Need expert advice with 1994 (MB) PCM pinout and o2 sensor troubleshooting | Page 2 | Ford Explorer Forums - Serious Explorations

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

You need to take a step back and stop loading the parts cannon with bigger and bigger items. Tossing an entire harness at it is going to be a hell of a lot of work and may not even fix the problem or create new ones.

The first thing I would check is your ground integrity. It looks like you have original cables with replacement terminals, which is a bad sign for the overall cable condition. It is very common for the cables to corrode internally, especially near the terminal and near the frame ground. The negative battery terminal has two parts to it. One is a fat cable that goes to the frame and then the engine block. The other goes to a two pin connector, which is extremely important. One of the pins in that connector carries all the grounds for the PCM and many sensors. There is a ground from the PCM to the body as well (located right near the PCM), and very strange things happen if there is a bad ground connection in this system. You can use a high current test light and your meter to do voltage drop testing of all of these grounds to the battery negative post.

I attached some schematics that should help. With the PCM and sensors unplugged, you can also use some probes and a test light to check each wire for the ability to carry current and check for shorts to power/ground/other circuits. Hopefully you got good quality sensors but it cannot hurt to check for any shorts from the heater to sensor wires in the sensor as well.


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You need to take a step back and stop loading the parts cannon with bigger and bigger items. Tossing an entire harness at it is going to be a hell of a lot of work and may not even fix the problem or create new ones.
I will definitely use these to test vehicle ground. I'd like to use this reply to provide an update, however.

I recently took a trip to Galveston from a suburb west of Houston. During the highway drive I managed to average about 17.8 mpg which is a huge jump from where I was getting about 14 on the highway. I was typically driving between 65 and 70mph.
I've been junkyard hopping and had been replacing a bunch of parts on my explorer that I either bypassed, ignored, or incorrectly deleted.
I think a multitude of issues have been leading to my ****ty fuel economy, and the wiring issue is just the icing on the cake. I'll list off some things I've taken notice to over the last couple weeks:
  • I replaced the factory, leaking, duct taped air intake hose with this cheap pile of ****, since I couldn't find a factory hose anywhere. Thankfully I now have 2 in my possession, and one is installed with the factory air box. The crappy aftermarket system had no port for my crank case vent hose, which I am assuming the ECU assumes is there and tried to compensate for when mixing the fuel/air ratio.
  • When I originally deleted the factory air box, I had no idea what those two air hoses were for that plugged into it, so I initially plugged them off but in the winter, I thought it might be used to open a flap at the bottom to let more air in, so I used a coupled to plug them together. I now know that was probably letting hot exhaust gas into my air intake the entire time. While I no longer have the part on the intake for the hoses to plug into, I just have the two plugged off in the meantime, considering I don't think it gets cold enough in Houston for the system to need to dump hot exhaust air into the intake to warm the system.
  • My canister purge valve was snapped at the plastic, and I noticed it when I had the intake plenum off for **either when I did my injectors, or the crankshaft synchro** a year or three ago, and I saw how expensive it was for an emissions part. I figured the emissions part probably wasn't that important, so I just capped it at the intake, and cut the wire since the plug piece snapped when I was disconnecting it. That is why there are new wires running out the plenum. I ended up soldering new wires, and wanted to run them out of under the intake manifold, so I could have easy access to test it, which I'm glad I did, because there's a signal issue clearly. As of right now I have the system on a restrictor coupler sucking in air from the canister the whole time, until I can figure out a solution. It seems as if I just don't have a solid connection to the PCM ground switch for the purge system pin.
  • I did take a hit on MPG when I replaced my failing fan clutch with the one from Hayden that is notorious for being EXTREMELY SEVERE DUTY. The fan likes to be engaged when I start my car and drive for the first 3 minutes or so, and my Explorer sounds like a damn city bus. The A/C finally blows colder, not as cold as I'd like, but my engine doesn't overheat at idle in the summer, and I don't need to be on a freeway for my A/C to work acceptably. During the summer, however, it seems to be engaged almost always, and I can 100% notice it sucking an incredible amount of power out of my engine during accelerations. I'll take recommendations on what I should do about the fan clutch. I'd pull one from the junkyard, but they aren't that expensive, I could just buy one, if someone has a good recommendation.
  • I had split the wiring harness at the location around my passenger wheel well to look at the wires and see if I notice anything obvious, but I didn't notice anything out of the ordinary, however, it was while I had the wrapping off and the cables sort of separated from each other, I noticed the three stages of getting my computer to read the correct data from my bank 1 sensor, so I haven't rewrapped them yet, but I will when I dig back into this project after my Cal II summer course.
  • Also I noticed the noise coming from my tire was from uneven wear. I had gotten my stuff balanced when I installed my new tires 2 years ago, but had just been rotating them since. After replacing the sloppy, leaking steering gear box, I noticed pretty excessive road noise. Originally thinking it was an alignment issue, I had that done, but with no avail, I ended up getting some dude at discount tire show me that the tread is uneven, and high on some spots leading the the road noise. I'm assuming the old sloppy steering gear box hid some of the noise from me. I want to get them balanced, but the tires are from 2017, as I got them at a crazy steal price when Walmart was discontinuing them, and I know most placed wont touch them once they are at 8 years old. I might take it to this hole in the wall shop down the road and see if they can balance them and trim the high spots, as I have PLENTY of tread left in them, and I don't wanna buy new tires yet. When I do buy new tires, I will probably end up going from my Rugged Terrains to an expensive set of those Michelins, or BF Goodrich. I forgot which ones they were, but they are known for low road noise, long life, and good fuel economy. I think I wanna stick with my 31" tires though. It just looks way better than stock. I can't imagine 31 inch tires on 3.73 gears is hurting my fuel economy THAT much, especially if it drops my RPM's when I'm on the highway. Maybe in the city.
I guess with all that being said, I did have a follow up question to the tires and height thing. What do you think hurts my fuel economy more, the 1 inch lift, or the 31" tires? I have the stuff to delete my lift, and I am pretty sure the tires wont rub on stock height. It just looks so nice, and I do occasionally need it when I'm trail riding. My truck see's about 70% city driving and about 30% highway driving right now. I do occasionally take trips in it though. Oh, and don't forget to give me suggestions for a fan clutch!

