need helpful tech help

randy88stang

New Member
Jul 11, 2020
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Port Jefferson, New York
so today is my first day on here. hopefully everyones thoughts will point me in the right direction. about 8 years ago i built an 87 stang and had it dyno tuned with an sct chip. it had alot done to it 342 stroker,vortech SC,full aeromotive fuel system and tank, a ford mass air computer and all the rest you would need . anyway car ran perfect. 2 years later i sold car but i removed everything from it. i only left the iac motor, the 02 sensors, the map sensor and the factory 88 wire harnesses in that car. fast forward to today i am struggling to recall everything i used to know about mustangs. medical issues thats why i have not got back to this. anyway i used a donor 89 mustang to put a different 88 mustang together using all wires from that car dont know if it was a auto or manual car had no engine or tranny when i bought car. i ordered new iac,02 sensors, used the 89 map sensor, 89 factory harnesses . set base timing for 15 degrees car starts and sounds good however after initial start when computer idles car down. i seems to go below 500 rpm and of course shut down( 1 to 2 min). i have not installed the dash yet but i plugged in everything i could find. there are a few things not plugged in, but they are to the left of steering column and dont think they have to do with engine management. my assumption is if did this right the tune should still be dead on. however i dont remember how the computer was programmed. i would assume the computer is not looking for egr,and emmisions ect. i would think the guy who tuned car would have turned all the regular stuff off. which is one of the things i do not recall. so i am not sure what should be plugged in and what does not need to be plugged in. i have not tried anything yet nor have i driven car yet. i hope just doing an idle reset will work. hopefully i have giving enough info for people to chime in. i am hopeful for lots of good suggestions to jog my memory any help would be appreciated thanks in advanced
 
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jrichker

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Dump codes sticky

Look at the top of the 5.0 Tech forum where the sticky threads are posted. One of them is how to dump the computer codes. Codes may be present even if the CEL (Check Engine Light) isn’t on. You don’t need a code reader or scanner – all you need is a paper clip, or if your lady friend has a hair pin, that will do the job.
I highly suggest that you read it and follow the instructions to dump the codes. http://www.stangnet.com/mustang-forums/threads/how-to-pull-codes-from-eec4.889006/
 

randy88stang

New Member
Jul 11, 2020
2
0
1
48
Port Jefferson, New York
Dump codes sticky

Look at the top of the 5.0 Tech forum where the sticky threads are posted. One of them is how to dump the computer codes. Codes may be present even if the CEL (Check Engine Light) isn’t on. You don’t need a code reader or scanner – all you need is a paper clip, or if your lady friend has a hair pin, that will do the job.
I highly suggest that you read it and follow the instructions to dump the codes. http://www.stangnet.com/mustang-forums/threads/how-to-pull-codes-from-eec4.889006/
I would but that is a new problem. I bought that code reader but it is not reading any codes. Nor does the check engine light come on in the dashboard. I will try a test lite but i think something may not be hooked up or grounded.
 

jrichker

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I would but that is a new problem. I bought that code reader but it is not reading any codes. Nor does the check engine light come on in the dashboard. I will try a test lite but i think something may not be hooked up or grounded.

Computer will not go into diagnostic mode on 86-90 models 5.0 Mustangs .

Revised 23 May 2020 to include warning about jumpering the dark brown connector with a black/orange wire.(12 volts) to the test connector.

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 any signal ground on the engine mounted fuel injector 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
63347.gif


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.

WARNING!!! There is a single dark brown connector with a black/orange wire. It is the 12 volt power to the under the hood light. Do not jumper it to the computer test connector. If you do, you will damage the computer.

OR

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 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 than 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 than that and the computer’s internal ground has failed, and the computer needs to be repaired or replaced.

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.

See https://www.stangnet.com/mustang-forums/threads/computer-issue.749974/#post-7490537 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 than 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.
eec-iv-computer-connector-for-5-0-mustang-gif.gif


a9x-series-computer-connector-wire-side-view-gif.gif


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.

88-91_5.0_EEC_Wiring_Diagram.gif


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

See the graphic for the 10 pin connector circuit layout.
?temp_hash=3ef2497fff29a7a9daee955cf93e5805.jpg
 
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