Will turn over, but won't start?

zerogspacecow · May 12, 2011

'98 Cherokee Classic
4.0L straight 6
Auto w/ 4x4
158k miles

Alrighty, here's what happened:

While driving home today, the engine just suddenly dropped to 0 rpm. No stuttering or sputtering, just died. All the lights stayed on, all the other gauges read normally. No check engine light.

When I tried to start it back up, it will turn over normally, but won't actually start. I figured the problem is either no spark or no gas. I wasn't sure how to check for fuel on the side of the road, but I checked for spark (the way my manual said, but pulling off the wire and putting it against the engine block). As far as I can tell, it's not producing any spark.

Anyone have any idea what the problem could be? Something that would just happen suddenly like that while driving?

old_man · May 12, 2011

By far and away the most likely failure is the Crankshaft Position Sensor mounted on the bell housing on the driver's side. Other than that, check the ground from the rear of the head up to the fire wall.

the_weirdo · May 12, 2011

Yep, check the CPS. I always buy from the dealer - I have found that ones from auto parts stores are dead about 60% of the time. Do a search, there is a way to test the CPS.

birchlakeXJ · May 12, 2011

The #1 cause for no spark on your 98 is a crankshaft position sensor. A very common failure.

The #2 cause is the ignition coil.

Both of these parts can be tested with a meter and a manual.

And I agree with above, I buy plenty of aftermarket parts, but the crank sensor is one part that I ALWAYS buy from Jeep for best reliability. Worth the extra money. No lack of threads on this thread and others of people having trouble with some (not all to be fair) crank sensors. Why take the chance with such a critical sensor.

zerogspacecow · May 12, 2011

Thanks guys, I'll check that today and post back with the results.

Will any Dodge/Jeep/Chrysler dealer normally stock the sensor, or is it a special order type of thing?

zerogspacecow · May 12, 2011

Alrighty, so I called the Jeep dealership near me, and they do keep them in stock. However, they're $115. So, I definitely want to test it before I go and spend that much money.

Searching this forum and reading my Haynes manual says that you should connect an ohmmeter to terminals B and C (which will be marked) on the sensor plug. If it reads "OL" (open loop?), then the sensor is bad.

Does that mean that any other reading means it's good? Like if I connect the ohmmeter and get 5kohms resistance, does that prove the sensor is good?

zerogspacecow · May 12, 2011

Whoops, got that backs. Open loop should indicate the sensor is good, correct?

I dunno, that test just doesn't seem very conclusive to me. Anyone know the theory behind it? It seems like the sensor should be able to be bad, while still reading an open loop?

Tim_MN · May 12, 2011

How much is tow to the shop vs. a new CPS that you can be confidant will work right and last another 100,000 miles ?

A simple search finds:

CPS Testing

Crankshaft Position Sensor Connector (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.

.

old_man · May 12, 2011

While this test will show one failure mode, it is far from a conclusive test.

Tim_MN · May 12, 2011

old_man said:
While this test will show one failure mode, it is far from a conclusive test.

True, but this test covers the most common mode of failure.

Since every Cherokee will face CPS failure at about 100-150,000 miles, the smart thing to do would be simple preventive maintenance and replace the CPS with a JEEP sourced part and keep the old one as a spare (if it is not failed). I have owned 5 Cherokees with 125,000-180,000 miles and 3 had CPS failure in that mileage range.

Dragonslayer · May 12, 2011

I read somewhere to use an analog ohmmeter (needle gauge) - not a digital ohmmeter -
to test the CPS.

Hook up the meter to the pigtail and check for resistance.

Then spin the starter.

The needle on the gauge should flicker back & forth if the CPS is good.

zerogspacecow · May 12, 2011

Okay, I checked the resistance on the pins. But they weren't labeled, so I don't know which pins were "B and C". Here's an illustration of the connector with the pins labeled as 1, 2, and 3. If someone could let me know which pins are A, B, and C, I'd appreciate it.

When I tested between "1" and "2" I got ~.3 ohms. When I tested between "2" and "3" I got OL. Does this mean the sensor is good? The engine was cold when I test (obviously, since it hasn't been running since yesterday afternoon).

Dragonslayer, I don't have an analog ohmmeter, but I can see if someone at work does. Can anyone else verify if I should be testing with analog or digital?

zerogspacecow · May 13, 2011

Yep, it was the crankshaft position sensor!

I went and bought a new one from the Jeep dealership, installed it, and it started right up!

Thank you guys so much for your help!

Will turn over, but won't start?

zerogspacecow

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old_man

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