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Late 80's Ford Mustang GT 5.0 with a T-5 tranny
Posted to Technical Discussion Forum on 7/31/2023 17 Replies

Hello Iatn folks,

I am currently working a 1989 Ford Mustang GT hatchback project with a 5 speed T-5 transmission. The 2 previous owners stated they were told it needed an ECM because no scan equipment would communicate. Car runs rough and the idle is hunting up and down. All my additional testing led back to the ECM. So, I purchase the project from the retired recent owner and decided to take a trip down "memory lane". I almost forgot how much of a headache these could be.

At first thought, I figured I could find an ECM for cheap, and they're all the same. Boy, was I wrong. This car (I learned) has the reputable "A9L" computer. This computer is a manual transmission only computer and used only in the V8 sedan and hatchback models. They are hard to find unless a person wants to risk $700.00 plus for a used unknown takeout on Ebay. (Several looked like they were beat up and run over, junk. No thanks)

This is not my first rodeo with early Ford computers, so I opened it up to inspect. It had the 3 typical electrolytic "blue" capacitors leaking slightly so I decided to take it to my local electronics & TV repair guy who is equipped to replace them and that worked out fine. The car now ran much better.

Wait, the story goes on, I still had no communications with scan equipment and every time I connected a scanner the engine would raise the RPM to around 2500 RPM. My older MODIS scanner did not even have to be turned on for this problem to happen.

I went online and searched the Fox Body Mustang owners' sites and found a couple of pin tests to run. Fortunately, I have an old Thexton Ford 60 pin breakout box so, I could easily do a pin out test. On this computer pins 60 and 40 are incoming grounds and pin 46 is an internal ground that goes back out to many of the engine sensors. Testing proved I had a problem with pin 46 and no continuity when measured across to pin 60 or 40.

I pulled the PCM and studied the PCB tracers and found a very tiny burn through about the size of a pin head. I cleaned the board tracer and tested across the break to verify it was indeed an open. Well back to my electronic repair guy I went. They were able to solder in a very small jumper wire in and repair the issue.

Two days later back in the car the computer goes. Wow! now I can now communicate KOEO, but that is all. It would not run a KOER with my scanner so just for kicks I borrowed another old OBD 1 scanner from a buddy of mine, and I had the same results.

Now comes the interesting part of this drawn-out search. The T-5 transmission has a neutral position switch in the top cover. I lifted the car and found the switch housing is there, but the wire leads and pins are long gone. I have to get the switch assembly and replace it and then I can connect to the underside vehicle harness This switch connects to pin 46 and 30 in the ECM to let the computer know the transmission is not in gear during testing.

This vehicle was designed with that special tranny switch to let the computer know the tranny is not in gear. If it does not see the switch operate, the computer will not allow KOER testing. As it turns out the computer is looking for the clutch pedal safety switch and the neutral switch at the same time in series.

No, a cannot simply jumper the harness connector because that will keep the computer in test mode all the time a create drivability issues. I could use a jumper for testing only to get KOER info and all passed.

Yes, this car is older and not as common anymore except to Fox Body collectors, but this goes back to when engineers were just getting started with complex computer systems. It was not until 1995 that actual live data could be read using the newly implemented OBD-2 systems on Fords.

Most vehicles prior this age had to be manually tested with a DVOM and diagnosed by proving out a bad component before just throwing parts at them. These computers can be considered primitive to what we have today. Modern techs are lucky in some respects but now overwhelmed by modern CAN systems that can shut a vehicle's systems down.

Glenn A. Hunt
Consultant/Owner
Automotive Doc's
Devine, Texas, USA

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