89 Gt 5.0 Not Communicating W/ Code Reader

Skyler Wilson

Mar 8, 2014
I bought this GT from a friend that installed an aftermarket intake manifold, throttle body, tranny, oil pump & cam among other things. It was supercharged at some point and he removed it before i got it. When i bought it the AC compressor/pump was bypassed, tons of electric connectors that don't go to anything are scattered around under the hood that i know nothing about, vacuum lines are plugged in random spots such as the main line for AC and B/R aswell as plugged on the throttle bodies 90 Degree nipple that sits before the throttle plate and the nipple on the oil filler is connected to nothing nor plugged. Anyways, i've tried using a newer code reader to diagnose a check engine light awhile back and had no luck as far as gettin it to even register that it was connected to the car. I bought one of those cheap 30 dollar ford OBD1 reader and had the same problem. Anyone know what could cause no communication between a reader and the car? Or a checklist that i can run through of things to look through?
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Mar 10, 2000
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Computer will not go into diagnostic mode on 86-90 model 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 anyplace 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 that 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 that 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 that 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 oldfuelinjection.com

The following is a view from the wiring 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.
Diagram courtesy of Tmoss & Stang&2birds

See the graphic for the 10 pin connector circuit layout.

PM me your email address and I will send you a complete Ford Factory 89 Mustang electrical diagram set. The zip file is 2.5 MB and is too big to fit through Stangnet's email gateway.
89 Mustang wiring diagrams zip package–

They are in a zip file format to reduce the size of the package. If you don't have Windows 7 or Windows 8, you’ll need WinZip or other Windows archive tool to extract them from the zip file.. See www.majorgeeks.com - Download Freeware and Shareware Computer Utilities for a free download.
The diagrams show the location area and the connectors are drawn to the same shape as those in the car.
You will need the Adobe Acrobat viewer which is also a free download – www.adobe.com/products/acrobat/readstep2.html - Adobe Reader download - All versions

Diagram courtesy of Tmoss & Stang&2birds

Vacuum diagram 89-93 Mustangs

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.

Complete computer, actuator & sensor wiring diagram for 91-93 Mass Air Mustangs

Complete computer, actuator & sensor wiring diagram for 88-91 Mass Air Mustangs

Ignition switch wiring

Fuel, alternator, A/C and ignition wiring

O2 sensor wiring harness

Vacuum diagram 89-93 Mustangs

HVAC vacuum diagram

TFI module differences & pin out

Fuse box layout

87-92 power window wiring
http://www.veryuseful.com/mustang/tech/engine/images/mustang87-92 PowerWindowWiring.gif

93 power window wiring


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