Questions and Answers
Jeep Grand Cherokee Crank, No Start
Q. Hi Vincent: I recently purchased a used 1994 Jeep Grand Cherokee LTD, 5.2 liter engine, for my son to drive. The mileage is 100,114 has ABS brakes, P/S, A,C, cruise control and automatic transmission as well. The Vehicle has a weird problem where it is not starting sometimes because the fuel pump is not pre-priming sometimes.
I noticed this because I had a 1999 Cherokee and I would always notice the fuel pump priming in the one second open loop PCM sequence when the key was put in the RUN/ON position. Several times I have turned the key off then back on, without starting, and let it sit, I hear the ASD relay and the Fuel Pump relay clicking rapidly at first, then slowing to about ½-second clicks, then the fuel pump would prime and it starts right up after.
I have an ODB code reader and it shows no fault codes. I changed the fuel pump relay and the ASD (Auto Shut Down) relay. I also tested the fuel pump pressure when it was running and it was steady at about 31 LBS, with no vacuum it rose to 40 as per the specs. I also tested the fuel pressure regulator with no issues found.
The dealer I bought it off wants to change the fuel pump but I do no think it is the pump, this is an electrical issue. I also ran a Carfax on this and it had a note at 62,000 miles that "fuel system replaced/repaired." So I think this problem was being chased for a while. I also noticed that the dash lights when the key is first turned on are not working in the right sequence.
I am under the impression that I was supposed to see the "Check Engine" light for three seconds, then the bells and other lights go out. Sometimes when It does not start I do not see the check engine light come on. I am thinking of a few problems, but I am not sure.
- Crank Position sensor? I cannot tell if this is looked at by the PCM when in open loop mode.
- CAM position sensor? I cannot tell if this is looked at by the PCM when in open loop mode.
- Heated O2 sensor. This is NOT supposed to be looked at when in open loop mode, but its design has a spring in it and it could explain the jittering of the solenoids? But I think this is a long shot.
- Ignition Key switch is very loose and wobbly. I am wondering if this has something to do with it? I checked the ignition sense with the Key ON and OFF and the ODB diagnostic tool showed a change in state from low to high when the key was on and off respectively
- I have noticed flakiness in the auto security system, door locks only work in one direction sometimes (unlock), then other times they work bi-directionally.
- And Finally, a faulty PCM.
Any help would be greatly appreciated!
A.There are nine modes of operation for this engine management system. But in this case we only need to concern ourselves with the first two. And they are:
Ignition Switch On:
This is an OPEN LOOP mode. When the SFI system is ACTIVATED by the ignition switch, the following actions occur:
- The PCM prepositions the idle air control (IAC) motor.
- The PCM reads atmospheric air pressure from the manifold absolute pressure (MAP) sensor and calculates the basic fuel strategy.
- The PCM monitors the throttle position sensor (TPS) and engine coolant temperature (ECT) sensor input. The PCM uses these inputs to modify fuel strategy.
- Intake air temperature (IAT) sensor is monitored.
- ASD relay is energized for 3 seconds.
- Fuel pump is energized for 1 second, unless engine is running or cranking.
- Oxygen sensor heating element is energized.
This is an OPEN LOOP mode. When the starter motor is engaged, the PCM receives input from:
- Battery voltage
- Crankshaft position (CKP) sensor
- Engine coolant temperature (ECT) sensor
- Intake air temperature (IAT) sensor
- Throttle position sensor (TPS)
- Camshaft position (CMP) sensor
- Starter relay
Based on these inputs the PCM will:
- Monitor crankshaft position sensor. If no signal is received within 3 seconds, the PCM will shut down the fuel injection system.
- Supply the injectors with a ground path, firing all injectors until the PCM determines crankshaft position.
- Determine proper ignition timing.
What we need to determine if there is a fuel or ignition problem. Pull a vacuum line off the intake manifold and shoot some carburetor cleaner into it and try to start it. If it starts and runs as long as the carburetor cleaner lasts, we know we have a fuel problem. If not, we have an ignition problem.
