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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