In reply to the mention of my replacement terminals, yes I do agree with you. however that is very fresh solder, as anything cut and soldered on the wiring harness has been done by me recently. I will check those frame grounds though. I am just very confused on how all the other sensors who use that ground wire work perfectly fine, except for the bank 1 sensor. And even more so how the voltages at the PCM are always the same no matter the configuration of my reader, but plugging in the extra pin on my reader drops only the bank 1 voltages.

This will be a project I will be working on for a while. I will definitely check all the things you said when I get a chance, but for now I'm gonna drive it like it is, until I finish with this summer course and pay off my credit cards. **** really added up quickly.

I had time to go out and troubleshoot more with my Explorer. I'd like to give an update about the gas mileage; the highway MPG seems to have been mostly fixed. I seem to average about 17.5 mpg on the highway, however, in the city, its still absolute dogsh*t at roughly 9 to 10 mpg. I'm thinking it has to be my Hayden severe duty fan clutch that makes my truck sound like a fu*king city bus, and absolutely kills my acceleration when engaged. I'd love to know if anyone has a better suggestion for a thermal fan clutch that will actually disengage when the RPM's climb higher than 3000rpm, so when I need to get on a highway quickly, I don't have to hold up traffic just because its hotter than 75F outside.

Moving on to the electrical issue:

I did some troubleshooting at the ecm today. I cut the gray and yellow wire at the ECM for the canister purge valve. I checked the continuity to ground from the wire going into the ECM. Looking at the live data, I noted when the computer completes the circuit for the purge valve, which seems to be at idle up to about 25% throttle. Regardless of the computer telling the purge valve to be on or off, the continuity and/or voltages (I ran plenty of tests and configurations) never changed. I had my friend stand by the solenoid to listen to see if it clicked on or off when I gave it enough gas for the computer to send a shut off signal. Nothing happened.
I then connected my alligator clip to the wire going away from the ECM, to ground out that wire to the frame. Sure enough, that triggered the vent valve to open.
This leads me to believe there must be something wrong with the ground wire that connects that part of the ECM circuitry. Can someone tell me what ground wire leaving the ECM would be in control of that? Also, where can I trace that ground wire to the end? I'd like to check to make sure it isn't open.
Now if that ground wire is on the same circuits as the transmission solenoids, then I can't imagine the ground wire being the issue, because those work fine now, and I had actually troubleshooted them a while back, and fixed the mess of melted wires on the driver side of the engine under the brake booster. Those solenoids ground out perfectly when the computer commands them to.

