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91 Mustang GT O2s

posted May 14, 2016 11:59:38 by Noah
I have a 91 5.0 5spd mustang I got as a basket case, and I've just been putting it together as parts come my way. I've driven it a few weekends without the O2s hooked up, and it runs fine, kinda rich smelling, and the cel comes on after driving a bit, but no big deal.
So I'm digging through my Ford Hoard, and I find an O2 sensor harness. I cleaned it up, disconnected the battery and plug it in. The car starts right up, immediate cel, and wants to idle at 2500rpm. So I shut it down, loose the harness, now when I start it, it finds its idle but has the cel on.
I remember something from when I was making a 5.0 5spd Ranger that if you use an automatic PCM in a 5spd application, the 02s are wired different and it spells imminent doom for the PCM. I was checking diagrams last night, and no diagram makes any reference to a 5spd or automatic specific 02 wiring. Anyone else ever heard of this? I'm bringing my pre OBD connectors to check codes today
Massachusetts, USA
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12 replies
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Noah said May 15, 2016 13:53:32
With the clutch depressed, idle returns to normal and the cel goes out. I'm thinking this is the wrong harness, but I usually don't hold into automatic parts. Can't imagine where this could have come from lol!
Massachusetts, USA
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Tyler said May 15, 2016 17:01:19
So I'm browsing the wiring diagrams, and I've got NO idea how the O2's factor into this so far, lol. BUT, I do see the clutch switch as an input in to the "Speed Control Amplifier", which I think is Ford-ese for a cruise control module. The automatic version uses a jumper connector (according to this diagram, anyway). Long shot, but you could try disconnecting the amplifier or the cruise servo, see what happens.

I've heard of similar problems with O2's on Dodge products, but not on a Ford. Any chance of getting flash codes out of it? If the OBD adapters don't want to play nice, that is.
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Noah said May 15, 2016 17:40:56
How'd you know the scanner wasn't going to play nice? 😊
The koeo test would run, but at the end would just dump me back to the main menu. Didn't say anything like no codes found or anything. The koer test made it up to a point where it said code 10: snap throttle, so I did just that, and no response...
I just got back from a friend's house so the whole back of my Navigator looks like a Mustang hospital. I need drop this stuff off at the shop, I'll see if I can get flash codes, didn't even think of that honestly
Massachusetts, USA
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Tyler said May 16, 2016 00:28:22
How'd you know the scanner wasn't going to play nice? 😊


I SABOTAGED IT, MUAHAHAHAHAHAHA

Lol, I dunno, just a guess based on the basket case nature of the vehicle. That really sucks that it didn't work right, 'cause I kinda liked the old Ford KOEO/KOER tests.

I just got back from a friend's house so the whole back of my Navigator looks like a Mustang hospital. I need drop this stuff off at the shop, I'll see if I can get flash codes, didn't even think of that honestly


EMT = Emergency Mustang Treatment? I'm not mean spirited enough to come up with Ford jokes on my own.

Man, I *think* you can get flash codes out of those? But I've never had to do it myself. I'm sorry if I mislead you!
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Noah said May 16, 2016 01:35:28
You're supposed to be able to but I couldn't get any. Oh well, still does donuts just fine.
Massachusetts, USA
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Noah said May 16, 2016 12:27:29
The first pic I found online.
The second one is my car. The pigtail on the right is the one I found and installed and caused the high idle, the one on the left I got from a friend. The car has a proper high cold idle and settles down into what I guess it's closed loop. Still got cel.

[Last edited May 16, 2016 20:33:58]
Massachusetts, USA
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Noah said Jul 04, 2016 20:20:39
Fixed it today. The one with the blue jumper from top to bottom is for a maf 5spd car, but I already had the one with the purple jumper connected and burned the 02 signal return on the board in the computer.
So here's my "Jenky" repair for the week.

Maybe I should add this to the "don't try this at home" thread.

No more immediate cel, nice low idle in closed loop now.
[Last edited Jul 05, 2016 03:11:58]
Massachusetts, USA
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Noah said Jul 04, 2016 20:23:04
I bet I could even pull codes from it now...
Massachusetts, USA
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Tyler said Jul 05, 2016 00:23:59
Lol, jenky! I dunno man, I've heard of electrical engineers fixing their design flaws by having the PCB manufacturer just add jumper wires to completed boards.

Congratulations on the fix, man, jenky or not. I think it's pretty cool that you opened up the PCM and added the jumper directly. Based on the images you posted, I'm guessing you're not the only one that encountered wiring differences?
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Noah said Jul 05, 2016 00:33:09
It's a common problem for people going from auto to manual, or 4 cylinder to v8. One you wouldn't know about if you weren't right in the middle of a build or swap. There's no diagram or pin out from the manufacturer of the 02 sub harness
Big credit to corral, a mustang forum. The edited pic is one I found through Google. The other is of me holding the two connectors I had. It took some digging, but I finally found a post where someone showed all 4 different 02 sub harnesses and what cars they fit. All the PCMs have the same pin out, and all the maf efi engine harnesses are the same, but that 02 sub harness will wreck your day. Not to mention, these PCMs are getting hard to find! I have 2, but wasn't ready to cook the second one until I knew I had the right sub harness installed
[Last edited Jul 05, 2016 00:36:19]
Massachusetts, USA
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Noah said Jul 05, 2016 00:41:20

Massachusetts, USA
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Noah said Jul 05, 2016 17:17:50
So after driving the mustang for the long weekend, the navigator got jealous. She was a crank no start this morning! Found it pretty quick, no rpm on the tach, same on the scanner. Went under the truck, worked the crank sensor harness and she fired right up. Looks like I'll be dealing with that this weekend.
Massachusetts, USA
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