Electrical Aod-t5, Speed Density To Mass Air, 02 Harness Differences, Etc


That is…until I whipped out my Bissell
Mod Dude
Feb 18, 2001
In doing research for an upcoming SD to Mass Air swap, I've come across some info that suggests there may be a few extra steps that are rarely mentioned. At first I thought I was overthinking this, but I've come to wonder just how many cars out there that have done a Mass Air swap are not technically wired correctly. Maybe this will trigger some new discussion on how a traditional mass air swap is performed vs how it should be done. If anything, I just wanted to get what I discovered down in writing somewhere for future reference.

Guys with mass air swapped 1987's....pay close attention

First off, I want to apologize to the 1986 cars. I didn't do enough research on the quirks of this model year, but I feel it may be similar to 1987. I'm still doing research

What I've discovered is that the 1987 engine harness is completely lacking the proper wiring for a MA conversion. This issue is located in the O2 harness. If you look at a 1987 O2 harness, it lacks the jumper wire that's frequently brought up in O2 harness discussions. No big deal, just add the jumper or change the harness? Well, if you take a peek at the engine side of the harness, it does not have all 8 wires that the 1988+ harness would have. 1988 is when ford added MA to the cali cars, and they prewired the engine harness for them no matter what EEC was present. This wiring is completely lacking in the 1987 cars as mass air was not used at all this year.

So what does this mean? Not quite sure yet. The o2 harness only contains a jumper that links two of three wires added to the 1988+ harness when ford added mass air to the mustang (in Cali). Those wires are related to the NSS and indicating to the eec that there is no load. Symptoms may include hanging idle.

From what I've seen, there are 4 O2 harnesses:

1987 only harness for both 5spd and AOD (no jumper at all)
1988-1990 5-spd harness (jumper from pin 1 to pin 5)
91-93 5-spd harness (jumper from pin 1 to pin 6)
88-93 AOD harness (jumper from pin 5 to pin 6)

Now discussion about o2 harness differences is nothing new. Big thread on another popular Mustang website about checking for voltage at pin 30 and 46 and swapping the jumper if needed, BUT it appears 1987 didn't even have this option. So I wonder how many mass air swapped 1987's are out there that may not be functioning 100% correctly?

Here's a 1987 O2 sensor harness. Notice..only 5 wires. While I don't have a pic of the body side harness, it will only have 5 wires as well, while the 1988+ version will have all 8.

Now, to change gears so to speak, ever since I converted my car to a 5-spd, I never changed any of the harnesses or computer. Still running an Automatic SD computer, AOD o2 sensor harness and AOD trans harness. What I notice however is that the idle logic does change depending on if I push the clutch in or not. The clutch switch, and the NSS on the t-5 are wired in parallel. Now on a 5-spd this generates the code 67 to tell the car to dump codes. When running an AOD computer however, it acts a little differently. With clutch in, idle goes up slightly. However, if I let the clutch out, even with T5 in neutral, the idle drops a tad. This is pretty apparent on my car. Why? Because the AOD transmission harness does not have a plug for the NSS of the T-5 in it's harness. If you follow the harness back to the pass kick panel, the pins for the NSS are missing on the AOD harness. As a result, the EEC never sees that the t-5 is in neutral. For the AOD's, this logic might be slightly different than on the 5-spd computer?

What to do? Well, you could jump the pin out in the driver's kick panel and have the car "forever in neutral" idle quality will go up, but since I like things to be as intended by the factory, my solution was to pick up a 5-spd trans harness and will be installing that. The trans harness is not that 1 foot harness that's commonly claimed to be all that's needed for a 5-spd swap, but instead a 6-7 foot long harness that runs from driver's kick panel (8-pin, square plug) to the trans hump, and has three plugs on it (NSS, VSS, and backup harness). This will bring NSS wiring back into the scope of what the EEC can see. Now, with clutch in and/or T-5 in neutral, idle quality will be better.

That brings us to another interesting tidbit of info. The vehicle speed sensor. All Mass air cars have this sensor, but during the speed density years, it was only installed on cars with cruise control. 1986-1987 cars don't even have the wiring if they didn't have it. The trans harness doesn't have the wiring at all. Ford started adding it in 1988 when MA was installed on the CA cars. So once again, 1987 cars (without cruise control) need to do a bit of extra work to really make MA operate as intended. They need to add the VSS (and compatible speedo cable) as well as run the wiring down to the sensor on the trans, or change out the trans harness and tap into the VSS wires at the driver's side kick panel.

None of this is mentioned in the usually talks about Mass Air swaps. It's always add the 4 wires to the MAF, swap some pins around, and (optional) add two wires to the VSS and one to the FP relay and you are done. It may be a bit more than that, especially if you have a 1987 car. You may need to get yourself a 1988-1989 O2 and engine harness to properly do the swap.

And also, in my opinion, guys doing AOD to t5 swaps should change out that trans harness to get the NSS wiring, or at least jump it out in the driver's kick panel. You can still drive around fine, but you may notice your idle quality is inconsistent.

EDIT: Just discovered that early 1988 models may also be lacking the proper mass air wiring, and have the same harness problem as 1987's
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Adding more info.

The transmission harness. This 8-pin connector is located in the drivers kick panel. Main difference is the AOD harness lacks the NSS wires. You can see this in the side by side


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I have a 87 that I have done both a Mass Air swap and a AOD to Tremec 5 speed swap. I have done all the 5 speed trans harness swaps you have mentioned, but the factory O2 Harness remains in place. My question is, if my 02's were not functioning properly wouldn't there be a code present for it??
That's sort of what I'm trying to figure out. Kinda why I bring it up in this thread.

One gentleman on another forum claimed that he was unable to really tune the car, until he replaced the O2 and engine harness on his 87 with 88+ harnesses. After that, he was able to tune the car better and made more HP on the dyno.

Here's the harness on my '88. This connector is located under the MAF right next to the AC WOT relay. You can see the purple jumper loop in my 02 harness at the bottom, as well as 8 wires in the engine side harness. If oyu look at your harness, if I'm correct, you will see no loop at all in the O2 harness, and only 5 wires in the harness on the engine side. Starting in 1988, there are 8 wires in this plug


I have an '88 EVTM I've been trying to study since it lists the wiring for both MA and SD cars. Trying to see what the MA computer does with those extra 3 wires in the harness. The o2 harness only contains the jumper between the three wires on the engine side of the harness. So ford routed the wiring to the harness starting in 1988, and then jumpered them differently based on trans. 1987 is missing this completely
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