Wouldn't that just be fu*king ironic if it actually is a bad ECM for that one component? I just got done explaining to AutoZone that I don't actually need their remanufactured ECM that they Fedex overnighted to me, and got them to refund me on it. Oops.
One last thing I need to do is replace the rest of my relays in the underhood fuse box. A year ago, I diagnosed my ABS light down to a bad relay for the ABS computer.
Recently, I swapped some relays around, and noticed now my AC blower doesn't always turn on right away. I had already replaced the resistor to it, so that's not the issue. I think it might be the resistor I swapped in there.
Need to pull some from the junkyard, or find some cheap online.

Been pretty zoinked on Vyvanse i've been prescribed while taking my Calculus 2 summer course (learn all of calculus 2 in 5 weeks), so as a side effect of becoming a genius philosopher while on this sh*t, I've also had a few 🤔 moments while driving my Explorer, and reviewing the live data, while also pondering the evidence presented in the past.

Here's one tid bit of data I calculated from experimenting with the live data: If you look at the RPM and pulse length for the injectors when in drive vs neutral, and do a few calculations, I learned I'm using 1.47x the fuel sitting at a light in drive, than if I put it in neutral. For some of these lights where I sit for up to 10 minutes in traffic to get to the front after a few light cycles, that's a lot of gas to be saved by throwing it in neutral.

I also had a "That's So Raven" style vision while driving home from class today, and remembered how when I used to use the scan tool with the purple and white wire plugged in, and the tool plugged into the cigarette lighter, essentially totally ****ing my O2 bank 1 sensor, I'd also have an issue of it sometimes randomly starting a KOER test, and I'd have to shut off the car to get it to stop. It would even do that while I was driving, and looking at the data, occasionally. Now, since I stopped using the purple and white wire while reading the datastream, I no longer have that issue. Given the purple and white wire gets shorted to the ground with the paper clip method, I'm thinking the wiring harness issue has to be localized to between the fuse box, and firewall.

I still don't have time to pull it, as I'm currently balls deep in this calculus 2 class, but I'll be done by early July, and have the chance to inspect it. While I'm at it, I really wanna chop the part of the harness for the OBD1 port, and relocate it to inside the car, under the passenger side of the dash.
Unlike the driver side of the wiring harness that was melted by power steering fluid and maybe brake fluid, that I have since completely fixed, there didn't seem to be any goop or oldness to any of the wires from under the fuse box and above the wheel well when I stripped it of its wrap, and inspected it. I'm assuming it has to be closer to the firewall.

Someone said I didn't have to pull the dashboard to get the wiring harness out from the firewall, so I'm hoping that's true, and it'll be easy to get that part out in the open so I can inspect it clearly.

I'll be curious to see what wires are actually affected. If I had to put a guess out there, I'll say there's a section where wires have rubbed down to their bare copper, and are touching each other without my consent. Pretty sure that's a felony charge. Jokes aside, I'll be suprised if it is touching the gray and red wire, since a bunch of the sensors tie into that ground, yet only one gets affected. Not only that, when the computer would read 0.00v, probing the exact pins at the motherboard would show me the normal voltages on my meter, but the computer wasn't reading what was there. This will be an exciting conclusion to the mystery. Maybe Netflix will make a 12 part docu-series on it. Anyone wanna take bets now on to what is the actual cause of all the hoopla?

I'm gonna post this, then go outside and quickly check what voltages I'm getting between the purple and white wire and the battery ground. If it's anything unusual, I'll report back.


My suspicions were correct. Now what the hell does this all mean?
For those who can't access YouTube or don't feel like watching the video, when the car is off, I am getting 4.89v consistent between the purple and white, and battery ground. When I turn the vehicle on, it starts oscillating between 3.50v and 5.00v, at about the same speed the O2 bank one sensor is supposed to oscillate 🤔🤔🤔🤔.

TAKE YOUR BETS NOW!!!! The series finale should conclude sometime mid July!

