Vehicle: 2007 Jeep Grand Cherokee 3.7 L
Crank - No Start Intermittent Pulse of Injector & Coil(s).

Recently rebuilt top side of engine after fire. We are getting an intermittent pulse on injector and coils. When cracking engine, (testing with noid lights on spark and injector) systems get normal pulses, engine tries to start, then pulses stop. Continued cranking gets intermittent on/off pulse signal surges. We get a blast of pulses the engine might turn two revolutions and then another blast.

We are seeking help and answers to what would cause intermittent pulse signals on the coil and injector circuits.

Useful Information:

• ASD auto shutdown relay working normal i.e relay closes for 2-3 seconds, if engine is cranking stays closed. If you stop cranking relay opens 2-3 sec (normal). Because it is working normalwe assume the ECM is providing ground.

• we have a problem with no ground on Starter Relay. Likely problem with internal transmission NSS. With ground jumper on relay it works fine. We get 12V pilot but no ground. Will addressthe NNSproblem once we solve the no the start problem.

• Engine wire loom was switched out with matching vehicle/loom (plug-in replacement).

• Two ECM were tried (original & and donor vehicle matching the loom)

• all other functions work (to the best of our knowledge)

• I should note we rebuilt the heads i.e. new timing chain, but we have checked the timing marks a number of times.

Can anyone recommend what would cause a intermittent coil and injector circuits pulses?

Your help is greatly appreciated.

Will



PS also looking for a PIN-OUT diagram for the 3 ECM plugs in order to trace grounds from the under hood relay boxes to ECM.

Because I do not have any better ideas I am wondering if it could be security related?

 

The problem sounds as though the Ignition Control Module (ICM) .. also known as the switching module, would be the most likely suspect. I would be suspicious of having a faulty ICM. It could also be a fault with the ICM circuitry and wiring that is external to the module. The ICM provides required inputs for producing ignition and fuel injector pulse. Spark plug fire and injector pulse does not occur without it.

Certain chain auto parts stores are able to test an ICM. If you pull the ICM and take it to Advanced, AutoZone, or O'Reilly's they can test it.

 

Thanks for taking the time to respond.

Now at the risk of showing my ignorance the ICM and ECM are one in the same? If this is the case we switched out to different ECM’s, i.e. the original and the ECM from the donor vehicle along with the engine compartment wire loom. The wire loom by the way was all plug-in so I don’t think we have a problem with it.

You raised an interesting point that I don’t fully understand which is the “circuitry and wiring that is external to the module”. I’m assuming the inside of the module which I would think is all PCB printed circuit board.

What I would like to do is to check wire circuit between the ECM plug(s) and say injector plug but I’ve not been able to find a ECM PINOUT diagram.

Also I’m wondering if I should check the characteristics of the ground at the injector plug? But I’m not certain what the characteristics should be other than closed-ground loop providing engine is cranking and the crank and cam sensors are sending a signal to the ECM.

I did pick up one of these Power Probe 4 tool but I haven’t yet tried to use it.

Thanks again,

Will

 

Yes.. but, not really in the same sense you are asking and thinking. A ICM is an ECM (Electronic Control Module). An ICM is not the main ECM. An ECM is just a different nomenclature for computer. The ICM is one of many ECMs throughout the Jeep (30+ ECMs). The main computer is the Powertrain Control Module (PCM), which is probably what you were referring to and have replaced.

The Ignition Control Module is the computer to regulate sparks and the fuel injector on/off pulse width modulation. It is the computer designated to control both of these functions.. seeing as how they are naturally timed or 'switched' on and off together in a synchronous relationship to one another. This is the reason for why the ICM sometimes gets labeled as the Switching Module. When known as only the 'Ignition' Control Module; this kind of leaves out the fact that it also is the computer holding the responsibility of providing neceasary inputs to the PCM for creatuon of fuel injector pulse.

If any external wiring to the ICM is with faults, it is somewhat possible for this to cause false signals to arise from the ICM (or a momentary lack of signal production). If the module itself got superheated in a flame? Something like this is also reason to believe internal damage of the ICM itself may have occured, and this would be able to produce the same kind of results. Both of these are likely considerations in my mind for how you have described the symptoms.

I think it is likely something to have an effect on both, spark and fueling equally, since they normally happen together. And since they are abnormally not happening together now.... If it were a ground, wire, or pin specific to either or, then the other would still take place normally how it should.

 

Very well written and explanatory, I thank you very much. I will endeavor to locate this module and have it tested or replace it.
I believe you're spot-on regarding the pin or wire issue. As the problems with the intermittent pulse signal is simultaneous with the coil and injector signal. With noid lights on each they both pulse/pause together.

Thank you again, I will test all of this and report back.

Will

 

While chasing the ICM module possibility. I do have another question that may be related. We for reasons I do not understand (since the fire) after replacing all of the wire looms have a issue with the NSS switch. The starter relay gets a pilot signal from the ignition switch, but there is no ground on the relay coil side pointing to the NSS. Now it happens that this transmission has the NSS switch inside of the oil pan so it's not a easy test of the switch.

While looking at the bigger picture Crack No Start , we hooked up a workaround i.e. jumper wire from the relay ground terminal to ground point near the relay boxes. This works fine and the relay now works with the ignition key.

My Question: is there any interlink between the NSS functionality and the ECM with the exception of turning on/off the starter relay ground. Is there any possibility this problem may be tied to the intermittent injector/coil pulse signals? My gut feeling is they were independent of each other, but I thought I would ask.

Another thing I've thought about but ruled out is the security issues with the key. I read a lot about this being faulty, but my thought is that it would not cause an intermittent pulse, it would be more likely to cause the ECM to open ground and stopped the engine. But that said, I thought I would ask.

Thanks again for your support

 

With the understanding I have of these things, I say no to both of the questions you posed regarding the functions of the security system and for the NSS to have such an effect as this.

Unless things have changed dramatically from all of the years prior.. the NSS should have zero effect towards the injectors or sparks. The only purpose for it should be in providing power to ground for completion of the starter circuit.

The same similar reasoning also applies for how the functions of the security system are designed to work. Unless things have changed dramatically from all of the years prior.. the security system should have zero effect on the sparks. The security system should be designed in such a way to shut off only the fueling side of things. The Jeep should be allowed to fully engage and initially it will run. Then, only just the fueling gets shut down. This will be what causes the Jeep to stall out and shut itself down a few seconds afterwards, once the fuel supply becomes cutoff by the security system.

The NSS should have no effect towards the fueling or the sparks. The only effect of the security system should be that of disabling the fuel supply. The security system should have zero effect on the production of sparks.