My wife's 2007 Expedition has been running rough lately and making a ticking sound when first started. There are misfire codes on #2 cylinder. A few months back when we first noticed this, I found the heater hose connector dripping into the #2 spark plug well. Well that is fixed and all of the plugs were changed. I swapped #1 COP with #2 COP to see it the problem changed cylinders and it didn't. I checked the resistance on the fuel injectors and #2 was consistent with #1 at 14.7 ohms. So, I hooked up a 12V LED to check it the fuel injector was getting a pulse. The light was on solid with #2 connecotr, but not bright only when the engine was running. I checked #1 and the light would flash on and off. When #1 was on it was bright, much brighter than the solid light from #2. Just wondering if there is a way to rule out either the wiring harness or PCM. Any help would be appreciated.

Thanks,

 

Prolly depends on how the LED was connected.
I'm assuming you connected the LED in place of the fuel injector.
When you cranked the engine the PCM would provide the ground when it was time to fire that injector, and the LED would turn on. When the PCM releases the ground the LED would turn off.


If the LED stayed on solid on #2, perhaps the harness has found a ground path, maybe a weak ground since the LED is dim, or perhaps the PCM driver is shorting out.


You could provide power to the + side of the LED, and touch the LED - side to the unique colored (i.e, control) wire. With KOEO, the PCM should not provide a ground to turn the LED on.

 

That is what I am thinking. Is there a way to eliminate either of these two possible causes. When I place the (-) wire of the LED to the purple wire on the connector then connect the (+) side to the positive terminal of the battery the LED turns on. This is the same for cylinder #1, so it appears to be behaving at is should.

 

KOEO, is that Key Out Engine Off??? That
is how I did the check

 

Right. KOEO is Key On Engine Off.


Is the purple wire the control wire for #2 injector?
If your KOEO, the engine isn't spinning, so there is no crank signal, so the PCM wouldn't be grounding (i.e, turning the injector on) so the LED should not be on.


If the LED is on steady at KOEO, something is providing a ground. You could unplug the connector where that purple wire goes into the PCM. If that makes the LED turn off, the PCM is prolly grounding the purple wire. If the LED stays on, then some where in the harness the purple wire has found a ground.


If you crank the engine, then the LED should begin blinking on and off.

 

With KOEO there is continuous connection to ground on the purple wire for all cylinders. This is true even with both connectors to the PCM unplugged. I would figure this is not the control wire. The control wire must be switching the +12V while the -12V or ground is always connected.

With KOEO the LED does not turn on if I connect to both wires on the fuel injector connector. This LED only comes and stays on with the engine is running when connected to both leads of the connector.

What is involved in replacing the PCM? Can I just get a used one and replace it, or is there another step (flashing) that is required?

 

The common wire (i.e, the same color on each connector) is battery voltage.
The control wire (different color on each injector connector) goes to the PCM driver module. The PCM switches the control wire to ground when it wants to open the injector and removes the ground when it wants to close the injector.


So if you put your LED in place of the injector (+ LED to the common color wire and the - LED to the control wire), the LED will light when the PCM grounds the control wire, and will be off when the PCM "ungrounds" the control wire.


If the LED lights at KOEO, somehow the control wire is finding ground, either someplace within the vehicle harness or within the PCM drivers.


If the LED lights at KOEO, unplug the control wire plug at the PCM connector. If the LED is now off, the PCM was grounding it. If the LED stays on, the PCM is not providing a ground (because the PCM is out of the equation) and the control wire has found its own ground.

 

I see where my confusion came from. With KOEO the common side which would be + when the engine is running is somehow pulled down to almost 0V when measured to the - battery terminal. This is true even when the PCM connectors are pulled. When the engine is running the common side is the + side. All connectors behave identical with KOEO. However, with the engine running, #2 fuel injector connector has a solid 10.43V across the two wires.

 

If I replace the PCM, does it need to be programmed, flashed or whatever?

 

The common wire is battery voltage. Doesn't matter if KOEO or KOER.


If your volt meter is connected red to the common wire and black to the negative battery terminal it should read battery voltage in both KOEO and KOER.


But if your volt meter is connected red to CONTROL wire and black to negative battery it should read battery voltage when the injector is off and nearly 0 volt when the injector is energized. (When current is flowing through the injector, the injector uses most of the available voltage and that is why the voltage on the control wire is much lower when the injector is on.)


I don't understand this statement - how does the engine run when the PCM connectors are pulled?


The common wire color is always the positive side, engine running or not.
That's good.
That's about how much voltage an injector should use. (I assume you are back probing the connector, red probe on the common wire and black on the control wire.) But it should be pulsing between 10.4V and 0V with the engine running.


So, unplug the injector from its connector and unplug the PCM. Switch to your ohm meter and check the resistance of injector #2s control wire, harness side, to a good ground. If low resistance, the control wire has found its own ground some place. If open, then the PCM must have been holding onto the ground and not pulsing the injector.


Yes, the PCM will have to be programmed. Ideally you would dump the program from your PCM to a scan tool, swap out PCMs, and then load your software from the scan tool back to your new PCM.

 

I'll do the resistance check for #2 control wire.

You are correct, the engine does not run with the PCM disconnected. What I meant was with KOEO the common side is 0V or very close to that. So when I tested with the LED in reverse polarity (before I knew which wire was common) the led would turn on if the positive lead went directly to the positive terminal on the battery and the negative lead was connected to the common wire of the injector connector. However with KOER the common wire is positive as it should.

Also with KOEO connecting the negative lead of the LED to the control wire and the positive lead to the positive terminal of the battery does nothing. I think this is how it should be, which indicates the control wire has not found a short to ground. Correct me if I am wrong.

 

Oh, okay. The common wire also supplies voltage to a number of other engine controls (MAF, IAC, etc.) so it may be finding a ground through those devices.
Agreed. Is that true for injector #2 also?

 

Yes, that is for #2 also. I have a used PCM on its way and a garage is going to flash it for me.

 

OK, Installed a new used PCM and the same issue exists, so I ripped apart the wiring harness and traced the signal wire all the way back to the PCM connector. There were no shorts to be found. I cut the signal wire a few inches away from the PCM connector and hooked up the negative lead to to the LED directly there and the positive lead for the LED is connected directly to the positive post on the battery. I get the same dim constant light as I did when connecting to the injector connector.Either both PCM's are bad (very unlikely), there is an issue with the PCM connector or something which I have no clue about. Any help would be appreciated. By the way I can wiggle and wiggle and wiggle the signal wire at the PCM connector and get an occasional bright flash on the LED. How would you go about fixing the wiring inside the PCM connector.

 

Problem solved. Turned out it must have been the connector on the wiring harness that connects to the PCM. I replaced the wiring harness and everything ran great.