used to be pin 51, not sure exactly for your year. Unplug the ECU/PCM, turn on the ignition switch, jumper a ground to both the relays and see if you can get 12 volts to the coil.

Click to expand...

So if I understand you correctly, run a jumper to the #85 pin of both relays to any ground with the ECU unplugged and ignition to on.

Is this basically taking the ECU out of the equation to test for the 12v at the coil?