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Unread 08-19-2012, 08:33 AM   #1
SpaceCowboy70
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Jeep Cherokee Won't start

You've heard it before. I've got a 1995 Jeep Cherokee (4.0 In-line six) that my wife loves. It starts every time, first time, until Friday. It turns over, it does not catch, not even a cough. Engine light comes on, you can hear the fuel pump go through its whine. I replaced the cap and rotor, and ultimately the coil. I have another Cherokee, and I switched out all the related fuses to test if that was the issue. No joy. I read on another post where some knowledgeable guy suggested replacing the crankshaft sensor. I have no idea where that is. All help greatly appreciated.

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Unread 08-19-2012, 10:20 AM   #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 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 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.

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.

If you buy a new CPS, get a genuine Jeep CPS, or the premium one from NAPA. Cheap crappy “Lifetime Warranty” parts are often out of specification or even failed right out of the box. They usually also have a shorter service life than better quality parts. Buy good quality repair parts and genuine Jeep sensors for best results.

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/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 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.

TESTING PROCEDURE 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, near or on top of the Transfer case, not as shown in the diagram below. Simply follow the wire from the sensor to the connector.
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Unread 08-19-2012, 11:09 AM   #3
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A wealth of information. I certainly do appreciate it.
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Unread 08-19-2012, 11:47 AM   #4
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Unread 08-19-2012, 12:23 PM   #5
moored194
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If you do end up changing the sensor which isn't a bad idea. Two tips 1. Get a oem one from the dealer I went through two bad ones from the parts store what I saved in money was long lost on labor and time. 2. I hope you have small hands it's at an awkward spot for any grown mans hands to get to. I had to use a 3" extender and an angle on my 1/4" socket. I couldn't get wrenches up there.
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Unread 08-20-2012, 03:38 PM   #6
jserna
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I helped my brother change the CPS on his 94 chero. It still does not work. We did use a CSP from Autozone. Let say the sensor is fine, what next to look at. We also checked all the same stuff that was mention before. Could it be the factory alarm?
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Unread 08-20-2012, 06:34 PM   #7
CJ7-Tim
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Quote:
Originally Posted by jserna View Post
I helped my brother change the CPS on his 94 chero. It still does not work. We did use a CSP from Autozone. Let say the sensor is fine, what next to look at. We also checked all the same stuff that was mention before. Could it be the factory alarm?
What was the reason for changing the CPS ? IdoitZone CPS are the cheap crappy ones.

There was no factory alarm sytem. A poorly installed or faulty alarm system could cause a no-start.
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Unread 08-20-2012, 09:14 PM   #8
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before you change the cps you might want to test it:
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Unread 08-21-2012, 06:38 PM   #9
rajacks
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Jeep Won't Start!!!!

I have similar but seemingly more serious problems. My 98' Jeep Cherokee XJ (4.0L inline 6) will not start as well. I don't know if the fuel pump is working while trying to start (don't know what that sounds like). I also don't understand the starting termionolgy used on this site such as the words "crank", "turn-over" and "spark". The best way I can discribe the way it starts is that it "Chugs" but will not take the last step and "fire-up". After I replaced the Camshaft Position sensor all my guages went dead. Bottom line......my jeep will not start. I have changed all the ignition related parts as shown below:

In order of replacement:
1. CPM
2. Distributor Cap
3. Rotor Button
4. Spark Plugs
5. Plug Wires
6. Ignition Coil
7. Camshaft Position Sensor
8. Crankshaft Position Senor (This was a BEAR to replace)

At this point I have no idea what else to do next. I am obviously new to mechanical work and this forum. Please HELP and education in simple terms I can understand.

Thanks in advance
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