Drivetrain Help with Check Engine Light after Transmission Swap - 1990 Mustang LX 5.0

Mustang5L5

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Mod Dude
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You did it right!

So that new trans harness, PLUS changing the O2 harness jumper are all you need to do to run the A9L after swapping from AOD to T5. So wiring-wise, you are all set.

Have you had a chance to test continuity yet on the pins of the ECU?
 
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jglass3

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Haven't tested continuity yet...looks like I'm going to have to be a contortionist to remove the ECU! I assume that I need to remove the connector at the bottom of the ECU and try to move the harness out of the way before attempting to remove the ECU? Any other tricks to removing it short of a sabre saw?
 

Mustang5L5

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Think you can test it in place once you unplug it?

Really, it's not that big of a deal. Remove the connector, lif thte plastic retainer up, and pull straight down. Might be a few vids on youtube that can show you the general idea of how to get it down. Don't overthink it. It's tight but it comes out pretty easily
 

jrichker

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Yes, the T-5 rebuilt transmission kit from NEO Mustang (Northeast Ohio Mustang) did include the NGS that I installed on the top of the transmission. I remember it well because the trans. was shipped to me without the switch and It was shipped to me separately (with the little pin), so I had to install it. Unfortunately, I already had the transmission installed so it was an absolute nightmare to access! The kit came with a brand new harness that replaced the auto. trans. harness. I recall that the harness had connectors for the fuel pump relay (under the drivers seat), VSS, reverse light switch and the NGS on top of the transmission. I remember having to connect the harness to an existing connector behind the drivers side kick panel. All connectors were correct and I installed them all. Did I do right or wrong? I guess I'm wondering how someone might have installed an A9L ECU into an AOD car and not trip the CEL, especially when the O2 harness was in the configuration for an AOD setup?
The computer and the transmission have very little real time communication capability. All the computer wants to know is if the transmission is in engaged and if the car is slowing down to stop. This is to prevent the engine from stalling when the car does come to a stop. The CEL does not know or care what kind of transmission is in the car.
 

jglass3

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Well, I pulled the ECU and checked continuity between pin 46 and 60 and got no continuity at all. Then, I checked continuity between pin 46 and 40 and, again, no continuity at all. What's next?
 

Mustang5L5

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How good are you at soldering?

your gonna want to open it up, locate the burned trace in the circuit board, and solder in a jumper.

or buy another A9L for $200-250
 

jglass3

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Well, I opened it up and sure enough, where your post shows a burnt circuit, that's where mine is burnt as well. I've always been pretty good at soldering, but printed circuit boards are pretty tough. Have you done this yourself? Were you successful? If I decide to buy a replacement A9L ECU, any recommendations on where to look for one that I can be fairly certain that it will not have the same malady as my own? Also, should I purchase another A9L or an A9P? For sure, the warranty label was sliced on this one meaning I'm not the first one to have opened this one up. Recommended next steps?
 

Mustang5L5

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I’ve not done this particular repair but I have done plenty of soldering and built my own aftermarket ECU for these cars.

See this thread for some photo guide on where to solder your jumper.

I would also inspect the three capacitors on the board as well and replace if necessary while you are there


As to where to buy a good working a9l, it’s tough to say. 30 year old electronics are a crapshoot.
 

jglass3

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I have found a company on eBay that will go through the ECU and make whatever repairs are necessary and give a lifetime warranty (not sure how that's possible on electronics) and the price is $98. I looked and the number of transactions for the company is over 700 and their feedback score is 100%. They also indicated they would diagnose whatever concerns and if it does not look like your ECU is repairable within reason, they would refund the full purchase price.

Here's the dilemma. At some time during this car's life, it had an ECU issue which resulted in its replacement. Had the person who swapped the ECU not installed an A9L, I may never have known it was replaced. Secondly, the replacement had to occur after bumper-to-bumper warranty expired. Otherwise, the dealer would have installed a replacement A9P. The big question is this: If I have my ECU repaired and reinstall it, is there any likelihood that the repaired ECU will fail similar to this one? In other words, is there something else lurking that could cause the ECU to fail again? Remember, even with this failed ECU, everything works,,,even if it is in limp mode (and runs like crazy). The only real objection is the CEL...and the bulb can be (difficultly) removed.

You are much smarter than me on the electronics of a Foxbody. What would you do?
 

Mustang5L5

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ECU failures are becoming common on these cars. It’s become more prevalent in the last 5 years or so. Usually it’s the caps that fsil, which can be fixed for under a couple bucks. I would be surprised if the ECU rebuilder will repair the trace. They might, but I gave yet to see one done.

due to this ECU attrition rate, many guys are going aftermarket ECUs.

Id just Fix or locate another a9L and keep going.

With a burned pin 46 trace, run the risk of the ECU failing completely one day and leaving you with nothing
 

jglass3

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Call me a coward, but I threw in the towel and reinstalled my defective A9L ECU. The car seems to run the same as it did before I removed and opened up the ECU. I figured something went wrong for someone to have replaced the ECU in the first place, and whatever went wrong, may still exist that may cause a refurbished ECU to fail as well. I guess I'll either live with an illuminated CEL or pop out the bulb when I get tired of looking at it.

