Need expert advice with 1994 (MB) PCM pinout and o2 sensor troubleshooting | Ford Explorer Forums - Serious Explorations

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Need expert advice with 1994 (MB) PCM pinout and o2 sensor troubleshooting

mcfixstuff

Well-Known Member
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February 23, 2021
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Location
Katy, TX
City, State
Katy, Texas
Year, Model & Trim Level
1994 Explorer XLT 4WD A/T
This is my third and last forum post about this, as this title should cover all the bases. I am getting very conflicting information about the pinout for the 1994 automatic, federal emissions (MB) eec-iv pcm ecu computer, whatever else I need to put to hit all the tags for people searching this in the future.
My Hayne's manual says 44 and 46 are the two pins for the bank 1 o2 sensor 44 being the gray and light blue, which is correct, but it shows it shares 46 as the other lead, with bank 2's o2 sensor. There is another gray and red wire next to it in pin 49. You'll see in the photo I provided. Ignore the wires, I am poking and prodding at them, and will properly reseal them when I'm done.

What pin wires are for the bank 1 o2 sensors, and what voltages am I supposed to look for at the pins right at the pcm?
What about bank 2? I'm having no issues with bank 2, but I wanna make sure I am double checking my issue.
My Bosch 1300 is showing me ~0.12 & ~0.82 for bank 2's o2 sensor, but bank one sits at 0.00 and sometimes fluctuates to 0.32, but mostly sits at 0.00 and doesn't oscillate like bank 2 does.
Housekeeping things to say to not waste anyone's time: I already replaced the o2 sensors on both banks and the readings from my live data haven't changed. I switched sides of the sensors, and the data stayed the same. I dropped in a new ecu this morning, and nothing changed.

Thanks again for the help. I really don't wanna drop it off at a dealership. They will probably charge me more than the truck is worth.




PXL_20220523_164433635.jpg
 



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To further elaborate on troubleshooting I've done...
When testing voltage at pins 44 and 46, on the actual PCB board solder points of the pcm, I get ~0.12 and ~0.82 switching, same with pins 43 and 46, however the computer, both the old and remanufactured new one, are showing me ~0.00 and occasionally a jump to ~0.40 for bank 1 o2 and bank 2 is showing the same I'm getting on my volt meter.
I even hooked up a dead AAA battery showing 1.1v to pins 44 and 46, and the computer just showed me 0.50 to 0.54v.
PXL_20220523_170915879.jpg
 






Further troubleshooting: I cut the wire at 46, and it affected the bank 2 voltages, they rose to about 1.2v, when I complete the circuit again, bank 2 goes back where it was, however, bank 1 seemed unfazed. Cutting the wire made no difference at all.
 






Okay, we are getting spooky here. Now I haven't driven it long enough to see if this is actually making an impact on my fuel economy, but check this out:
When I use the 12v socket on a portable jump starter I have to power my bosch 1300 reader, my o2 sensor bank 1 reads 'normalish' ~0.08v to ~0.70v, and my stfueltrm jumps around -4 and 2.
When I plug the reader into my vehicle's cigarette lighter socket, the reader shows bank 1 at 0.00v and my short term fuel trim jumps up to 43, and stays there.
Bank 2 does not get affected by any of this.

WHAT THE ACTUAL F***
 






So what is the problem you are having?

Is there a trouble code in memory? Is the engine not running properly?
I understand you are trying to troubleshoot your live data readings on the bank 2 hego switching, but is there anything actually wrong or are you just questioning the lack of switching in the bank 2 sensor?
 






So what is the problem you are having?
The computer compensates for a lean condition it thinks it has in bank 1, and dumps way too much fuel in that side of the engine. I end up with 10mpg combined, when I used to get 16.
If I do a KOEO test with the code scanner plugged into my cigarette lighter, I get a bank 1 lean code, if it's not powered by my car, I get a 111, or all systems passed code.
 






Could it be it needs the ground of the vehicle for correct data? Idunno it is kind of weird.

On a hunch, if you could, crack the ecm open, and look for evidence of corrosion or leaking capacitors, bad solder joints.
 






He has same condition with two different pcms

What makes you think it is not actually lean on bank 1? Sounds like the 02 sensor is doing it’s job?

Have to ask
 






He has same condition with two different pcms

What makes you think it is not actually lean on bank 1? Sounds like the 02 sensor is doing it’s job?

Have to ask
Because despite what the computer says, when I hook up a volt meter to sensor 1, it shows me 0.12 to 0.82v, yet the computer is reading something different, whether it be 0.00, or 0.09-0.54v. Not only that, the exhaust smells very rich.

I believe it is a wiring issue the more I look at it. I might swing by a junkyard and pull the whole harness from the PCM to the gray and black connectors. There goes $40 in gas, plus however much it costs for a harness :(
 






Could it be it needs the ground of the vehicle for correct data? Idunno it is kind of weird.
That's what I would think, but just for the bank 1 sensor? All the other data stays the exact same.
Plus, with no vehicle ground to the reader, I'm getting closer to the actual data I'm reading with a voltmeter on the pins at the pcm.
 






