No start, brief power to coil

[Evan_in_CA]

Hi all, brand new to the forum here, been working with my son on resurrecting a 98 4 door XJ for the last few months. We’ve done quite a lot of work to get the engine tip-top and it runs great now. Starts up immediately and runs smooth after a doing a lot of deferred maintenance.

This evening he went to go to the store and the Jeep will not start. It cranks over but will not fire. I tested power to the coil and it gets a brief spike to about 10 volts then drops back to zero, so it looks like something is going on with the power being sent to the coil. Fuses all look good, it has fuel etc. It ran great last night so this is pretty frustrating! Any advice on what to look at would be appreciated.

Thanks,
Evan

 

[Tim_MN]

The most likely cause of it cranks and cranks but won't start up is the Crank Position Sensor (CPS). CPS failure is very common. The CPS can stop working with no warning or symptoms and the motor will not run.

Symptoms -

Fuel gauge and voltage gauges may not work or display properly.

You sometimes will have No Bus on the odometer after 30-60 seconds.

NO spark.

Fuel pressure is OK

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

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

You might 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, replace the CPS.

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.

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?

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 and replace as needed. Do the same for the grounding wires from the battery and engine to the Cherokee's frame/body.

CPS Testing


Crankshaft Position Sensor Connector



TESTING PROCEDURE 1991 – 2001 4.0L H.O. engines
Near the rear of intake manifold, disconnect sensor pigtail harness connector from main wiring harness.
Place an ohmmeter across terminals B and C (SEE Image). Ohmmeter should be set to 1K-to-1OK scale for this test.

The meter reading should be open (infinite resistance). Replace sensor if a low resistance is indicated.

 

[Evan_in_CA]

Hey Tim, thank you! While you typed out this awesome reply I actually found a thread in teh FAQ about no start troubleshooting and unplugged/replugged the CPS a few times and it started right up. Looks like I’m headed to the parts store to get a new CPS. Thank you for the speedy reply, much appreciated!

 

[Tim_MN]

Genuine Jeep engine sensors are very strongly recommended. Most auto parts stores sell cheap crappy Chinese made parts, some that even come with a "Lifetime Warranty". These parts are poorly manufactured and/or made from inferior materials. They are often out of specification, or even failed, right out of the box. The ones that are not faulty many times will have a short service life before they fail. Always buy top quality replacement parts and genuine Jeep engine sensors. Numerous threads detail long and frustrating searches for a "problem" that ended up being cured simply with genuine Jeep repair parts.

Cheap parts are cheap for a reason.

 

[Evan_in_CA]

Roger that, I am all about the top quality parts. A good example - we did the fuel pump since it wasn’t maintaining pressure in the rail and I sprang for the complete Delphi unit. I hate having to fix things twice.

 

[Runnin'OnEmpty]

I have a theoretical question:

When a bad CPS causes a no-spark situation, would
"hot-wiring" the coil with 12v allow the engine to start?

 

[lawsoncl]

I have a theoretical question:

When a bad CPS causes a no-spark situation, would
"hot-wiring" the coil with 12v allow the engine to start?

No, because the crank sensor is needed to determine when to spark and inject fuel.

 

[boncrshr]

Hey Tim, thank you! While you typed out this awesome reply I actually found a thread in teh FAQ about no start troubleshooting and unplugged/replugged the CPS a few times and it started right up. Looks like I’m headed to the parts store to get a new CPS. Thank you for the speedy reply, much appreciated!

If you have the cash I'd go ahead and get two and put one in the glove box and carry the tools necessary to change it in the vehicle with you. That is the one part on your most likely to leave you unexpectedly stranded.