Dave51

Unless your ignition switch is bad...

Noah911

Having power to the injectors with the key off may be normal as most systems run the return side of the circuit through the ECU and no current flows in the circuit until the ECU commands the injector to turn on.

With the key off measure the resistance from the low side of the fuel injectors to ground. If it is really low like less than 100 ohms there is the problem. It should be open...

Next, disconnect the computer. Measure resistance again. If the resistance is still low there is a pinched/shorted wire. If there is high resistance now, the problem is in the computer (bad drivers).

(edited):
It is difficult for me to think it could be all of the drivers are bad like that at the same time...

Jordan Double

Ok thanks u guys I’ll try some multi meter reading today and ecm shenanigans and get back to you, I just cleaned the dipstick ground and the headstud to body ground to bare metal and that didn’t help so

Jordan Double

Well we voltage tested the injector with the ecu plugged in and neither terminal was ol but after we unplugged the ecu one went down to ol, are u saying that means the ignition is bad or the ecu?

Noah911

Let me understand what you did?

You tested both wires of a fuel injector..? You did the testing on the ground feeder circuit wire of one of the fuel injectors? You also tested for resistance on the power supply circuit wire on the same fuel injector too? At the connector?

You tested both of these, and neither one of them showed infinite OL with the key off and the PCM remaining still plugged in? There was a low resistance reading?

You unplugged the PCM, and re-tested both of these again with the ignition key still remaining turned off.. and now one of these went to display infinite OL?

Jordan Double

Yes that is what we did, we grounded the one side of the multimeter and put the other in the harness side of both prongs of the fuel injector with and without ecu plugged in that’s all we tested really

Noah911

This indicates the injector driver in the PCM (ECU) is faulty. The ground circuit of the PCM should be open unless the PCM is making the command for the injectors to spray fuel... It completes the circuit by 'Closing' the fuel injectors ground supply (it feeds in the ground power supply in this way), which energizes the injector coil causing fuel to spray out of the injector.

This is why it should read as OL (Open Loop) with the key off while the PCM is plugged in. The PCM keeps this aspect normally in an open loop status whenever the Jeep is turned off. If it does not read OL this essentially means the PCM injector drivers are internally shorted to ground.

Unplugging the PCM always normally causes this same ground circuit to become an open circuit - (unless there is an external short to ground in the wiring, in which case it would remain as showing to have resistance)

Jordan Double

Ok so my ecu is probably bad, I just pulled it out of my dash and am gunna try with it not touching anything just Incase and so the wires and in there good for sure and everything

Noah911

The testing you did is definitely telling you your PCM needs replacing...

There is some more things to worry about now though before you put in a replacement computer. PCM injector drivers do not normally just go bad on their own. Even when it is just one of them (because there are six injector drivers one for each fuel injector).. there is still ussually a reason for why that injector driver went bad. A lot of times it will be a short pinched wire to that one specific injector. Or, the fuel injectors diode failed.. and it caused the PCM injectors driver to get zapped by an electrical field of opposing polarity.

If putting in a new PCM without first fixing the reason for why the PCM injectors driver failed in the first place.. may result in doing the same thing to the new PCM killing that same injector driver of the new computer.

Have all of your Jeeps PCM injector drivers failed, do you know? You have to test all of them. The same test at every fuel injector needs to be done...

Noah911

You should then pull each injector and find what is the resistance of each one...

Look it up specific for the fuel injectors you have in your Jeep, to see their specifications and test them to see what is their Ohms. Replace any injectors which test bad is really important.

Trace all of the fuel injectors individual wires and along the whole main harness for them and repair any faults you find. If it is all of the PCM injector drivers are faulty, I would suspect maybe there is a problem in a part of the main wiring harness for them...

Jordan Double

Ok good to know we only checked injector #1 it’s too cold and I’m done for the night tonight but I will check the others tomorrow, and the zap thing sounds possible with the crappy old injectors that we’re in it that’s all I think it really could be, all my wiring seems good and I now have all new 4holes in it so if it’s that I should be good

Noah911

Just to clarify.. You do not have to test all of them, since you cannot replace any single one failed driver. The whole PCM needs replacing wether it is one or all six drivers that fail.

It is just a little strange to have all of them to be failed. I was just wondering if you knew? I thought earlier you were saying that you could tell all of them were remaining on and were spraying. Maybe this was not what you meant though?

Jordan Double

Idk how many were staying on I just know u could hear at least 1, tomorrow I am picking up a ecu from the parts car I got the motor out of and hopefully my problem is fixed and before we put it in we will try and see if anything that could cause that to go bad and fix it. If it happens again I know to dig deeper, I’ll just have to actually buy one next time 😅 lets just hope there is never a next time

Dave51

So it sounds like you tested for resistance rather than volts.

Renix fuel injector firing is not controlled by completing the circuit at the grounds but rather by simply providing power. See schematic:



By disconnecting the ECU cable and measuring resistance, the only thing you have proven is that you have disconnected the ECU cable. I believe a better test at this point would be to measure voltage at the injectors with key on and watch in the 30 seconds following key off when the injectors still appear to be spraying.

Jordan Double

But wouldn’t that just confirm the injectors are being told to spray not that the ecu injector drivers are bad, either way I’ll probably end up trying the new ecu tomorrow and we’ll see