If it is an ignition problem it could be due to a bad Crankshaft Position Sensor (CKP), Camshaft Position Sensor (CMP) or a bad Ignition Switch.
To do a complete test of the CKP circuit, you'll need a scan tool. But you can do a quick test of the CKP with an ohmmeter.
- Near the rear of the right cylinder head, disconnect CPS wiring harness connector from main wiring harness.
- Place an ohmmeter across terminals B and C of the CPS connector. The meter reading should be open (infinite resistance). Replace sensor if resistance is indicated.
NOTE: For this test, an analog (non-digital) voltmeter is needed. Do not remove the distributor connector from the distributor. To perform a complete test of the Camshaft Position Sensor and its circuitry, refer to the DRB II diagnostic tester. Also see the appropriate Diagnostic Charts. To test the sensor only, refer to the following:
Using small paper clips, insert them into the backside of the distributor wire harness connector to make contact with the terminals. Be sure that the connector is not damaged when inserting the paper clips. Attach voltmeter leads to these paper clips.
- Connect the positive (+) voltmeter lead into the sensor output wire. This is done at the distributor wire harness connector.
- Connect the negative (-) voltmeter lead into the ground wire.
- Set the voltmeter to the 15 Volt DC scale.
- Remove distributor cap from distributor (two screws). Rotate (crank) the engine until the distributor rotor is pointed towards the rear of vehicle. The movable pulse ring should now be within the sensor pickup.
- Turn ignition key to ON position. Voltmeter should read approximately 5.0 volts.
- If voltage is not present, check the voltmeter leads for a good connection.
- If voltage is still not present, check for voltage at the supply wire.
- If voltage is not present at supply wire, check for voltage at pin 7 of Powertrain Control Module (PCM) 60 pin connector. Leave the PCM connector connected for this test.
- If voltage is still not present, perform vehicle test using the DRB II diagnostic scan tool.
- If voltage is present at pin 7, but not at the supply wire:
- Check continuity between the supply wire. This is checked between the distributor connector and pin 7 at the PCM. If continuity is not present, repair the harness as necessary.
- Check for continuity between the Camshaft Position Sensor output wire and pin 44 at the PCM. If continuity is not present, repair the harness as necessary.
- Check for continuity between the ground circuit wire at the distributor connector and ground. If continuity is not present, repair the harness as necessary.
- While observing the voltmeter, crank the engine with ignition switch. The voltmeter needle should fluctuate between 0 and 5 volts while the engine is cranking. This verifies that the camshaft position sensor in the distributor is operating properly and a sync pulse signal is being generated.
If sync pulse signal is not present, replacement of the Camshaft Position Sensor is necessary.
When an unauthorized entry into the vehicle occurs, the VTSS sends a message via the CCD bus to the Powertrain Control Module (PCM) that it is not OK to start the engine. The PCM then zeroes out the pulse width to the fuel injectors after the engine has been started, thus shutting down the engine (start and stall condition). The engine will not start until the system is disarmed.
If this is what's happening, a noid light plugged into one of the injector connectors will not flash.
Hi Vince: I just wanted to let you know on the Jeep what fixed the issue.
After I got your mail I found a break in the drivers door wiring harness. I noticed that the door locks were not working and when I pulled on the door to door jam harness, the door locks would once again start working.
I suspect that the security system, as you mention in your mail, was also intermittently getting a signal from the door locks that were malfunctioning and in turn was sending a no start signal to the ECM. If I remember correctly, the door alarms on this year are pretty simple and need to work in conjunction with the locks.
So In the end the dealer that I bought the car off, changed the ECM, then the door wiring harness and all is fine now. Although I bet the door wiring harness would have done the trick by itself. But it cost me $0.00 so I cannot complain that I have a new ECM in a 10 year old car.
Thanks again for the help,
Additional Information provided courtesy of AllDATA
Additional Information provided courtesy of AllDATA