NewKindOfClown

Guys, would that be indicative of CPS failure? I always forget whether or not it will crank when those go out

salad

CPS is crank, no start

Cranking = starter motor rotates engine when key is turned to START

StuckBuck

In the case of a no start you need to check three basic things. Fuel, Spark, and Air. If an engine is missing one of these three it will not run, at least for very long that is. You have already stated you have fuel and most likely its not an air problem so that pretty much leaves us with spark. In this post I will cover most of the common problems that keep a I6 4.0 from sparking properly. I apologize in advance this is going to be a long comment lol but I'm almost positive I will mention your problem in this post. Hope I could help! Have fun!

In your case the first thing I'd tell somebody to check with an irregular cranking issue is kinda dumb but believe it or not a very common problem. Check the braided ground strap that goes from the driver side center of the firewall to a head stud. You'd be surprised how much trouble this can give a guy. If that's not it start going through the following symptoms and diagnostics, don't worry you'll find it!

The CPS is the most common failure, but not the only one that can cause a no-start.



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 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 or cannot read codes because the CPS/CKP has failed. Disconnect the code reader, disconnect the CPS/CKP , 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 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 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


(CPS/CKP)



.



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 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, not as shown in the diagram below. Simply follow the wire from the sensor to the connector.

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NewKindOfClown

I knew I liked you

StuckBuck

LOL I try!

VTJeep

Issue might be NSS but I wanted to check with you guys about some specifics cause I got some odd issues.

Facts:
-Battery (and whole jeep) sat all bitter cold winter. It is dead and needs to be jumped after a short sit.

-Doesn't like to run without a little throttle at first. But only a few seconds, then its fine. Basically bogs down without some gas. This is a new issue.

Brand new issue:
-Discovered my chassis ground wire from neg terminal was showing signs of, well, melting/shorting. Replaced it with a 4ga lead and a refreshed the mount point.

-Went for a drive.

More new issues:

-Now no 4th gear (aw4) and when in 1/2 gear it doesn't want to stay in 1st.

-Twice in about 10 miles the voltage shot to 19v (max on dial) stayed for a few sec then by the time I could pop hood, look at engine and go back to cab it was off.

-No reverse lights.

I guess that's it. What do you think?

StuckBuck

Could be a NSS but with all the other odd things going on I think it sounds more like another bad ground or short somewhere. Could be all those put together.

VTJeep

It's due for a tune up. Sitting for a year was no good, but thems the breaks.. It's had, like any good jeep, its share of electrical gremlins. But all this **** is pretty outta the blue. I suppose it's just time to clean the NSS and probably starter as its coated in oil. Thanks man!

StuckBuck

Ya clean everything up, fix anything obvious, check all your grounds, and if that don't work report back to us and we'll try to help best we can. Good Luck!

VTJeep

Unfortunately, grounds are good, and I've been all over the wiring in the past few days tearing apart the loom and inspecting. All looks good there too. But that still leaves the TPS, IAC, MAP, NSS, starter, cap/rotor, plugs/wires, fuel, oil and whatever else I forgot to list, still questionable. Time to be methodical and do as you suggested!

StuckBuck

Well I wish you luck, I hate tracking down invisible problems!

VTJeep

Ha! Hate is such a strong word.. And accurate. Took me way longer than it should have to discover my ignition coil was falling apart.. Pretty much the only thing I didn't replace in my attempt to figure this all out. Too bad that wasn't the final piece of the puzzle but I guess I've got something to do after work!

StuckBuck

Oh I hear you! It took me 4 lower radiator hose replacements before I realized not one of them had leaked. It was simply a bad water pump. Not one of my brightest moments LOL!

CAT_Tech2

hello i am a newbie and would like to do a simple write up on an issue i imagine other have had. how/where do i put it?

Dumajones

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