Hello all, new member here. Joined because you all seem very knowledgeable and figured I’d give this a try. Recently purchased a 2000 Cherokee 2wd 4.0 A/T.. got her cheap. $500 bucks. However has an issue preventing me from being able to enjoy the drive.

Cylinders 2 and 4 have no injector pulse. When using a noid light I get nothing. However there is constant 12V on the supply side of the injector connector, and when I probe the ground side of the injector with engine running I get a constant 8-9 V going through the multimeter to the pcm grounding circuit for the fuel injector drivers. Was told by previous owner that pcm was replaced and no improvement. Harness in good shape. Replaced the fuel injector connectors and the pcm C2 with a known good one. All wires check good for resistance. Can’t find anything grounding out between C2 from the pcm to the injector connectors. All injectors measure between 12.3-13.5 OHMs. Am I missing something? I’ve got a new PCM coming from FS1 (roll the dice I know).

 

Switch the #2 injector out swapping it with any of the other injectors besides for the #4. Take the #2 injector and swap it with and putting it into the #1 injectors current location and put the #1 into the location where the #2 injector was just removed from and see what happens..

I am not sure but, maybe all of the test results seem to me they are indicating the fuel injection system is operating normal on all of the circuitry as it should. This leads me to believe it could be with the actual injectors themselves as the reason for your problem. Maybe not though if you do not get a noid light response? The meter read from the injector ground may not really be able to tell good information.

The PCM pulls ground to energize the internal coil of the injectors. The injectors all look like they test fine for Ohms. There is power. There is ground circuit activation by the PCM drivers.

If the Internal portion of the injector that moves as a result of the coil being energized has gotten stuck in either the up or the down position this could possibly explain it.. maybe? Maybe it is causing the coil to be stuck on energized? Maybe the PCM injector drivers for these two injectors is stuck closed or has gone faulty?

Swapping the injectors out like this will definitely give you more information to tell you something about what is happening no matter what.

(edited the above in case you read it befor..)

 

Please elaborate on this. Where is the meter grounded for this measurement, and where are you connecting the hot side of the meter?

Also, don't completely trust resistance measurements from the injector connector to the PCM connector. If there is a single strand of wire intact, you'll get a good measurement with a meter, but it won't properly carry the current to fire the injector. So, the test looks good, but nothing works.


Yep. This is excellent advice.

 

I disconnected the injector connectors and put the positive lead on the 12 constant pin and negative lead on the pcm ground side of the injector pin. When I tested all the connectors this way all others except 2 and 4 jump bewtween 0 and 12v in a pulsing fashion. 2 and 4 float around 8-9 volts. When using a noid light on the connectors the bulb doesn’t illuminate. Injectors have been replaced.

 

I found on the pcm wiring schematic that there are two ground wires that join together into one wire label g117 but I find no mention anywhere else for the location of ground g117. I cleaned all the grounds I can find.

 

Also in response to first comment, I pulled the harness apart to replace some bad injector connectors that had wire showing and physicalled inspected each lead going back to the pcm connector C2 which I also replaced due to some loose pins. I even went a step further and ran tempora wires from each injector straight back to the pcm for testing purposes and still no change. 2 and 4 did not pulse or illuminate the noid light. Just a constant floating 8-9 volts.

 

It sounds like you have a short to ground, pinched wires, or the PCM injector drivers have failed to me. I am leaning towards it being in the PCM since you have run separate wires to them like you say you did. It could still be one or both of the injectors causing this though...

With power off, unplug the harness from the injector and check the resistance of the pin on the harness (not on the injector) that the ground wire is attached to and a nearby chassis ground point. There should be no continuity. If there is a low resistance on the wire for the injector ground (harness side only), then the driver transistor in the PCM or the wire to it has shorted out. Do this for all of the fuel injector ground seeker wire circuits and compare them all!

In the above; If it is low or high on all the ground seeking wires, then test again but, with power on. (WARNING: If you do this in this way.. you 'may' destroy your multimeter if there is an abnormal voltage that should not be there on the ground wire) - first, check for an abnormal voltage on the ground to protect the meter. For this test all of them should normally read having a high resistance to ground.

If the #2 and #4 only reads a low resistance on the first test, unplug the PCM and retest. If it is still low, then the problem is with the wire and not the PCM.

