Crank, no start even with new CPS...

[Xtreme XJ]

Last eve splashed through some rain puddles the XJ is running fine, parked and idling fine then the XJ just shut off... probably sitting & idling for 4/5 mins. when it shut down. It's a 96 Sport 4.0L auto 4WD...
This AM I noticed the tach. wasn't registering so I R & R'd the CPS (Napa Echlin) and still no start or tach. RPM.
I understand the value of the Mopar sensor or even a NTK, but neither are quickly accessible and time is a HUGE concern as I need to use it early next week.

As far I can tell from pulling a plug wire & testing for spark there is none...

I do seem to be a magnet for getting bad sensors... so am I back to pulling the new sensor cursing & swearing and getting another ?

Any other suggestions ? While I'm OK with turning wenches I'm no master troubleshooter.

 

[Tim_MN]

Any Check Engine Light trouble codes ?

Testing the CPS is simple.

When the ignition key is first turned ON, 12 volts travels from the ignition switch to the ASD relay. The PCM provides ground to the ASD relay to energize it. The ASD relay sends the 12 volts to the primary side of the coil. If after a few seconds no signal is detected from the faulty CPS, the PCM opens the ASD relay and 12 volts power to the coil and fuel pump is removed.


If the CPS/CKP is failed sometimes the OBD-II code reader cannot make a connection to the computer or cannot read Check Engine Light/MIL codes because the CPS/CKP has failed.

CPS Testing
TESTING PROCEDURE 1991 –2001 4.0L H.O. engines
1. Near the rear of fuel rail, disconnect sensor pigtail harness connector from main wiring harness.
2. Place an ohmmeter across terminals B and C or 2 and 3. Ohmmeter should be set to the 10K-or-2OK scale for this test.
3. The meter reading should be open (infinite resistance). Replace sensor if a low resistance is indicated.

 

[Xtreme XJ]

No CEL...

I'll see if I can manage to test it...

Thanks for the input...

 

[Anak]

Besides looking at the CPS, have you pulled the distributor cap?

 

[Xtreme XJ]

I did, but nothing looked obvious... cleaned it out.
It probably needs to get a cap & rotor, but I cant see it being my issue... in the top center of the cap is a tit that is spring loaded that presses down on the rotor, it seemed a bit sticky...
It was just like flipping a switch when it shut down...

 

[kastein]

Any Check Engine Light trouble codes ?

Testing the CPS is simple.

When the ignition key is first turned ON, 12 volts travels from the ignition switch to the ASD relay. The PCM provides ground to the ASD relay to energize it. The ASD relay sends the 12 volts to the primary side of the coil. If after a few seconds no signal is detected from the faulty CPS, the PCM opens the ASD relay and 12 volts power to the coil and fuel pump is removed.


If the CPS/CKP is failed sometimes the OBD-II code reader cannot make a connection to the computer or cannot read Check Engine Light/MIL codes because the CPS/CKP has failed.

CPS Testing
TESTING PROCEDURE 1991 –2001 4.0L H.O. engines
1. Near the rear of fuel rail, disconnect sensor pigtail harness connector from main wiring harness.
2. Place an ohmmeter across terminals B and C or 2 and 3. Ohmmeter should be set to the 10K-or-2OK scale for this test.
3. The meter reading should be open (infinite resistance). Replace sensor if a low resistance is indicated.

Excellent advice - only two things to add.
The CPS will not send the ECU a useful signal with the engine not turning, thus the ECU will turn the fuel pump and ASD relays off after a few seconds if you just key-on without turning the key to start, or if the engine is cranking but the ECU has not seen enough correctly sized (length and voltage) to recognize where the crank is in its cycle.

This test for the CPS is also a failure test - it cannot tell you a CPS is *good*. If it fails this test, it is definitely 100% bad. If it passes this test, all you know is it is not dead in that exact way that the test checks for. It can still have multiple other internal failure types, just not the one that shorts the sensor out completely. The only way to prove a CPS is good is to hook up an oscilloscope to the sensor ground and CPS signal output pins with the vertical and horizontal scales set correctly and watch the signal appear on the screen, then verify it is the pattern that should appear.

 

[Xtreme XJ]

Thanks for the response... I had to take it in to have it looked at...
Hopefully they'll get back with me soon...

Curt

 

[lawsoncl]

The only way to prove a CPS is good is to hook up an oscilloscope to the sensor ground and CPS signal output pins with the vertical and horizontal scales set correctly and watch the signal appear on the screen, then verify it is the pattern that should appear.

Backprobing the connector with a voltmeter should show something. Older Renix had a/c spikes that varied in voltage with rpm. Newer, it's a nice clean square wave. Measuring on a/c you might have to swap the leads on some low-end meters if it's polarity sensitive (ie only measures one half of the wave).

 

[Xtreme XJ]

Thanks to those who offered direction...
To follow up on the issue... it was a coil... which had very weak spark...

Thanks again for the help...
Curt

 

[Tim_MN]

Thanks for the follow up. The CPS is the most common and most logical primary suspect for a no-start and/or random stalling. Once the CPS is tested, and passes in your case, one can move on to the next most likley suspects for testing, ignition issues.

 

[kastein]

I'm not sure why I didn't think of that, because I've actually had more bad ignition coils cause this than CKPs. I did the CKP on my 91 because everyone refused to accept that it could be something else until I did it and it fixed nothing and the coil eventually died all the way, and I did my CKP on my 98 because I bought both and wasn't walking to the parts store twice, and it ended up being the ignition coil on that one too.

 

[Jam Master Jay]

Is the engine attempting to fire? stumble?

 

[Tim_MN]

I've actually had more bad ignition coils cause this than CKPs.

I have owned seven 97-01 XJ's and five required CPS/CKP replacement to cure a no-start/stalling condition. One was purchased from an exasperated owner with no-start condition and laundry list of new parts. I fixed it within an hour with a fresh CPS, and re-sold it in a few days for 3x more than I paid for it. I carry a spare brand new genuine Jeep CPS in both my current XJ's.

 

[Xtreme XJ]

Is the engine attempting to fire? stumble?

It is now fixed... but no it wasn't stumbling...

Curt