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Unread 06-04-2010, 12:31 PM   #1
rangers5
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96 cherokee wont start

From one day to the next 96 jeep 4.0 v6 wont start. Fuel pump is ok. Fuel filter changed and relays checked. Rolls but just wont turn over and keep running. Going to check for spark but after that what next? Thanks for any help

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Unread 06-04-2010, 12:36 PM   #2
CJ7-Tim
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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 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, 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. 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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Unread 06-05-2010, 09:26 PM   #3
emosher
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Awesome explanation, and great timing. My 93 HO died today, with the symptoms you've outlined. I will head to autozone for a CPS.

Thanks!
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Unread 06-06-2010, 08:29 PM   #4
emosher
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OK, attempting to remove the bolts from the top I can't seem to get a socket head on them. Does anyone know what size the mounting bolts are?

is it easier to get them from underneath?

Thanks,

Eric M
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Unread 06-06-2010, 08:34 PM   #5
postaldave
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unbolting the bar that bolds the the transfer case up helps. put a jack under the trany, unbolt and lower the drivetrain down.

i would do some testing before changing the CPS.
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Unread 06-06-2010, 08:35 PM   #6
xjfever
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Yes, underneath is the easiest way. You will need a swivel adapter and around 15" of extension. I want to say they are 13mm, but it has been a while.
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Unread 06-06-2010, 09:30 PM   #7
emosher
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Quote:
Originally Posted by postaldave View Post
unbolting the bar that bolds the the transfer case up helps. put a jack under the trany, unbolt and lower the drivetrain down.

i would do some testing before changing the CPS.
thanks, that is sound advice. However, I already got one so the damn thing is going on
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Unread 06-07-2010, 07:39 PM   #8
emosher
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I got it.

I had to go from underneath with a n 18 inch extension and a wobbly head. It is an 11mm bolt head, just for the record.

It was harder to get out than to put in, taking out took a while, one little click at a time on the ratchet. Putting it back in I was able to get my ogre hands up in there and spin the bolt most of the way with my fingers.

Fired right up.
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Unread 06-07-2010, 07:40 PM   #9
emosher
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Quote:
Originally Posted by xjfever View Post
Yes, underneath is the easiest way. You will need a swivel adapter and around 15" of extension. I want to say they are 13mm, but it has been a while.
Thanks, this was the "formula" I followed to get it replaced.
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Unread 06-07-2010, 07:52 PM   #10
xjfever
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You can get in some really awkward positions trying to get your hand in there, but I'm glad you got it.
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  2. Sold'99 WJ limited, 4.7L, 3" IRO lift. Bilstein shocks, JK rubicon wheels, 255/70/17's, carolina driveshaft, IRO adjustable control arms, IRO adjustable A-arm
  3. Sold '00 TJ, 4" lift, rusty's adjustable control arms, skyjacker shocks, rock krawler adj. trac-bar,rugged ridge SYE and RE driveshaft, soft 8's, 35" mud king xt's,XJ HP D30 w/aussie locker, 4:88 yukon gears, hella 500's, rock crusher diff covers
4. 04 Dodge ram 1500 quadcab, yes, it has a hemi.
5. '08 BMW X3, Mods in progress.
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