cherokeesportlbi

so my shop called the other day and said they cant figure my jeep out and that the only thing that could be wrong is the computer. he did a compression test and all was fine so i guess he has to be right. but would a bad computer make my jeep not just run one day out of the blue?

Running Free

need to know year and motor size

BLACKSPORT

Crankshaft position sensor i have heard if they go bad the jeep wont start up and that would be cheaper than the ecu. double check but that could be a possibility

ASNFMOM

X2 on crankshaft position sensor

JGCORNEL

X3 on the Crankshaft position sensor. They just go out... I pulled into a gas station shut my jeep off pumped my gas and when I went to start it.... No spark. I would definitely check that first.

cherokeesportlbi

ok should of been more specfic 1996 xj 4.0. i replaced the tps,cps,iac new plugs, new dist. cap ,new wires, new sarter,new alternator,new fuel filter changed oil. i have done every thing. i have fuel pressure my compression is good it will start and run rough and shut off if below 1500 rpms. so it must be the computer right cause its throwin all the same codes still and there all brand new parts.

ASNFMOM

All you can do is try it. I worked at a parts store for six years and the cps is one of the most warrantied sensors on Cherokees. I'd say try the computer if that doesn't work then you got a bad cps.

cherokeesportlbi

the cps i bought was brand new and i replaced it so it cant be bad

ASNFMOM

I have had brand new ones warrantied same day hell its happened to me twice one cps and one o2 sensor it could be a manufacture defect it happens all the time what brand sensor did you buy. All I'm saying is from your description of your problems it could still be the cps or it could very well be the computer. Like I said earlier your just gonna have to try it.

freegdr

i would check connectors at ecm remove and clean also check grounds by oil dip sticl

dukie564

it could very well be the computer, but i wouldn't overlook the fact that a "new" CKPS sensor can still be bad due to manufacturing defects. Have they checked out the Camshaft sensor also? What codes is it throwing?

mentalbreakdown00

First what codes does it have? Second are you only missing spark or both spark and injector pulse? Also lastly, I run my own shop and only buy OEM crank, cam, and MAP sensors. I have put new ones in from varying stores, and still had the same problem or a no start condidtion, beat my head on a wall for a day to find out had a bad new sensor.

JeepingDan

I've had problems with new cps before.
I say test it and make sure it's working.

BTW the one's I had trouble with were from AZ if you want it to be right get it from the dealer for a few bucks more

cherokeesportlbi

how do i test it to make sure its not bad?

dukie564

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 engine gets hot, but works again when it cools back down.


Symptoms-
- 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 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 OBDII code reader cannot make a connection to the computer or cannot read Check Engine Light/MIL codes because the CPS/CKP has failed.


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, 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 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.

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

CPS Testing


Crankshaft Position Sensor Connector (CPS/CPK)


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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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