The problem has steadily been getting worse. To the point, now, my truck wont stay running for more than 5 seconds. I pulled both the main harness and secondary one under the intake, and stripped and inspected them. No issues on either. However, I just tested the diodes in the fuse box, and both are bad. I'm trying to source a good one.

diode allows current to flow in one direction
Junkyard good source for good ones
odd to have two bad diodes??

Only one of the diodes were bad, actually. However after replacing both diodes, the issue still persists, and nothing has changed. I will be testing all grounds once more, once this rain stops, and will try slapping in yet another PCM. I already pulled and inspected the entire wiring harness, so I have ruled that out. I replaced the diodes, and ruled that out. It has to either be a bad ground or bad computer. I am thinking bad ground, because the canister purge valve's ground pin on the PCM is not fully grounding when engaged, and not ungrounding when disengaged.

I have seen quite a few bad PCM in the last two years especially from the older trucks, age is starting to get to them it seems

So, interesting enough, I put the computer into output switch test, via my handy Bosch 1300, and with the switches and solenoids set to the on position, I was able to get the canister purge valve to engage. It would engage at 12.35v the same as the battery, and when disengaged, it would go to 10.45v. Not sure now if its a bad ground or computer. I still don't know. I was able to get the EVR% on the datastream to wake up, and could drive it around the block, however, both o2 sensors like to stay asleep, when they wake up, they show between 0.92 and 1.02 volts, and the engine runs like absolute dog ****, then they fall back asleep, and the truck continues to fun adequate.

More updates to come

Oddly enough. I got it put back together last night and it still had all the same issues. A cold front blew in, and with it 40° outside this morning I tried it, and all the sensors were alive, and the truck got me to work and almost back with no issues. Started stalling about 3 minutes from home.
I mean this has gotta be a computer issue at this point. You used to wake dead hard drives by either heating them or throwing them in the fridge.
I got the Explorer God™️ sending me a confirmed working PCM. Will update more

New PCM dropped in, and exactly the same issues.
Close to giving up and throwing the towel in.
Driving to Austin this Sunday to visit Kris and Russell to see if it can stump them too.
It seems to not stall if I'm driving on the freeway, so fingers crossed I can at least make it there.

if outside air temp change correlates to system change between working right and not, highly suspect a connection issue/solder joint. You swapped to known good computer. and no change.
It did work after you had the harness out and back in, inthe cold. this seems a gremlin untraced in the harness / connector issue. did you do just continuity test or resistance & wiggle with the harness out

I already pulled and inspected the entire wiring harness
re-read... was this visual inspecton or thoroughly metered& measured?

re-read... was this visual inspecton or thoroughly metered& measured?
This was a visual inspection. I stripped all the wrap around the wires, and visually inspected every wire, then rewrapped it. Any component I was having issues with, I physically checked the continuity with a meter, and I was having zero wire continuity issues. The only thing I haven't checked was all the ground wires going to the PCM.
For example, and issue I could have, is the computer could tell me it is reading 0.05v on my o2 sensor, but if I checked the pins at the PCM for that same sensor, I would be seeing something like 0.95v on my voltmeter.

.95 reported to bosch 1300 , yes? was the .05 measured at the sensor, and verified on the pin-out-to sensor at pcm?
I think at this point it would be prudent to provide more detail regarding the pin/wire you are measuring voltages at pcm/sensor, and if you are seeing same measurements between original and replacement pcm.
After seeing the condition of the harness degradation from the fluid many months ago, would assume that, no assumptions can be made. Would consider visual inspection to be inadequate. Wiggle test with resistance meter on evey wire is due. tedious, but you know you have had previous damage to harness.
to restate, continuity may not be sufficient measurement. actual resistance should be measured. you know there is a difference of at least ≈.9 volt,
You know that there was a condition under which your vehicle ran correctly, with outside air temp at º40.
truck got me to work and almost back with no issues
You haven't really ruled out the harness it you only/mostly did visual.

Swapped the MAF sensor with a Motorcraft one, and the problem went away. Would have never guessed it in a million years. The voltages coming from it were perfectly fine, they must have just been dirty signals. Truck runs and drives fine, however the original issue for the o2 bank one, and the canister purge valve persists. Will tackle that down the line however. I'm just happy to have a working truck at the moment.