no pcm start cycle?
#1
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Year: 1997
Model: Cherokee
Engine: 4.0
no pcm start cycle?
Seams I may have been barking up the wrong tree, in past threads. What would cause PCM not to do its pre-start cycle? Had been trying to check O2S heater circuit, and ended up tracking and tracing half the wiring harness and connectors in the process. Due to no voltage at the O2S connector. It tested fine when running, just thought bad internet info. Just checked fuel pressure and once again nada with key on engine off. On start-up it quickly built up 25psi (problem#2) and held for about 5 min still above 20psi. Problem #1 is why does my PCM not energize my O2S and fuel pump before I actually start the engine? ASD relay tested good from round 1 with the O2S. Hoping its not a bad PCM
sry: once again 97 Cherokee 4.0 AW4 neutered(4x2)
sry: once again 97 Cherokee 4.0 AW4 neutered(4x2)
#2
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Fuel pump should spin up when the key is turned to "on". Does it? Everything else power up in "on"?
IIRC there is a wire that tells the PCM the key is in the ON position. I might check that out, and if warranted, the ignition switch itself.
IIRC there is a wire that tells the PCM the key is in the ON position. I might check that out, and if warranted, the ignition switch itself.
#3
If the PCM fails to receive a crank signal it triggers (or fails to trigger) the ASD relay and prevents the motor from running (No sparks, no fuel, no joy). I believe there is also a need for a cam sensor signal as well as the crank, never really dug that deeply into it.
#4
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Year: 1999
Model: Cherokee
Engine: 4.0
Seams I may have been barking up the wrong tree, in past threads. What would cause PCM not to do its pre-start cycle? Had been trying to check O2S heater circuit, and ended up tracking and tracing half the wiring harness and connectors in the process. Due to no voltage at the O2S connector. It tested fine when running, just thought bad internet info. Just checked fuel pressure and once again nada with key on engine off. On start-up it quickly built up 25psi (problem#2) and held for about 5 min still above 20psi. Problem #1 is why does my PCM not energize my O2S and fuel pump before I actually start the engine? ASD relay tested good from round 1 with the O2S. Hoping its not a bad PCM
sry: once again 97 Cherokee 4.0 AW4 neutered(4x2)
sry: once again 97 Cherokee 4.0 AW4 neutered(4x2)
Fuel pump: The fuel pump has its own relay, activated independantly from the ASD relay. When the PCM sees engine rotation, via the CPS, it will activate the fuel pump continuously until the engine is shut down. And only momintarily when the key is turned to RUN inorder to prime the fuel rail. Power to the fuel pump comes from fuse F21 in the PDC, and the relay control coil is powered from fuse F11 in the JB. The key must be in RUN inorder to get power to F11 (JB) and F21 (PDC).
The ASD relay is powered from fuse F18 in the PDC. When the ASD relay is activated (F11 JB) the power is split off to fuse F23 in the PDC. F23 powers the O2S heaters. F20 is also powered through the ASD relay, it powers the Fuel Injectors and the Ignition Coil. The ASD relay control coil is powered by fuse F11 in the JB and is activated the same way as the ASD rela.
Check your PDC fuses. See '97 PDC below.
If the fuses n the PDC are good, check fuse F11 in the JB. See '97 JB below.
#5
If the PCM fails to receive a crank signal it triggers (or fails to trigger) the ASD relay and prevents the motor from running (No sparks, no fuel, no joy). I believe there is also a need for a cam sensor signal as well as the crank, never really dug that deeply into it.
#6
Seams I may have been barking up the wrong tree, in past threads. What would cause PCM not to do its pre-start cycle? Had been trying to check O2S heater circuit, and ended up tracking and tracing half the wiring harness and connectors in the process. Due to no voltage at the O2S connector. It tested fine when running, just thought bad internet info. Just checked fuel pressure and once again nada with key on engine off. On start-up it quickly built up 25psi (problem#2) and held for about 5 min still above 20psi. Problem #1 is why does my PCM not energize my O2S and fuel pump before I actually start the engine? ASD relay tested good from round 1 with the O2S. Hoping its not a bad PCM
sry: once again 97 Cherokee 4.0 AW4 neutered(4x2)
sry: once again 97 Cherokee 4.0 AW4 neutered(4x2)
#7
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Year: 1997
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It will start and run, I think my lack of power is due to low fuel press. I was thinking fuel injectors thus the press test. But when I tur. On the key engine off the pump does not prime and no power to the O2S heater either. Both work with engine running.
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#8
Check power to fuel pump with ignition on. I do not believe it's possible for power to go to the fuel pump when it running but not when the ignition is on. However it is possible for a bad pump to act this way.
#10
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Year: 1999
Model: Cherokee
Engine: 4.0
The only thing you should be looking for with the key to RUN, before starting the engine, is for the fuel pump to run for a few seconds to prime the fuel rail.
How do you know the fuel pump isn't priming?
#11
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Year: 1999
Model: Cherokee
Engine: 4.0
Seams I may have been barking up the wrong tree, in past threads. What would cause PCM not to do its pre-start cycle? Had been trying to check O2S heater circuit, and ended up tracking and tracing half the wiring harness and connectors in the process. Due to no voltage at the O2S connector. It tested fine when running, just thought bad internet info. Just checked fuel pressure and once again nada with key on engine off. On start-up it quickly built up 25psi (problem#2) and held for about 5 min still above 20psi. Problem #1 is why does my PCM not energize my O2S and fuel pump before I actually start the engine? ASD relay tested good from round 1 with the O2S. Hoping its not a bad PCM
sry: once again 97 Cherokee 4.0 AW4 neutered(4x2)
sry: once again 97 Cherokee 4.0 AW4 neutered(4x2)
#13
He has low fuel pressure, rail pressure for his year should be in the neighbourhood of 49psi.
His lack of power is obviously the 25psi, other than that he's just not hearing what we are telling him. CCKen made it clear enough.
His lack of power is obviously the 25psi, other than that he's just not hearing what we are telling him. CCKen made it clear enough.
#14
Thing is I never heard where he said it was running bad. Maybe I missed the post?