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1999 XJ - No start - PCM?


NAXJA Forum User
My buddy dropped off his 1999, 4.0 auto XJ for a no start for me to look at.
Cranks but does not start.
Turning the key i notice the volt gauge on the dash and the check engine light does not come on.
I look at the ASD relay and it and the fuel pump relay are not clicking.
All fuses check out good.
I go the PCM and back probe A22 on the black plug and it's got battery power, A31/32 both have ground.
I pull the connector to visually inspect, all looks ok, no loose pins on the PCM, no corrosion that I see.
While plugging it back in, I accidently left the key on, the relays all went nuts. The ASD and/or fuel pump started rapidly clicking like they do when they are low on voltage, but if I wiggled the plug just, not seated all the way into the retainer, the the ASD and fuel pump relay worked, the fuel pump primed, and pin A2 on the PCM had power, the volt gauge on the dash started working (The fuel gauge did not, not sure if it did before however). The check engine light now comes on with the key and I am able to pull codes.
p0113 - TPS
p0123 ECT
p0118 IAC
p1492 - Battery temp
None of those really meant anything to me for a no start, but I am a Ford tech, not a Jeep tech.
I put a noid light on the fuel injector, it is not pulsing/ no signal.
I did not check for spark, it was late...

Not really sure where to go from here.
I will check to see if those sensors are getting 5v reference.
See if fuel injectors are getting power.
See if the coil has power?
Any other ideas?
Does the CPS take away spark or fuel pulse?
If it was a Ford computer I could open it up and look for the brown stains, but I can't .
If feels like a PCM issue, but I want to be sure before I tell him to get that repaired, the whole not seating the connector all the way is strange, but I will clean it all tonight.
Does the CPS take away spark or fuel pulse?


PCM failure is very highly unlikely and should only be considered after a thorough and in-depth diagnostic of the more logical and more common causes for an XJ Cherokee no-start.

Fix any known or suspected faults like a failed relay or damaged relay socket. The receptacle plugs for the relays will occasionally break loose and only make intermittent contact. I have had to use JB weld to secure a fuel pump relay socket.

The symptoms suggest a failed CPS, or another one of the other 5 volts OBD engine sensors. ECM/PCM failure is very highly unlikely and should only be consider after thorough and in-depth diagnostics of the more common and more logical suspects for the symptoms described.

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.

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. Typical genuine Jeep CPS lifespan is about 150-200,000 miles.

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

Typical CPS Symptoms, (not all symptoms may be present, or occur at the same time) -
- Random stalling
- Starter cranks and cranks but engine won't start up
- Fuel gauge and voltage gauges may not work or display properly.
- You sometimes will have NoBus displayed on the odometer after 30-60 seconds.
- A failed CPS/CKP may or may not throw a CEL trouble code.
- Fuel pump should run and prime for 3-5 seconds.
- No spark at the spark plugs.

Diagnostic steps to help 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, 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 these relays as the cause of the no-start. Confirm that the fuel pump 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 lose communication.
-Check the ground connection for the PCM. Make sure it is clean, snug, and has no corrosion.

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 (See Image). 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.

If you buy a new CPS, get a genuine Jeep CPS. Most auto parts stores sell cheap crappy Chinese "Lifetime Warranty" parts that are poorly manufactured from inferior materials and are often out of specification, or even failed, right out of the box. The ones that aren’t faulty often have a short service life before they fail. Always buy top quality replacement parts and genuine Jeep sensors. Numerous threads detail long and frustrating searches for a "problem" that ended up being cured simply with genuine Jeep repair parts. Buy good quality repair parts and genuine Jeep sensors for best results.

You must also perform 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. Jeeps do not tolerate low voltage or poor grounds and the ECM/ECU will behave oddly until you remedy this.
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