Interesting I think you are on the correct path inspecting the wiring harness possible dead short or Mia ground
 






Interesting I think you are on the correct path inspecting the wiring harness possible dead short or Mia ground
I've got until 06/06 until summer courses start up and I gotta devote all my time and energy to calculus II 🤮, so I'll try to figure out this puzzle before then. I think tomorrow I'm gonna pull and strip that part of the wiring harness wiring harness of it's wrap and look and inspect all the cables. Solder issues if I find any, and rewrap it. I don't feel like driving to the junkyard yet. That's just so much gas, and the Texas heat is would end me.
 






Anyone know how to remove the PCM wiring harness from the firewall? Tried to look through some posts, but couldn't find any information. Is it easy, or do I have to remove part of the dash? I'd like to know what I'm getting myself into before I go to the junkyard.
 






The harness can be pulled straight through. Pinch the rubber grommet on the inside of the cab, push into the engine bay to feed through. youll need to disconnect the fuel shutoff/intertia switch, the pcm and ...something else. Don't recall the other connector.
To continue pulling from the other side though, removing the ac fan /heater resistor cover, or maybe the right fender may make it easier.
 






The other connector is for the heater/ac it’s a square plug w 4-5 wires also will have to remove the pcm ground wires

Helps to remove or at least pull back the plastic inner fender liner
 






I did not realize the harness goes all the way to the driver side without a disconnect. I was thinking I was just the passenger side area, and I could just unplug a bunch of stuff. Boy was I wrong. I left the junkyard with a brand new looking air hose, the canister purge valve with connector, and a really solid looking third brake light. The explorer looked like it had been garage kept, however the PCM had a different code on it, so I didn't wanna take my chance wasting my time and money on the wiring harness if it wasn't exact.

With that being said, I'm leaning more and more to a bad wiring harness, because after hooking up the canister purge valve I notice it's not working because its only getting 10.8v, plus its not turning off when the computer says it's supposed to be off. It just stays at 10.8v no matter the throttle position. I bench tested the purge valve and it does indeed work if supplied 13.8v.
Odd, I have more work to do.

Also, I averaged ~15.8mpg on the highway to the junkyard and back home. A step in the right direction, but not where It was at 19mpg.
 






Okay, so I found another piece to this puzzle.
Haven't had the time to pull the wiring harness yet. Seems like its gonna be a pain in the absolute ass, but also I'm thinking I might not have to pull the whole thing.
Piece 1: The purple/white wire going to the code reader causes issues with reading o2 bank 1.
So here's where I'm at:
Combo 1: Purple/white wire plugged in, and reader grounded to vehicle (plugged into the cigarette lighter socket); I get 0.00v on the live data for o2 bank 1. (Keep in mind, literally everything else stays the exact same throughout these combos)
Combo 2: Purple/white wire stays plugged in, reader powered with vehicle positive, but negative disconnected (same as if running on separate 12v power altogether); ~0.08v to ~0.60v oscillation.
Combo 3: Purple/white wire disconnected, reader either plugged to vehicle power or separate 12v source; ~0.12v to ~0.84v. Exactly as it should be.

I am not stopping there, because this tells me that I do have a wiring issue, and I need to correct it. I wonder if this information can help me isolate it.

BUT THE WEIRDNESS DOESN'T STOP THERE!

Last year, I noticed my canister purge valve was broken, and just deleted it from the system. However, in light of trying to chase down my ****ty mpg culprit, I am returning all systems completely stock. I put a new purge valve in, and it isn't working. Not that the valve is broke, but the system is only providing 11v to it. Not only that, but the system always provides 11v to it, regardless if its supposed to be on or not. I'll give it throttle and watch when the computer tells it to turn off, and the volt's never change.
If I bypass the original wiring, and hook it directly to my battery, the purge valve functions normally.
This also tells me there absolutely has to be a wiring issue with my vehicle.

I know I need to pull the harness.
What should I look for when I strip the shielding? Do I just look for frayed wires, like what was the issue with my transmission, or is there more to look for?

PXL_20220526_220738726.jpg PXL_20220526_215312448.jpg PXL_20220526_220726452.jpg PXL_20220526_220731386.jpg
 






I'm pretty sure there will be 11-12 v present on both pins of solenoid. When ecm calls for tank venting, one pin will drop to 0v , which will cause the solenoid to operate. The fuel injectors operate the same way. Ecm drops one pin to zero and injector sprays.

It may be difficult to capture the pressure vent command
 






I'm pretty sure there will be 11-12 v present on both pins of solenoid. When ecm calls for tank venting, one pin will drop to 0v , which will cause the solenoid to operate. The fuel injectors operate the same way. Ecm drops one pin to zero and injector sprays.

It may be difficult to capture the pressure vent command
Thankfully it is really easy to capture the vent command as my bosch 1300 w/ live data shows me exactly when the computer commands it to vent. It says to vent when the truck is at idle, and when the truck passes ~10% throttle, it tells it to turn off. No matter the on or off state, the voltages stay the same, and no ground continuity changes.
 



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