87 T-bird W/92 5.0 Swap High Idle


Sep 25, 2017
Hello All,

New here. Ill do my best to explain the situation.

Bought a 1987 Thunderbird for my brother. The car was originally a 5.0 non-HO. The previous owner swapped in a 5.0 HO out of a 1992 fox body and a AOD trans. He also swapped in the A9P eec without doing any harness changes. The previous owner never did the MAF conversion to the 87 harness and could not get the car to run right.

So when I picked up the car, instead of wiring in the MAF, I decided to pick up a 1987 auto mustang EEC (DC model) in hopes it would match the speed density harness the car already has. The car started fine but has a high idle of 1200 rpm. If I disconnect the IAC the idle drops back to normal (750 rpm). So I replaced the IAC twice now with the same issue. I replaced the MAP, same issue. Ive adjusted the TPS. Did an idle reset procedure, when I kicked all the accessories on (head lights, radio, A/C) the car idled rough and quickly stalled out.

The car does not spit out any codes. Ive done the old school way with the test light and jumper wire and get nothing at all. With the key off I get a steady 1 light every few seconds. key on gives me nothing. With a snap-on scanner hooked up, its does not communicate with the car. Fiddling around in the engine bay I found some loose grounds. I tightened them and added an extra ground from the battery to the a/c bracket. Thinking the DC EEC is bad since it would not communicate with scanner or jumper wire, I had a local ECU remanufacturer test it out and it has tested GOOD.

Im lost at where I need to probe next. I need to get this car running asap.

Am I missing something in the harness. Is the EEC harness different from a 1987 tbird to a 1987 mustang? Do I need to change some pins around?

Please help. sorry for long post. Thanks in advanced
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Computer will not go into diagnostic mode on 86-90 models 5.0 Mustangs

Disconnect the battery positive terminal before making any resistance checks.
The voltage drop in the ground cable will cause incorrect resistance readings.

How it is supposed to work:
The black/white wire (pin 46) is signal ground for the computer. It provides a dedicated ground for the EGR, Baro, ACT, ECT, & TPS sensors as well as the ground to put the computer into self test mode. If this ground is bad, none of the sensors mentioned will work properly. That will severely affect the car's performance. You will have hard starting, low power and drivability problems. Since it is a dedicated ground, it passes through the computer on its way to the computer main power ground that terminates at the battery pigtail ground. It should read less than 1.5 ohms when measured from any place on the engine harness with the battery pigtail ground as the other reference point for the ohmmeter probe.

What sometimes happens is that the test connector black/white wire gets jumpered to power which either burns up the wiring or burns the trace off the pc board inside the computer. That trace connects pins 46 to pins 40 & 60.

The STI (Self Test Input) is jumpered to ground to put the computer into test mode. Jumpering it to power can produce unknown results, including damage to the computer. The ohm test simply verifies that there are no breaks in the wiring between the test connector and the computer input.

How to test the wiring:
With the power off, measure the resistance between the computer test ground (black/white wire) on the self test connector and battery ground. You should see less than 1.5 ohms.


If that check fails, remove the passenger side kick panel and disconnect the computer connector. There is a 10 MM bolt that holds it in place. Measure the resistance between the black/white wire and pin 46 on the computer wiring connector: it should be less than 1.5 ohms. More than 1.5 ohms is a wiring problem. If it reads 1.5 ohms or less, then the computer is suspect. On the computer, measure the resistance between pin 46 and pins 40 & 60: it should be less than 1.5 ohms. More than that and the computer’s internal ground has failed, and the computer needs to be repaired or replaced.

See http://www.stangnet.com/mustang-forums/749974-computer-issue.html#post7490537 for Joel5.0’s fix for the computer internal signal ground.

If the first ground check was good, there are other wires to check. Measure the resistance between the STI computer self test connector (red/white wire) and pin 48 on the computer main connector: it should be less than 1.5 ohms. More than 1.5 ohms is a wiring problem

The following is a view from the computer side of the computer wiring connector: it is for an A9L, A9P computer.


Diagram courtesy of Tmoss & Stang&2birds

Check out the diagram and notice all the places the black/white wire goes. Almost every sensor on the engine except the MAF is connected to it.


See the following website for some help from Tmoss (diagram designer) & Stang&2Birds
(website host) for help on 88-95 wiring http://www.veryuseful.com/mustang/tech/engine

See the graphic for the 10 pin connector circuit layout.
Well the eec tested good. The next plan is to test the ohms between the grounds from each connector to the harness.

I found a schematic for the vacuum plumbing and noticed vacuum lines are routed wrong in the car. So I'll fix that.

I will let yall know the results
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