5.7 Hemi, Crank no start

[NightRider84]

Checked all fuses and relays all good. Have P0685 and P0688. Both lead to ASD issue. Tested control circuit from relay to ecm and got .3 ohms. Relay powers up when key first turned on. Have 12 vdc at coils and injectors when jumping relay still won’t start. Have replaced both cam and crank sensors and no change. Fuel pressure good just installed new pump. Tested grounds at ecm. Have 5 v signal at both cam and crank, ground is good and getting signal back to ecm. Not sure what to check next before having to replace ecm.

 

[Fast69Mopar]

Checked all fuses and relays all good. Have P0685 and P0688. Both lead to ASD issue. Tested control circuit from relay to ecm and got .3 ohms. Relay powers up when key first turned on. Have 12 vdc at coils and injectors when jumping relay still won’t start. Have replaced both cam and crank sensors and no change. Fuel pressure good just installed new pump. Tested grounds at ecm. Have 5 v signal at both cam and crank, ground is good and getting signal back to ecm. Not sure what to check next before having to replace ecm.

With everything you have tested and inspected have you verified whether or not you have ignition spark at the plug?

Remove a coil and leave the connector plugged in and stick a screwdriver in the end and hold it close to ground while cranking to see if you have spark.

You said you just replaced the fuel pump and said you had good pressure. Did you verify fuel pressure with a gauge?

Can you hear the pump run when you first turn the ignition key to the run position?

 

[NightRider84]

Yes fuel pressure verified with a gauge. Changed pump do to not maintaining pressure after key turned off. Pressure dropped to almost 0 psi after 3-4 min. I’ll have to double check to see if there is spark at the coil.

 

[Fast69Mopar]

Yes fuel pressure verified with a gauge. Changed pump do to not maintaining pressure after key turned off. Pressure dropped to almost 0 psi after 3-4 min. I’ll have to double check to see if there is spark at the coil.

Yes. Good idea to see if you have spark coming from the ignition coils. It would be good to get a scan tool connected to this truck to see if you actually have a cam and crank signal.

 

[NightRider84]

Finally got a chance to test for spark and that would be a negative. I don’t have easy access to a scan tool.
I was reading in another forum that a bad ECT sensor can cause a no start. I do have a cheap code reader and when I look at the FF data, the ECT is showing -40. Haven’t had a chance to check the sensor yet. Could this really be the reason why I’m getting a no start.
I also read that adding an extra ground strap from the ECM to chassis to head to TB can help. Is this true?

 

[NightRider84]

Forgot to mention... I’m getting ahold of a compression tester tonight to be able use for a compression check.

 

[Fast69Mopar]

Forgot to mention... I’m getting ahold of a compression tester tonight to be able use for a compression check.

The only way the ECT Sensor could cause a "no-start" condition is if the sensor was shorted internally and bringing down the
5-Volt supply circuit from the PCM. If you want to see if this is the issue you can test for proper 5-Voly Supply at the cam and crank sensors. If you do not have the correct voltage at the crank sensor, disconnect the coolant temp sensor and recheck the voltage at the crank sensor. If the voltage returns to 5V while the coolant temp sensor is disconnected then you know the sensor is shorted. Try disconnecting the coolant temp sensor and cranking the engine to see if you have spark at one of the coils.

For your question about the extra ground, yes, it is always good to have a soild, reliable ground. It may not help your current situation but it would not hurt. If for some reason adding an extra ground fixes your no-spark issue then we know you have some poor grounds elsewhere on the motor.

 

[NightRider84]

Ok so it’s been a while now but finally able to check 5v at both cam and crank sensors. Both tested good. Also checked the input signal at pcm from both sensors and both tested good. Both had voltage between 5v and about .3v while cranking.
Occasionally while cranking it acts like it wants to start but just chokes. I noticed that while cranking, the tac doesn’t move off 0 rpms until it does its little choke then back to 0. Ran through the cluster check and all gauges work.
Back to being stumped again.

 

[bolko2021]

Sorry, What was the end of the story? I have a similar problem.

 

[lexusguy01]

Also having similar issues lmk what you did.