Computer issue?

Black Sun 5.0

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I spoke to my mechanic who is getting ready to put a bullet in my '90 GT. Apparently, the only way to be able to run codes or get the computer to come to life is by manually grounding some kind of signal return wire or something like that. Sorry, I wrote it down and left the info at home. Does this sound like it rings a bell with any of you gurus out there? Also, when the cluth pedal is depressed and released, my mechanic also indicated that the idle will fluctuate. Anybody?
 
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HISSIN50

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Knowing the issue or symptom would help (dumping codes requires connecting wires. Making the EEC come to life should not).

Idle fluctuations with respect to the clutch being depressed are not entirely uncommon. That's not to say the issue should be there however.

Good luck.
 

Mustang5L5

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That kinda doesn't make sense. What do you mean by "Makes the computer come to life". The car wouldn't run period if it was dead.


Is the car a T-5 swap? Sometimes not plugging the NSS on the top of the trans causes probs reading codes
 

Black Sun 5.0

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What exactly is the NSS? Anyway, I spoke to my mechanic again, and what was specifically said was that if the sigal return wire was not manually grounded, they could not get data, read, or clear codes, as if the computer was not properly grounding that wire. Any help or leads are appreciated, this has been going on a long time and I'd love to drive my car again this season. Thanks.
 

joel5.0

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What exactly is the NSS? Anyway, I spoke to my mechanic again, and what was specifically said was that if the sigal return wire was not manually grounded, they could not get data, read, or clear codes, as if the computer was not properly grounding that wire. Any help or leads are appreciated, this has been going on a long time and I'd love to drive my car again this season. Thanks.
From that little piece of information, looks like the SIGRTN circuit PWB line in the ECM is open/fried...... or something like.....

mustangcomputer001circle.jpg


Which can be fixed as shown below....... BTSTDTRT plenty of times.

mustangcomputerfixed005.jpg
 
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joel5.0

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FYI: The computer has been replaced twice for troubleshooting.
If a VMI check confirms the damage on the PWB, he needs to ID what is causing that circuit to blow + check the integrity of the SIGRTN wire + make sure there isn't a short to power causing the SIGRTN circuit to be damaged. If the root cause of the problem is not corrected, the SIGRTN will be damaged regardless of how many ECM's are installed.
 

joel5.0

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ECM disconnected, turn IGN ON, check for voltage at the SIGRTN terminal of the TPS connector, TPS disconnected....... or pin 46 at the ECM connector. LUK
 

93 LX

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Stangnet is always more help. Most shops just fix the easy but they dont drive them or care for Stang.. Stangnet people love the Mustang... we know how they work cuz we work on them....
 
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Black Sun 5.0

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OK, here's an update for all the curious folks out there: The problem with my computer has been resolved. Apparently there was a bad splice located somewhere in the harness. I guess the fella who helped me rebuild the car zigged where he should zagged. Either way, the car should be on the road this week. Next up: 5-lug, 4-wheel disc upgrade.
 

jrichker

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There are 2 primary causes for this problem:
1.) people try to dump the codes and use the brown connector (12 volts for the under hood light) instead of the gray STI connector. Both have the same shape and are located in close proximity to each other. My how to dump the codes notes the differences and dangers of the two similar looking connectors.

2.) Wrong O2 sensor harness; use an Auto Transmission O2 sensor harness with an A9L 5 speed computer and it will burn the trace off the computer PCB as shown. I have an "O2 Sensor harness interchange and modification" tech note that explains the differences and cautions about O2 sensor harness interchange.