This is my first post here. This looks like a big group so I'm sure there are a couple experts around. Thanks for any help you guys can provide.

'88 F150 5.0L 5 Speed Lariat. This is a project truck for me. I bought this truck for my nephew 3 years ago but he never learned how to drive stick. I like the truck and decided to buy it back. It had been sitting for a while so I've been working out quite a few issues. I'm currently sorting out a few electrical issues.

I had a code 42 (O2s Switching not detected). As I traced the circuit I found that the grey/yellow and green/purple wires from the O2 sensor led DIRECTLY TO THE BATTERY NEGATIVE POST lol; it's the one circled in yellow. Obviously, this is wrong.

Here's where it gets weird, the connectors on the harness circled in red are EXACTLY THE SAME as both the O2 sensor connect and the harness that leads up from the O2 sensor to the negative battery terminal. These two harnesses can be swapped and plugged into each other. On the harness circled in red, one side of the connections has the same wiring colors as the O2 sensor. If I swap that harness with the one that was connected to the battery negative terminal, both sides of this harness would now have the same wire colors. Seems like it's where it should be. But that leaves one connector open. The other side of the harness circled in red has black/white black/dark green. I can't find this color combo anywhere on the wiring diagram.

So I connected the to harnesses that matched and left the other one open. Went to turn the truck on and didn't here the fuel pump prime. Tried to do a KOEO test but it would not initiate.

For the harness that was left open, I checked the side that was originally plugged into that harness to see if any were grounds. The wire that would have connected to the black/dark green, which was now sitting open, lead to ground previously. So I ran a jumper from this pin to ground, turned the key, the pump primed, and I ran a KOER test. NO MORE CODE 42!!!

I know this is hard to follow, sorry guys. Stay with me, almost there lol

So as it sits now, the two harnesses have been swapped and everything seems to be working right. However, I still don't know what the black/white wire on the forward side of the harness circled in red goes to. I've tried to trace it to the wires leading to the ECU but I get nothing.

 

The O2 sensors are likely showing a symptom of the original problem. Drive it a few days, I will be surprised if the code doesn't show up again. Let us know.

The information I have with my manual, the three O2 sensor conductors should be connected, one to power ground, the second to pin 29 (HEGO SIGNAL) on the 60 pin ECU connector and the third to the ignition run circuit (12v)..From the ECU, pin 49 on the ECU 60p is the HEGO ground which should go to a lug on the chassis nearby.

 

I didn't drive it today and am going to far out of town tomorrow to take it with me. Will probably be Wednesday.

The code 42 has come up in every KOER test of done since I took possession of the truck about a month ago. As soon as I made this change, next KOER, code is gone. I had recently replaced the O2 sensor so it's in good shape as well. I think I solved the O2 sensor problem, if the code reappears then I'm wrong.

The question is what is the black wire with white stripe that is now left disconnected? What is the black wire with dark green that is now to ground?

 

See my post, I added to it. I cannot determine wire colors from diagram in my manual.

 

I reread your post. In regards to the origin of the O2 sensor wire, my manual says the same. I need to go back and see if the connector that now has the O2 sensor circuit does indeed end up at pin 29/ignition switch/ground. If they O2 sensor is indeed working, then I expect that the will. Pin 29 did have the correct color (green/purple) but I did not check the ignition switch for the grey/yellow.

So there are 2 circuits with identical connectors (O2 sensor connectors) close enough to each other that they can be swapped. The wiring looks mostly unmolested and factory. Is this typical on a Ford?

 

Usually adjacent connectors and harness lengths have some differences where they cannot be connected to the wrong point. I don't have the 5.0 in my truck and there is only one harness anywhere near the o2 sensor on my 4.9..

 

These harnesses aren't near the O2 sensor. There is a connector at the O2 then the circuit continues to another identical set of connectors by the battery (with ANOTHER set of identical connectors 12" away on a different circuit).

 

I know on my truck, there are some unused (few) connectors scattered in the engine compartment which hang open. They were like that from day one.I never was concerned about them. I assume they are for factory testing or Ford uses the same harness for every truck and depending on vehicle equipment, the connectors are used as needed.

 

That could be it. I'm still trying to get it running right. Just chasing down anything that might need attention. Idle is erratic and I have periodic losses of power. The O2 sensor could fix the loss of power part. I finished late last night and didn't even get it on the road. Will do it this evening. Hoping to see some improvement. I'm also going ahead and changing the IAC since it's not very expensive and known to fail.

This truck is a learning platform for building a car my way from scratch. I plan on doing a MAF conversion and roller block but I need to have all the bugs sorted out before I even think about that. I have a lot of parts from my '90 Mustang that had a 408 just sitting around. Including a Tremec TKO600 (haven't researched if that can fit yet).

 

So the O2 sensor circuit rewire made the truck run much better. After about 40 miles of driving, no CEL, no O2 sensor code.

So it was on to the erratic idle. Last night, I determined that it was caused by a bad wire in the power steering pressure switch circuit. I replaced the sensor and nothing changed. Traced the wires and found the one going to the diagnostic port was open. Ran a jumper and it idled perfect. Wired in a replacement wire and this truck now runs damn near like a new truck!!!

The bad wire was a black/white stripe and according to my diagram should be tied in with the mystery wire I was trying to figure out. However, now I have no codes (other than emissions equipment) and the truck runs great. On to the next issues, Brakes pulling to one side and slop in the steering.