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Marc
Marc, Technician
Category: Car
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Experience:  33 years exp. automotive repair NYS emisisions repair cert.
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Jeep Wrangler no start

Customer Question

1995 Jeep Wrangler 4cyl. will not start buts turns over fine. Fuel pump not running and no spark, checked fuses, replaced distributor cap and rotor, relays, coil,crank sensor,and even computer. Checked all connections and cleaned with electrical solvent. I do not hear the fuel pump coming on and do not have a spark, are they related or different problems?
Submitted: 11 years ago.
Category: Car
Expert:  Marc replied 11 years ago.
That system uses a crank sensor to signal the PCM to activate the ASD relay if that relay is not activating you will loose spark fuel pressure and injector pulse .It is a common problem on Jeeps ,The crank sensor goes bad all the time especially on the 4 liters.






Customer: replied 11 years ago.
Reply to Marc's Post: I have replaced both of these, shutdown relay came from my other jeep that I know works and also a new crank sensor was replaced, is there anyway to check the sensor?
Expert:  Marc replied 11 years ago.



OPERATION


See Figures 1 and 2


The Crankshaft Position Sensor, or CKP sensor provides the PCM with information about engine speed and crankshaft position. It is located near the bellhousing.


The CKP sensor contains a Hall effect device which sends either a 0.0 volt or a 5.0 volt signal to the PCM depending on the position of the distributor shaft.


The PCM uses the CKP sensor signal to determine fuel injection event time among other things. The engine will not run without the CKP sensor signal.


Click image to see an enlarged view
Fig. 1: View of the crankshaft position sensor and its location-1996 5.2L engine shown



Click image to see an enlarged view
Fig. 2: View of the crankshaft position sensor and its location-1996 4.0L engine shown




TESTING


See Figures 3 and 4


Click image to see an enlarged view
Fig. 3: Connector terminals for the Crankshaft Position (CKP) sensor



Click image to see an enlarged view
Fig. 4: Checking the CKP sensor voltage between the wiring harness side and battery ground





  1. Backprobe with a high impedance ohmmeter between the CKP sensor connector middle terminal and battery ground.



  2. Verify that the resistance is less than 5 ohms. If the resistance is not as specified, repair or replace the wiring as necessary and continue the test.



  3. With the ignition ON and the engine OFF, backprobe with a high impedance voltmeter between the sensor connector middle terminal and either of the end terminals.



  4. Verify that a 5 volt or greater signal is present at one of the two terminals. If not as specified, repair or replace the wiring as necessary and continue the test.



  5. With the ignition ON and the engine OFF, backprobe with a high impedance voltmeter between the sensor connector middle terminal and the end terminal that did not have the 5 volt or greater signal.



  6. Crank the engine and verify that the voltage reading alternates between 0.0 and 5.0 volts or verify that the voltage reading is 2.5 volts (averaging voltmeters only).



  7. If the voltage readings are not as specified, the sensor may be faulty.












Customer: replied 11 years ago.
The crank shaft sensor seems to check out. You keep referring to a 6 cylinder but this is a 4 cylinder (2.5) and I still do not hear the fuel pump and the relay checks out on my other jeep. I have even put a small amount of fuel in the throttle body and it will not fire. Do you have any other ides? According to the Haynes manual, since there are relays and given it is a 95, there should be no in line fuses.
Expert:  Marc replied 11 years ago.



It is not firing because you have no spark the automatic shut down relay enables spark, fuel pump, and injector pulse.


It is energized by the ECM when it receives a signal from the crank shaft position sensor.


The info is from a 4.0 engine because the info on the 2.5 doesn't include a crank sensor test proceedure

Customer: replied 11 years ago.
Reply to Marc's Post: I will recheck it again and get back with you.
Customer: replied 11 years ago.
Reply to Marc's Post: Sorry to say, but all the information that you gave me I had already stated in my original letter has been replaced or checked out and the jeep still will not start! Do you have any other ideas at this point?
Expert:  Marc replied 11 years ago.



