Fuel injector open circuit fault. Low voltage, but continuity?

[jivemiguel]

Hey y'all first post here, but you guys have been a huge resource for me since I bought my XJ a few years back, so thanks for all of the great resources...


So today I am officially stumped. The other day I fired it up and the engine was rough and threw a CEL out of the blue. I put my scanner to it and there's an open ground on injector 4.


What I don't understand (I'm handy but not a mechanic) is that the wire to the injector has continuity all the way to the pcm, but it's pulling lower voltage. Looks like the other injectors were firing around 3v, and the number 4 was round 0.2v.



Now I had some squirrels in there a couple years ago, and they did in fact eat the wire to the number 4 injector, but I got a new pigtail and soldered it in, shrinktubed it and it has been fine ever since. The first thing I did when I saw it was the number 4 injector, was check my soldering and the pigtail. I even tried soldering in a new one. The issue is not there.

One other thing is the second wires to the injectors (green and orange- I am assuming these are grounds) have all apparently been soldered together in the harness. Or at least it looks like injectors 3-6 are all glommed together and stuck into a large piece of shrink tubing. One assumption I am making here is that since the other injectors are fine, this ground wire is not an issue. So far as I can see the little bit of ground wire that comes out of that mess into the injector appears undamaged.


Here's a pic of that for the curious.


So I don't know where to go from here. Injector wire lights up the test light. There's continuity all the way to the PCM. But the voltage is low.

I also swapped lines and rescanned the car, and it definitely looks like the injector is fine. This is also supported by swapping lines and watching the engine.



Sorry for the long text here. I have been down for a couple days, and I am kinda losing my marbles over here.

 

[lawsoncl]

Well, based on the wire colors, the mention of a CEL, and the throttle cable position, I'm to assume it's a 1991 or later?

In which case, the common dark-green/orange wire is power from the ASD relay. The PCM controls the ground side of the injectors (the earlier Renix engine computers controlled the positive and they shared a common ground). What was the code? If the PCM is complaining about the circuit, then it's likely not seeing 12-volts when it's not trying to fire the injector, meaning a open circuit or bad injector.

Verify that you have 12-volts to the injectors with the key-on, and measure the resistance of the injectors themselves. Check the wiring in the harness closer to the firewall where it tends to rub and flex with engine movement. If it's not wiring, then I'm going to guess you might have a bad injector, and it's time for a set of refurbish Bosch 4-holes.

 

[lawsoncl]

In addition to looking for voltage, I'd measure resistance between the green/orange wires across two different injector connectors to check for a high resistance connection/splice.

 

[Tim_MN]

Failure of the splices inside that shrink tube can happen, especially if you have been wrestling the wires about.

 

[jivemiguel]

Hey guys- thanks for the input. I was going to head out in a few and have a look, but I have a couple of questions (again, no mechanic so bear with me)


and first off, to answer lawsoncl, this is a 98 sport. THE CEL Was a P0204.

As to the ASD relay, I looked it up and found this diagram which indicates it was for someone with a 95. I believe the 95 is pre OBD-II, but for our purposes with this wire is it cool to refer to this?

https://www.cherokeeforum.com/attac...g-power-fuel-injector-pcm-95fuelinjectors.jpg


If so, what is this junction on the green/orange wire labeled S 115? I am trying to figure out where on the power line I should be measuring. Again I had kind of made the assumption that because these lines are appearing to be soldered together that the fault was after that point since the other injectors are fine. And in which case there's only a few inches of wire from that point to our pigtail.

Sorry to ask you to explain a little more. My lack of experience here is definitely showing

Thanks.

 

[jivemiguel]

okay guys just another update.


Traced the positive line and back probed it in a few spots all the way to the relay. There's continuity all the way down.



I grabbed the ground strap at the firewall and confirmed the voltage for the number 4 pigtail is the same as the others. I couldn't do this from the on position, the jeep had to be running.


Resistance on injectors 3, 4 and 5 is at about 12.5. This makes sense as swapping cables did not throw codes on two injectors. I think we can rule out bad injectors, although who knows how old these are- I am going to upgrade them soon.


So It seems the issue is either with the PCM or that wire to the PCM?

 

[jivemiguel]

Sorry for all the posts.



So I checked three different wires from injectors 3 4 and 5 to the PCM.


All had the same low resistance, and voltage from the battery were all the same as well.



I think at this point it's safe to say I need a new PCM.

 

[lawsoncl]

If the wires and the injector tested good, then it may be a blown injector driver in the engine computer.

S115 refers to were the wires are spliced together in the harness. (S=splice)

 

[jivemiguel]

Hey y'all thanks for the assistance and feedback. So after checking the ground to the PCM, and checking the power line to the splice and battery, I figured it was a bad PCM.


Orded a rebuilt unit and bam. Problem solved. Was hoping it would fix my dome light and passenger window regulator, but alas I am gonna have to chase that wiring down another day.

For now, we're rolling again, and Imma order some new injectors next week.


Again, thanks to the responsiveness of the community. I'll be checking back in with y'all when I hit some more projects this summer