simspace

My '91 Jeep Cherokee Laredo 4.0 has stopped running.

I did not hear the fuel pump prime, so I installed a new fuel pump. Now when I turn the ignition key, I still do not hear the fuel pump kick on.

• I pulled the fuel pump relay and applied 12 V across pins 85 and 86 and I can hear the relay clicking.
• I jumpered pin cavities 30 and 87 and I heard the fuel pump kick on.
• I pulled the fuel pump relay from my other running XJ. Still no fuel pump prime when I turn the key on.
• At this point, I'm confident the FP relay is okay and the fuel pump sounds good.

Upon further investigation I discovered ...

• There is no spark,
• No fuel pump prime
• No check engine light (CEL) to read the ODB-I codes.

First I checked the grounds ...

• All the grounds look good and are tight.

Next I tackled the CEL ...

• I pulled the gauge module from the dash and tested the check engine light bulb. It is good.
• I applied 9 volts to the traces to make sure there was no issue with them. All good.
• I installed the bulb and applied 9 volts through the traces. The bulb works.
• I disconnected the PCM 60 connector and found the "Check Engine" circuit cavity.
  I did a continuity check from the PCM 60 connector to the gauge module plug. All good there too.
• So, now I also need to figure out why I am not getting a "Check Engine" light, so I can get the ODB-I codes.

To eliminate the NSS as a cause of no start...

• I wiggled the shift lever while cranking the engine.
• I did the same in neutral. Still no start.
• My reverse lights do come on when the shifter is in reverse ... at least on the drivers side.
  I need to check the reverse light bulb on the passenger side.

I tested the CPS ...

• I unplugged the CPS and turned on the key. All gauges were fine.
• I unplugged and reconnected the CPS and cranked the car. The Jeep did not fire up.
• I checked the resistances and they were correct.
• However I still replaced the CPS, because the connection at the manifold was slightly melted and a wire was exposed.

O2 sensor ...

• I inspected the wires and wire connectors at the O2 sensor on the exhausts pipe. It looks good.
• The O2 sensor / wire harness connection under the motor looks good too.

I have been looking for sensor shorts to try and get the CEL to light up.
I checked voltages the following sensor/PCM connectors for the voltages sent from the PCM...

• MAP - 2.4v (to low, should be 5v)
• TPS - 2.4v (to low, should be 5v)
• CamPS - 8v (correct)
• CrankPS - 8v (correct)
• O2 Sensor - I don't get any voltage reading.
• I still need to check the speed sensor.
• I disconnected all the sensors (except the speed sensor), then reconnected one sensor at a time to see if the CEL would go on. Still no luck.
• I don't know what steps to take next, regarding the sensors with 2.4v.

I still need to check on the Ignition switch on steering column. I replaced that last year after a no-start condition.
​
Do I need to check these too?

• IAC, SKS, ICM, MAT, EGR, CPU
• Idle Air
• Purge Solenoid
• Temp sensor
• Tran Sensor

Is there anything else I can do to try and figure out wh​y there is no spark, fuel pump prime or CEL?

Thanks,
Chris

BlueRidgeMark

ASD relay. It controls spark, fuel pump, and injector power.

simspace

Thanks BlueRidgeMark!

Jeep is still not running, but after a lot of probing and voltage reading, diagnosing and reading the FSM, the Powertrain Diagnosis Manual and lots of posts, here's what I think I know so far, but I'm still not sure how to fix the problem.

First the PCM 5v circuit is only putting out 2.44v. I back probed all the PCM 60-way connector's 5v connections ... they're all 2.44v. Which is what I also see on the MAPS and TPS.

Also, I think there is a ground short.

When I tap the pin 86 on both the ASD and Fuel Pump relays, by connecting a jumper directly to the battery negative from the relay cavity, then I turn the key to run, I finally hear the fuel pump prime.

When I remove the jumper and turn the key ... no prime. That tells me there is a ground connection issue.

I know the PCM grounds both the ASD and FP relays on pin 86. So, that also points back the PCM.

Lastly, I don't know if it's indicates another problem or not, but when I tap pin 86 on both the ASD and Fuel Pump relays and turn the key to run, the fuel pump never stops.


Tapping the pin 86 with a jumper to battery negative

Thanks,
Chris

BlueRidgeMark

Okay, good diagnostic work!

Check the PCM's ground. It goes to the plate where the main battery ground connects, just forward of the distributor. If the PCM's ground isn't good, ain't nobody happy.

See my sig, #1!

simspace

Finally, an update ...

I received a new ECU/PCM from RockAuto yesterday. I installed it this morning and the Jeep started right up!

As soon as I turned the key and saw the CEL and heard the fuel pump I knew I was in good shape.

This whole process took a while, but it was not wasted time. I learned a lot about diagnosing potential electrical issues.

And now I have an extra fuel pump and CPS.

Thanks to everyone who posted comments and suggestions, they did help!

BlueRidgeMark

Thank you for following up! Glad you got it sorted out.