sumax

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)

Radi

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.

Rockfrog

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.

CCKen

First of all your fuel pressure is too low (25psi). This would be a failing fuel pump module. Should be 49.2 psi +/- .5 psi.

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.

holycaveman

Without the cam sensor working the fi will not operate but you will still have spark and fuel pressure. So yes you are correct.

holycaveman

Sounds like a bad pump. Have you checked for voltage at the pump when you cycle the key?

sumax

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.

holycaveman

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.

wamiii@comcast.net

Don't shoot me - brand new - how on earth to I ask a new question - i.e. where is is good repair shop 01 Jeep Cherokee for daughter in Boulder CO

CCKen

The O2S heaters won't be powered until the engine is running (ASD relay pulled in).

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?

CCKen

Other than all of his ^ what exactly is your problem?

holycaveman

Could the whole problem be he is not hearing the fuel pump even though it's working? LOL

Rockfrog

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.

holycaveman

Thing is I never heard where he said it was running bad. Maybe I missed the post?

Rockfrog

Yeah, he does mention a lack of power, but that's all really.