90 GT project


New Member
Aug 13, 2019
Hi Neighbors

Just newly Became a member and I excited to meet and talk to you all about my project. It’s a black 90 GT convertible automatic and she needs work. I’m very interested in what experience you all have with this or similar cars.

First and foremost I want a world class T5 to put in there so we be doin some component swapping but it’s the small things I need help with. Can I simply swap a automatic A9P eec with a manual A9l eec and keep the same wire harness that’s In there for a automatic? What needs to be added or subtracted to the harness?

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You have a nice rear end there Dave.
15 Year Member
Nov 29, 1999
Mililani, Hawaii
If you are swapping an AOD for a T5 you do not need to do anything with the EEC. The EEC is specific for the harness that it was installed with from the factory. A9L and A9P have different o2 sensor wiring so do not swap your EEC. The EEC has nothing to do with the trans in a foxbody as it controls nothing as far as trans goes. So, short answer, leave your exisitng EEC in place and just swap the trans. I have done it several times.


StangNet's favorite TOOL
SN Certified Technician
Mar 10, 2000
Dublin GA
Welcome to Stangnet... :spot:

Not all of us know how to fix everything, but some of us know how to fix some things with excellence!

Places to check out here on Stangnet:
http://www.stangnet.com/mustang-forums/threads/technical-thread-how-to-index.808661/ How to do it tips for some of the most common problems and upgrades for 5.0 Fox body Mustangs.
http://www.stangnet.com/mustang-forums/threads/the-official-progress-threads-thread.761371/ the collection of build/progress threads from Stangnet members. You get to find ideas and clues to what works well and what doesn’t.
http://www.stangnet.com/mustang-forums/resources/ Has tech tips for common problems on Mustangs.

If you are in California or some other area with strict emissions laws and buy a car, part of the deal should be that the owner takes the car and has it emissions tested. If it passes, you buy the car and pay for the emissions test cost, if any. If it does not pass, walk away from the deal. For a mechanically inexperienced owner, emissions related problems can be difficult and expensive to fix. Avoid any car that was originally EFI and has been converted to carb like the plague. That's often a clue to the fact the previous owner had problems and the resulting hack job has MORE problems.

Keep in mind that areas with strict emissions laws may make the go faster goals more difficult to reach. Some parts are OK to replace with aftermarket parts and others aren't. Check carefully before you get out your wallet and buy something you can't use in the area where you live.

If you want to do the fix up & power up thing, make sure that you have some other form of reliable daily driver. That way the stang can sit while your wallet and hands take a rest from the last project that didn't quite get finished on that 3 day weekend. Things always cost more and take longer the first time you do them. Having some other working vehicle makes life easier since it isn't the big crush to get it running for the Monday morning drive to work or class.

Plan on spending some money on tools it you don't already have them. The stang has both metric and American fasteners, so you really need two sets of wrenches. A timing light, digital voltmeter, vacuum gauge, compression tester, fuel pressure test gauge and fuel line coupler tools are some of the test & tuning tools you'll need. Visit the pawn shops; sometimes you can find deals on tools & test equipment if you stick to well known name brands.

A sheltered work area is almost a must, someplace that you can leave the car in pieces without upsetting anyone. Some guys here have changed a transmission in the parking lot of their apartment in a rainstorm, but they will tell you it wasn't fun. Depending on where you live, a warm dry garage is a nice place to do the winter projects that stangs tend to become.

EFI (Electronic Fuel Injection) Computer - every stang after 85 has or had one: don't let it intimidate you. The computer based EFI systems are not hard to fix and most of the time they tell you what's wrong with the engine. Here's a book that will get you started with how the Ford electronic engine control or "computer" works.

Ford Fuel Injection & Electronic Engine Control 1988-1993 by James Probst: ISBN 0-8376-0301-3.

It's about $35-$45 from Borders.com see http://www.amazon.com/ . Select books and then select search. Use the ISBN number (without dashes or spaces) to do a search

Use the ISBN number and your local library can get you a loaner copy for free. Only thing is you are limited to keeping the book for two weeks. It is very good, and I found it to be very helpful.

For lots of great ideas and tech notes on upgrades and repairs, be sure to check
out http://forums.stangnet.com/showthread.php?t=643651 “Useful Technical Thread Index” sticky at the top of the 5.0 Tech forum.

Things that break often:
T5 5 speed manual transmission (syncros go bad, mostly 3rd gear)
TFI Module (Thin Film Ignition module - mounts on the distributor)
Electric door locks (see the link in my sig for a cheap & easy fix)
O2 sensors(oxygen sensors) They are good for about 60,000 miles and start to go downhill after that.
TPS Sensor (Throttle Position Sensor) causes flaky problems with idle & acceleration.
Fog lights -they overheat the wiring and cause the headlights to flicker. The fix is cheap and simple if you can do electrical stuff.
Harmonic Balancer – they separate between the hub and outer ring. A harmonic balancer puller is a must have to change it. You can rent or borrow a puller from most of the larger auto parts stores.

Things that are very durable: Engine - as long as it hasn't been abused, it will run good for 150,000-200,000 miles without an overhaul
Rear axle - other than an occasional case of worn clutches in the traction lock, they almost never have problems.
Computer - believe it or not, the computers seldom have problems of their own. Most of the problems are with the sensors and the wiring.
Suspension – the front and rear suspension has very few problems if the car hasn’t been wrecked or seen a lot of drag strip runs. The drag strip runs tend to distort and tear the mount points for the rear axle control arms. Revving the engine up to 4000 RPM and dumping the clutch with slicks or drag radials tends to break things.

Things that don’t break often but are hard to fix:
Water pump mount bolts – they corrode and shear off when you try to change the water pump.
Rear oil seal on the engine – lots of parts to remove to get to a $20 seal.
Power steering pump – the pumps are noisy and the pulley requires some special tools to remove and install. If you have the tools, they are easy to do. Again, the larger auto parts stores will rent or loan the tools for the pulley.
Power Steering rack - it is hard to get the toe in set so that you can drive the car to the shop to get it properly aligned.
Starter – the top bolt is hard to get a socket on if you don’t have the right combination of socket, universal joint and extensions.

Everything considered, 5.0 Mustangs are not hard to work on. They just require some patience and thought before you get started.


i'm familiar with penetration
Mod Dude
Feb 18, 2001
Just to add onto the above post. If you do decide to change from automatic to 5spd ECU, there are a few wiring changes you need to make.

You need to swap out the trans harness

And you need to repin the o2 sensor harness, which is very easy to do