All 1993-95 2.5L and 4.0L Engines are equipped with multi-port fuel injection. The ignition system on these engines is operated by the Powertrain Control Module (PCM) and consists of:




  • Spark plugs



  • Secondary ignition cables



  • Ignition coil



  • Distributor (contains rotor and camshaft position sensor)



  • Powertrain Control Module



  • Crankshaft position sensor



DIAGNOSIS



Failure To Start Test


See Figures 1, 2, 3 and 4


Click image to see an enlarged view
Fig. 1: Special jumper wire construction for grounding the coil



Click image to see an enlarged view
Fig. 2: Common coil harness connector



Click image to see an enlarged view
Fig. 3: Coil terminals



Click image to see an enlarged view
Fig. 4: PCM 60-way electrical connector



Apply parking brake and block wheels before performing any engine running tests, including idle or timing checks and adjustments.




  1. Check battery voltage and determine that a minimum of 12.4 volts is available for operation of cranking and ignition systems.



  2. Crank engine for 5 seconds while monitoring voltage at coil positive terminal. If voltage remains near 0 during entire period of cranking, please refer to Fuel System in this guide for on-board diagnostic checks of PCM and auto shutdown relay.



  3. If measured voltage is near battery voltage but drops to 0 after 1-2 seconds of cranking, please refer to Fuel System in this guide for on-board diagnostic checks of distributor reference pick-up circuit to PCM.



  4. If measured voltage remains near battery voltage for entire 5 second cranking period, turn key OFF and remove SBEC 60-way connector. Check 60-way connector for any spread terminals.



  5. Remove the wire-to-coil positive terminal and connect regular jumper wire between coil positive terminal and battery positive terminal.



  6. Using a special jumper cable, momentarily ground terminal No. 19 of the PCM 60-way connector. A spark should be generated when ground is removed:




    1. If spark is generated, replace the PCM.



    2. If no spark is seen, use a special jumper to ground the coil negative terminal directly. If spark is produced by directly grounding the negative coil terminal, but was not produced with pin No. 19, trace and repair open condition within wiring harness. If no spark is produced when directly grounding the coil, the faulty ignition coil should be replaced.


Secondary Circuit Testing




  1. Remove the center wire from the distributor cap.



  2. Using insulated pliers, hold the terminal end about 1/2 in. (13mm) from a good engine ground and crank the engine.



  3. Check for steady arcing. If steady arcing does not occur, inspect the secondary coil cable. Also, inspect the distributor cap and rotor for cracks or burn marks, then repair as necessary.



  4. Remove the cable from one spark plug.



  5. Using insulated pliers, hold the terminal end about 1/2 in. (13mm) from the block or head and crank the engine.



  6. If a spark occurs, check ECU sensors using tester DRB II, or equivalent. If the sensors check out okay, the problem is probably in the fuel system.



  7. If no spark occurs, The rotor, distributor cap or spark plug wires are defective.











Customer: replied 11 years ago.
Reply to Marc's Post: Will run these tests on 7/20 in the pm and get back with you, thanks, XXXXX XXXXX help!
Customer: replied 11 years ago.
Now I am 2nd guessing on a few of the parts that I have replaced. The coil has sufficent fire coming to the plug but no fire coming from the coil but this would not have anything to do with the fuel problem. The crank sensor was replaced new and I do have power going to the connector but wondering if maybe I have a bad sensor. All the other tests with the PCM check ok. I think I will take the crank sensor back to the auto parts and get another at this point. You have been most helpful with more tests than my Haynes manual has given me to try. Will let you know when I pick up the other one and what the outcome is, thanks so far...
Customer: replied 11 years ago.
I ordered another new crank sensor and should be in on Friday. At this point, I am at a loss. Still no fuel pump kicking on and no spark.
Customer: replied 11 years ago.
Still am not able to get it started! Coil checks out ok, comp has been replaced with one that worked when it was taken out last week. Relays were switched with another Jeep and 2nd new crank sensor has been put on. All wiring checks out, all fuses and connections have been taken apart and cleaned. The fuel pump is not kicking on and still no spark! HELP....
Expert:  Marc replied 11 years ago.

Are you getting a 5 volt pulse at the crank sensor? if so Did you check the circuit back to the ECM?


Also are you sure you change the auto shut down relay ? Compare the relay wiring to the one in the schematic.(colors the same)



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