CPS Crank Position Sensor failing?

CPS would seem to be the main suspect.

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. Typical 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.

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.

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.

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.
Crankshaft Position Sensor Connector (CPS/CKP)
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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CJ7-Tim said:

CPS would seem to be the main suspect.

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.

Click to expand...

That comment right there leads me to think maybe it isn't the CPS. My reasoning is that it isn't hard to start and runs perfectly, even during the 'funny turns'. The only physical problem besides dash lights and gauges going crazy is that it will fire up easily but then stop 2 seconds later. I can let it idle or rev the nuts off in that 2 seconds but the end result is the same, it dies.

Someone on a UK forum suggested ignition switch might be the problem?

On Friday when this all started, once I got it home safely I messed around stopping and starting the engine (or attempting to start depending on what mood it was in!) and at one point I got the impression that if I pushed the key into the ignition a bit harder and gave it a waggle then the dash/dial problems changed. Maybe this means the ignition is the problem but I couldn't be sure it wasn't just a co-incidence. The vehicle actually drives perfectly when all the dash is going doolally, it just won't keep running if I attempt to start it during a funny turn. Could it do this with a misbehaving ignition switch or CPS?

I have been out in it tonight and done a few errands and it ran without a hitch. I stopped a few times and it always fired up without a problem but I am sure that whatever the problem is it will resurface at some point!

Money is a bit tight at the moment so I don't really want to start replacing components unless It is a definite cause. Any Ideas? In the meantime I will do the Ohmeter check on the CPS as soon as I get chance.

you need to pull apart your dash, and pull your gauge cluster. The connections on the back of the gauge cluster have gotten corroded and need to be cleaned, and possibly even tightened. Its free, and it only takes about half an hour. By "tightening" I mean bending the little prongs on the two electrical plugs so that they grip the gauge cluster plugs tighter, making a more secure connection. Combine that with some dielectric grease and you should be in business. Reassemble everything and see if this solved the problem.

From all of your symptoms, it doesn't really sound like an obvious CPS problem. The airbag light problem and the "no bus" code is very very common to the dirty plugs on the back of the gauge cluster. Give that a try and report back and let us know if it fixed it. All you need is a phillips head screwdriver and a pick of some sort to pry the little prongs out. It's almost like you have a couple different problems surfacing at once here.

2001 XJ Crankshaft Position Sensor Failure

Thanks to CJ7-Tim for the post on 11/28/08 on CPS Testing. My 2001 XJ with 101K miles stopped dead while driving two days ago. Backed out of a parking space, put it in drive, applied the gas and within seconds the engine just quit. After this cranked but would not start. Displayed Check Engine Light and "no bus" on the odometer. Did not notice other indications at that time.

Got a ride home. Searched the net. Came up with this thread. Returned to the Jeep the next day with a meter, a 1/4" socket set, an 11mm box end, a few extra extensions and a flashlight. Put the key in the ignition and switched on. Noticed the Volt Gauge and the Fuel Gauge inoperative. Located the CPS and disconnected the connector. Turned the key again and the Volts Gauge and Fuel Gauge operated. Checked the connector with the ohmeter but found open on all connections. This took about a half hour.

Called the parts store, located a new CPS (paid $120 at Bumper to Bumper).
Used the box end to loosen the top bolt and the 1/4" drive 7/16" socket to loosen the bottom. After bolts were loose used a 1/4" drive 7/16" sockect with a 6 inch extension to remove the bolts the rest of the way.

Installed the new CPS. Installed and started the bolts with my fingers, ran finger tight with the socket and extension and then tightened the bolts (box end on top, socket on bottom). I spent no more than an hour on the removal and installation. Put the key in the ignition and the Jeep started right up and works just as before.

My initial response when the Jeep started was "Thank God for the internet." Thanks to those of you who take the time to post. My friends who watched me mechanicing in the parking lot at work think I'm a genius.

my CPS measurements

Using the diagram above BUT reversing pins A and C. In other words i looked at the connector upside down versus what is shown in the diagram above, like this:

==============================================
|.............................................|
|.............................................|
|.............................................|
|....A...............B...............C........|
|....=...............=...............=........|
|.............................................|
==============================================

I found-

brand new BWD brand CPS, from AAParts ($40):

A to C, 6.9 M ohms
A to B, 6.9 M ohms
B to C, (3.6 to 4.0) k ohms

my bad CPS:
A to C, infinite (ok)
A to B, 8.0 (eight!) ohms (very bad!)
B to C, infinite (very bad!)

put in the new BWD and it started right up! btw, all measurement done with parts sitting on the bench

Can a faulty CPS cause shifting problems in a 1998 AW4?

I'm pretty sure my CPS is faulty, as I am experiencing the no start after a long drive issue. It fires right up after 10-15 minutes. There is another issue as well, I started having shifting problems (from a stop, no forward movement in "D" position) several months ago. I have to shift manually.

So far, I replaced the TPS, changed the fluid, cleaned the NSS, and the no-shift issue persists. I suspect the CPS because my AT shifts fine when it's stone cold, but after it warms up it will no longer shift on its own. I figured if the CPS is experiencing thermal failure then maybe these problems are related. Am I mistaken?

BTW, I have spent countless hours searching and reading threads, as well as a great deal of time thumbing through the FSM. So please don't tell me to 'search.'