No Spark

Hi,

Before I suggest what I think may be the issue, first confirm the rotor is turning in the distributor when cranking the engine.

If that is good, then we need to confirm the ASD relay is also good. The relay is mounted in the power distribution center. This relay supplies battery voltage to fuel pump relay, fuel injectors, ignition coil, and O2 sensor heating element when energized by Powertrain Control Module (PCM). Basically, when the key is in the start or run position, the PCM provides a ground to the relay but the PCM has to receive a signal from the distributor to work. If there is no ignition reference to the PCM, the ground is removed and fuel pump and components are shut down.

Now, regardless of replacement of the crank sensor, the fuel pump should be powered for a couple seconds when the key is first turned on, so if it isn't, that leads me to this relay.

I hope I explained that well enough to understand. LOL

Okay, I attached three pics. The first shows the ASD relay location, the second is from the schematic showing a fuse for the ASD, and the third shows the ASD fues. Interestingly, the schematic says it's a 20 amp and the location (pic 3) shows it as a 15 amp. Stay with what is there if it is bad.

Don't know if this will help, but here is a link that shows how a relay is checked. I realize you are a mechanics, but thought I would add it in case it would help.

https://www.2carpros.com/articles/how-to-check-an-electrical-relay-and-wiring-control-circuit

If it was me, I would switch the relay with one in the box that has the same part number. LOL

Take care and let me know if any of this helps.

Joe

Hi,

Did it start working after you replaced the ASD? If it did, recheck to see if there is corrosion, a damaged pin (pushed in), pull the ASD out and put it back in and see if it changes.

Also, there is an ignition ballast resistor relay. Check it as well. It is located under the hood on the outside of the power distribution box. See pic 1 You can use the aforementioned link to test a relay for this one as well.

The PCM controls ignition timing and coil dwell by controlling the ground path to the ignition coil. So the idea that you have power to the coil on one wire is normal. See pic 2 For power to flow, the PCM needs to provide ground.

Have you checked for diagnostic trouble codes? I have a feeling that if the ballast relay is good it may be related to the crankshaft position sensor. Follow this link and let me know if anything is stored. You can get codes without a scanner on this vehicle.

https://www.2carpros.com/articles/jeep-diagnostic-trouble-codes-for-car-built-on-or-before-1995-odb1

Next, make sure the connector to the crankshaft position sensor is good. Also, take a look through these three links:

https://www.2carpros.com/articles/how-a-crank-shaft-angle-sensor-works

https://www.2carpros.com/articles/symptoms-of-a-bad-crankshaft-sensor

https://www.2carpros.com/articles/crankshaft-angle-sensor-replacement
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Here are the directions for testing your crankshaft position sensor. Basically, you are making sure there is no resistance between terminals B and C. See pic 3

PROCEDURE

1. Near the rear of intake manifold, disconnect sensor pigtail harness connector from main wiring harness.

See pic 3

2. Place an ohmmeter across terminals B and C (Fig. 5). Ohmmeter should be set to 1K-to-lOK scale for this test. The meter reading should be open (no resistance). Replace sensor if a low resistance is indicated.

___________________________________________
If you determine the sensor is bad, here are the directions specific to your vehicle for replacement.

I1993 Jeep Truck Cherokee 4WD L6-242 4.0L VIN S MFI
Procedures
Vehicle Powertrain Management Sensors and Switches - Powertrain Management Sensors and Switches - Computers and Control Systems Crankshaft Position Sensor Service and Repair Procedures
PROCEDURES
Crankshaft Position (CKP) Sensor 4.0L

See pic 4

REMOVAL

1. Disconnect negative battery cable.
2. Disconnect sensor electrical connector, located near rear of intake manifold
3. Remove nut holding sensor wiring clip to fuel rail.
4. Remove sensor mounting bolt(s) or nuts.
5. Remove the sensor from vehicle.
6. Remove clip from sensor wiring harness.

INSTALLATION

1. Position camshaft position sensor flush against the opening in transmission housing.
2. Install sensor mounting bolts or nuts and torque to 19 Nm (14 ft-lbs).

CAUTION: On some models, two bolts are used to secure sensor to transmission. These bolts are specially machined to correctly space the sensor to flywheel. Do not attempt to install any other bolts.

3. Connect the electrical connector to the sensor.
4. Install clip on sensor wire.
5. Install clip over fuel rail stud and tighten mounting nut.

Let me know. As far as the PCM being bad, anything is possible. However, lets eliminate other common causes.

Let me know what you find.

Take care,

Joe

Hi tdgast. Welcome to the forum. I don't have a service manual so I'll try to do this from memory. You should have an automatic shutdown (ASD) relay. The first step is to determine if that is turning on. By '93, this was a Chrysler product and I think they were using the standard, fairly reliable circuit. You can measure the voltage feeding the ignition coil, (typically dark green with an orange stripe), the injectors, or one of the small terminals on the back of the alternator. Both small terminals on the alternator will have voltage at the same time, one will just be a little lower).

You should find full battery voltage on those places two times. The first is for just one second after turning on the ignition switch, then it will go back to 0 volts. The voltage will turn on again during engine rotation, (cranking or running). That voltage comes from the ASD relay which is turned on by the Engine Computer when it gets pulses from the two sensors you replaced.

As an alternative to measuring the voltage, you can pop the cover off the ASD relay and watch to see if it clicks when a helper cranks the engine.

When you replaced the crankshaft position sensor, was it new, and did it have a paper spacer or plastic rib on the end? One of them must be present to properly set the air gap. If there was no spacer, the sensor can be broken from hitting the ring on the flex plate or it won't generate a sufficient signal if it is too far away.

Caradiodoc

If the crank sensor came from the dealer, they should have provided the paper spacer. It will slide off when the engine is started. Many aftermarket sensors have a plastic rib molded in that wears away as the engine runs. When those are removed and reinstalled, you are supposed to cut off the remaining rib and use a new paper spacer.

Just thought I'd mention that in case you plan on removing the sensor to inspect it.

Caradiodoc

Check and test the crankshaft positioning sensor at left side of transmission bellhousing

You should be getting a fluctuating AC signal-sounds like you have to replace it or a problem with the PCM

Perform the no start tests below

Inspect and test the ASD relay it powers the coil/injector/fuel pump-

I see you changed the ballast resistor. In your system is a resistor bypass relay. My schematic doesn't tell me exactly where it is. I would assume it's close to the resistor. If this relay fails, you will have no spark. You may have already checked this, but we have to start somewhere. Also, I am again "assuming" you have fuel pressure and injector pulse. Please advise.