Electrical a9p dead pin 33 & 46

Good evening fellow fox... people.

I haven't been in the game in quite a while and Ive run into an issue...

Purchased a 89 with a "331" that runs, and drives. and that's about it. Also, vin number shows it to be a gt cobra, which whatever.
It appears to have
- gt40 heads
- cam unknown, but has crane rockers on top.
- intake- cheaper variety
-MSD ignition throughout
-70mm TB
-55mmMAF(I believe it is too small and swapped upon sale)
-rear end unknown
-fuel pump replaced and on a switch
-aftrmarket rad
-elec fan
- PURPLE injectors(24#??)
-no o2 harness and sensors are in with wires cut off

anyhow throttles high, checked timing, without spout, at 12d, pulled iac and checked tps and showed 5-6v on all wires. head scratcher. proceeded to check wire path from 46 to tps and ohm'd out fine. checked pins on a9p and couldn't get a read. now this was a week ago, and have since been online here and other places to get a better grasp on this situation.

pulled open the brain and lo and behold, 46 was burned. figured that was all, then looked across and noticed 33 is as well. I would assume 46 is a somewhat simple fix for someone with knowledge. I can solder wires together but this is out of my wheelhouse.

Question of my day would be concerning 33. I DL'd the cool diagram of the pins and what they're for etc etc, I can't see that 33 deals with anything other than the egr, but I know it is a factor in operation.

What would options be with 33 and is it necessary for that circuit to be 100%.

As for everything else with the car, that'll be a topic for another day.

Who here has any insight on this problem?

Thanks in advance
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EcuExchange.com that's the best advise I can give. They have been great with a few members ECUs. Not expensive and a quick turn around. They will rebuild your computer. These computers are so old that they really need certain things replaced. The capacitors are also way past their life span.
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My 89 convertible also was a GT Cobra. Come to find out, all the GTs are called Cobras in Canada. I wonder what they call the 93 Cobra ?

More than likely the previous owner noticed an issue with the wiring of the O2 sensors. There's a different harness for 5 speed or automatic. They can be repinned for either depending on the computer. You just need an O2 sensor harness for any 89 to 93. The jumper at the connector needs to be moved depending on your computer. I believe LMR sells new harnesses. They are listed as automatic so jumper will have to be unpinned and moved.

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An A9P is am auto trans computer and it will work OK with a standard trans
However, the internal wiring is different, so don't be too surprised that you get different resistance readings
when you compare them side to side

Computer will not go into diagnostic mode on 91-95 model 5.0 Mustangs

Revised Dec 23 2017
1.) To clarify signal ground connections on the engine mounted fuel injector wiring harness and add diagram for the engine mounted fuel injector wiring harness
2.) To add warning about using an automatic transmission O2 sensor wiring harness with a A9L manual shift transmission computer.

How it is supposed to work:
The grey/red 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. As long as you are successful dumping the codes by using the gray/red wire on the diagnostic connector for the ground when dumping, the computer’s internal ground on pin 46 is good.

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 ohm when measured from anyplace on the engine harness with the battery pigtail ground as the other reference point for the ohmmeter probe.

Engine mounted fuel injector wiring harness sensors for a 5.0 mustang

What sometimes happens is that the test connector grey/red 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.


If an O2 sensor harness from an automatic transmission Mustang is used with an A9L manual shift transmission computer. The 12 volts from the automatic transmission starter circuit will damage the A9L computer.
The A9P is the computer for automatic transmission cars. and has different start circuit wiring. It will work Ok with a standard transmission, but it may have a hanging idle when the car slows to a stop,

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 (grey/red wire) on the self- test connector and battery ground. You should see less than 1 ohm.


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 grey/red wire and pin 46 on the computer wiring connector: it should be less than 1 ohm. More than 1 ohm is a wiring problem. If it reads 1 ohm 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 ohm. More than that and the computer’s internal ground has failed, and the computer needs to be repaired or replaced.

See image below for help finding the burnt signal ground trace.

The fix is some careful soldering of a small jumper wire across the burnt section of copper trace.

While you have the computer connector disconnected from the computer, turn the ignition switch to the Start position and look for 12 volts on pin 46 of the computer wiring harness. If you see 12 volts then you have an automatic transmission O2 sensor harness. That will damage the A9L manual shift transmission computer.

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 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.


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/ Everyone should bookmark this site.[/b]

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

91-93 5.0 Mustangs

Complete computer, actuator & sensor wiring diagram for 94-95 Mass Air Mustangs

See the graphic for the 10 pin connector circuit layout.
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