Electrical Unable To Pull Cel Codes

IdaFox

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Jul 20, 2017
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Hello everybody,

I recently purchased a 1989 Mustang GT, and it has a check engine light. I don't have any history on the car, so I'm really starting at step one. I followed jrichker's post here on the process for checking the CEL codes but unfortunately the CEL light never flashes. It just remains solid.

This morning after some more Googling I discovered post #8 on this thread (https://www.stangnet.com/mustang-forums/threads/check-engine-light-on-after-eec-iv-switch.772570/). I removed my ECU and opened it up to discover that this trace was less than optimal. I installed a bodge wire which seems to have fixed something because now when I attempt to perform the code pull the vehicle recycles the fuel pump and relays after I turn the key on. This was not the case before.

I attempted to leave the negative battery terminal unhooked for approximately 30-60 minutes multiple times now to clear the CEL. However, this also didn't work. I'm not sure if there is another procedure.

This is my first OBD1 Ford; I'm usually running around in an OBD1 Honda so I'm more familiar with their acronyms and verbiage. So I'm not sure which sensors will trigger a CEL like this. I apologize if I'm doing something completely wrong and I appreciate everyones time and advice. Thank you in advance.

I have recorded a video since I believe it will further help troubleshoot the issue. You can view it here

 
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jrichker

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

attachment.php?attachmentid=58312&stc=1&d=1242744354.gif


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.

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 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.88243.gif


a9x-series-computer-connector-wire-side-view-gif.71316.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
 

IdaFox

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Jul 20, 2017
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I performed the tests as instructed. The first test from the black/white wire to ground showed 0.02ohms. The second test between black/white and pin 46 on the harness read 0ohms. The third test from pin 46 to pins 40 & 60 on the ECU read 0ohms and 0hms. Test four from grey self test (brown/brown) and pin 48 on the harness read 0ohms.

Test #1
View: https://youtu.be/RH5pEPX2Sfw


Test #2
View: https://youtu.be/afcKb63QWyk


Test #3
View: https://youtu.be/HagInjSXiOU


Test #4
View: https://youtu.be/ia3d82RpLEc



I attempted to pull the codes again and this was the result:
View: https://youtu.be/iQOLjzhezZ8


I performed the checks multiple times and confirmed my findings that I recorded. I'm not sure what else to look for or do. I also attempted to contact the previous owner and see if they ever investigated the CEL. No luck, but I was informed that they believed "it's because there is no mufflers or o2 sensors." Would that cause the computer to not pulse the codes? I would assume that it would still pulse the codes to inform me of a missing or malfunctioning O2 instead of remaining solid.
 

IdaFox

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Jul 20, 2017
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Oh really? I thought it would display just a 1 out to the side for infinite ohms. I've already performed the fix. I'll disassemble the ECU again and inspect my bodge wire at those points. If anything I'll attempt to resolder the connection. I'll reply shortly with my findings.
 

IdaFox

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Jul 20, 2017
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I removed the previous bodge wire and scraped the pins to verify they're clean. I then resoldered a new bodge wire in, and verified at the tip of the pin and the base of the pin.

I get anywhere from 0.4 to 0.6ohms when testing at the board or connector pins.

20269953_1785517074809125_1707880917_n[1].jpg

I'm pretty sure a lot of the testing that I did today was in vain because I believe I had the scale set incorrectly (20k :doh:). Unfortunately I'll have to wait until tomorrow night to verify my findings with the ECU in the car.
 
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IdaFox

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Jul 20, 2017
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Alright so on to testing phase two. I'm back with a new set of outcomes with the tests.

Test 1: Black/White to Ground - 0.6ohms
View: https://youtu.be/yTftxtSW8kg


Test 2: Black/White to Pin46 Harness Connector - 1ohms
View: https://youtu.be/vPfN2PKrgkM


Test 3: Grey Self Test (Brown/Brown) to Pin48 Harness Connector - 1.2ohms
View: https://youtu.be/fimqOKdWzEc


Test 4: ECU Test Pin46 to Pin60 & Pin40 - 1ohms & 1ohms
View: https://youtu.be/YbY6sBL8U1E


The final test was to see if I could pull codes since I made the internal ECU repair again and the previous tests seemed fine. Unfortunately that wasn't the case.
View: https://youtu.be/3Nv-6KBSkrQ


I've run the tests multiple times and verified the ohms readings. The only other thing that I can think is that there is a sensor somewhere that has failed that is preventing the ECU from outputting codes. I unfortunately don't have access to another EEC4 and they're quite expensive to purchase.
 

IdaFox

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Jul 20, 2017
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@jrichker do you by chance have anyy other ideas that could possibly assist me? I'm going to see if I can borrow a nicer DMM to verify my results, but from what I'm seeing here everything is/should be fine.
 

IdaFox

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Jul 20, 2017
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More information to report. I purchased an Innova Ford OBD1 Scanner and it also failed to pull the codes. I don't believe it to be an issue with the wiring harness, or the repair made to the ECU as the relays all click as they should. I have also verified the wiring and ohms readings with multiple much better/accurate DMM's.

I'm in the market for a known good/working EEC4 for AOD as I believe it to be suspect.
 

jrichker

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More information to report. I purchased an Innova Ford OBD1 Scanner and it also failed to pull the codes. I don't believe it to be an issue with the wiring harness, or the repair made to the ECU as the relays all click as they should. I have also verified the wiring and ohms readings with multiple much better/accurate DMM's.

I'm in the market for a known good/working EEC4 for AOD as I believe it to be suspect.

As a last try, connect the gray self test connector/white/red wire to battery ground and turn the ignition on and see if it dumps codes.
 

IdaFox

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Jul 20, 2017
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Afternoon Jrichker,

I just ran out and tried your most recent suggestion, and unfortunately I received the same outcome. Solid CEL that remains solid after the relays and fuel pump recycle.

Have you ever heard of someone using the ECU repair services on eBay successfully?