Very important to remember is what would have caused the PCM injector drivers to fail? Internal injector coils want to retain their charges. As the field collapses (millions of times over the course of their lives), this generates a fairly large very fast pulse of the opposite polarity known as 'backward EMF'.. This reverse voltage can zap the PCM drivers. There are protective measures set up to protect the PCM from this phenomenon, like diodes and such but, over time these things may fail. What I am saying is that if your PCM drivers went bad? It is likely there was something which caused this to happen.. If you end up having to replace the PCM? You should seriously consider on replacing the two injectors with problems on them right now along with it, so they cannot zap your replacement PCM and result in this same thing happening all over again in relatively short order.. just to make sure.

I would pull the injectors one by one and try cranking to see if it starts. And swap inoperable injectors out with good ones to try that too.

 

Noah911, thanks for the response. I hate electrical gremlins. I should note that the gentleman I bought the Jeep from told me that he replaced the pcm and the problem continued. When I Megan working on the Jeep I noticed the injector 2 connectors had bare wire exposed. My hypothesis is that the exposed wires shorted together damaging the pcm fuel injector drivers and perhaps when he installed the new one it damaged that one to. I have since repaired the connectors. I’m still waiting for the new pcm to arrive. I’m not the best with a multimeter, and all these resistance values confuse me. Just to clarify. I should probe the grounding side of the fuel injector connector with the positive lead of my multimeter, set to resistance, and put the negative lead on a good ground location. With the key off I shouldn’t see continuity? And if there is no continuity I should expect to see infinite resistance, since the key is off the pcm wouldn’t have opened the ground circuit for that fuel injector. Is that correct?

 

A short to ground would make smoke somewhere.


Agreed.


The main ground for the PCM is on the grounding plate near where the coil is on older models. Passenger side of the engine block towards the front. The main battery ground is there, too.

 

You can "backprobe" a connector by inserting a paperclip into the wire side of the plug and attaching your meter wire to it. That way you get the information while the circuit is intact and operating. The PCM only grounds the injector to make it squirt fuel. It controls the mixture by varying the "on" time pulse. With the key on, all the injectors are powered up with 12v, but only after the PCM gets a signal from the Ckps. (ASD relay on) The original Siemans injectors in my 2000 WJ 4.0L had case cracks in them that allowed moisture to corrode the coils. I replaced them with Bosch 4-port ones. I have a problem right now with my 2000 XJ with a leaking injector (I think) because if you shut it off when it's hot and try to restart it immediately it's fine, but once it sits for a few minutes, it stumbles like it's flooded. If you let it cool down for a while, it starts fine.

 

Regarding the last post, of the coils were corroded would they fail the ohms test?

 

That is right. It should be showing as a reading greater than what the meter can measure.. as OL (open loop) infinite resistance when checking for continuity with the key/power off. You can put either one of the meter leads red or black in either location to the chassis or on the ground seeker wire ends of the connector for obtaining these readings.

 

Yes, if the corrosion has removed the varnish from the wires and caused less impedance in the coil, that is making the length of the coil's circuit shorter. This can reduce the magnetic pull on the plunger that allows fuel to flow.

 

Update:

Got a new PCM reman unit. Now I have fuel injector pulse all 6 cylinders. Great. Except now I have a new issue. Rough idle, comes and goes and bucks hard at times while driving. Loss of power. Seems to clear up at times especially when driving for a bit. I have racked my brain trying to solve this. current DTC: P0132 o2 sensor s1 short to voltage
Po443 Evaporate emissions Purge solenoid

Checked 02 sensors harnesses and cleaned connections.
TPS sensor within Specified range when tested
Coil rail passes OHM test and verified spark.
spark plugs are clean and gapped.
No vacuum leaks that I can find.
coolant temp sensor seems to be working, accurate readings while driving.
Cyl 4 injector has been replaced recently by Previous owner.
All grounds are good, cleaned and tight.

I'm stumped. Sputters at idle, seems to come and go. Cylinder 4 is the only cyl I’ve seen a misfire on using my work equipment.

 

someone went went thru a hell of a time, and the purge solenoid was the issue..makes the vehicle run lie a dog apparently
replaced nearly every thing first...his was emitting a weird noise when he got real close up apparently.

think it was a cheap fix..I grabbed all that stuff of a wreck recently, just cause of the dudes post!