I'm sorry to have consumed so much of your time but really appreciate your technical advice and suggestions. I learned that on a 30-year old car, I'll be chasing wires, connectors and sensors forever, so I'll just drive and enjoy until something really fails. Thanks again for your help and hope I can repay the favor some day.
 

jrichker

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Call me a coward, but I threw in the towel and reinstalled my defective A9L ECU. The car seems to run the same as it did before I removed and opened up the ECU. I figured something went wrong for someone to have replaced the ECU in the first place, and whatever went wrong, may still exist that may cause a refurbished ECU to fail as well. I guess I'll either live with an illuminated CEL or pop out the bulb when I get tired of looking at it.

I'm sorry to have consumed so much of your time but really appreciate your technical advice and suggestions. I learned that on a 30-year old car, I'll be chasing wires, connectors and sensors forever, so I'll just drive and enjoy until something really fails. Thanks again for your help and hope I can repay the favor some day.
I have a home brew proverb: for you, but first a little personal history is in order. I an 74 years old and I still work a full time job as a computer service specialist for a Federal government agency. That give me the right to push and poke at people to try to get them to understand that you are never too old to learn something new. Half of everything I know about computers is obsolete every 12 months; after 42+ years I am still learning new things on regular basis.

I got into the computer service business because I have 2 year electronics degree and when I started in 1979, the repair work was mostly electronic troubleshooting and very little software. That has since flip-flopped: software is the big break/fix problem now. However, the electronics troubleshooting skills have found a home in the 1986-l993 EFI 5.0 3 Mustangs. After 20+ years activity here on Stangnet's 5.0 forum, I don't miss very often when I have all of the details to work with.

Finally, here's the proverb:
Persistence is often of greater value than genus.

Don't just give up because you don't have all the answers; Lean, grow, get good at what you want to do, and you will succeed.
 

NXcoupe

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It has really failed. Your signal return circuit fried. Have the pcm repaired, I do this all the time, it's actually very common. And it's the best repair. No telling what caused it, I've repaired them and never had it happen again.
If you are deadfast set on leaving your car broken, then take a male spade connector and crimp it onto a 18g wire or so, then insert it i to the ground slot on the data link connector and put a ring terminal on the other end and ground it to the battery negative. Light should go out and all sensors will come back into range. It's a backwoods fix but it works.
Usually what burns that pin 43 signal return out is a bad ground from battery to engine and battery to computer.
 

NXcoupe

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Call me a coward, but I threw in the towel and reinstalled my defective A9L ECU. The car seems to run the same as it did before I removed and opened up the ECU. I figured something went wrong for someone to have replaced the ECU in the first place, and whatever went wrong, may still exist that may cause a refurbished ECU to fail as well. I guess I'll either live with an illuminated CEL or pop out the bulb when I get tired of looking at it.

I'm sorry to have consumed so much of your time but really appreciate your technical advice and suggestions. I learned that on a 30-year old car, I'll be chasing wires, connectors and sensors forever, so I'll just drive and enjoy until something really fails. Thanks again for your help and hope I can repay the favor some day.
Coward!






Lol. Had to. You told me to....
 
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jglass3

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Yeah, I'm 68 with an engineering degree and retired from Ford after a 30 year career. Funny thing is that I used to work at the old Lincoln Mercury building on Schaffer Road in Dearborn, right across I-94 from the Rouge complex and Dearborn Assy. At lunchtime, some of the guys and I would often go over to Dearborn Assy. and watch Mustangs being built...Foxbodys! Remember the 1986 GTs? They had a flat black hood that was actually a decal. There was one lady, probably in her late 50's, who wet down the hoods and slid those decals on like she could do it in her sleep! The other job that was amazing was leading the roofs to the sides on coupes and hatchbacks (the last facility at Ford that still leaded bodies). They had a couple of guys that could take paddles and with about 2 strokes, the joint was perfect...didn't need any finishing at all. Unfortunately, their life expectancy probably wasn't very long. That's how I knew that college had done me well! Kind of deja-vu: In management, I used to have company lease cars and I remember in 1989 I leased a Mustang LX 5.0 hatchback. We abused company cars like mad. I remember, living in Michigan, loading a full half-cord of firewood in that car. Road like a Lincoln! My son was young and we were practicing baseball in the front yard. He threw one over my head and it hit near the top of the front fender...left a dent so defined that you could see the stitches of the baseball in the fender...no hiding that from the company garage! Fun times...

I assume the wire that you are mentioning that I might try grounding is the single ground wire that would normally be plugged into the OBD 1 tester? I thought about it, but didn't want to try it unless it was suggested. I realize its a hillbilly fix, but what the hay....I was born in WV! :D Thanks again for everyone's help and...yes...I am a coward around electronics!

I know this is opinion for another thread, but my biggest mod was going from a 2.73 to a 3.73 rear axle. Great ratio for the AOD, but the T-5....wish I had done a 3.55 as the car now feels like it could use an extra gear or two...:D
 

NXcoupe

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Yeah, I'm 68 with an engineering degree and retired from Ford after a 30 year career. Funny thing is that I used to work at the old Lincoln Mercury building on Schaffer Road in Dearborn, right across I-94 from the Rouge complex and Dearborn Assy. At lunchtime, some of the guys and I would often go over to Dearborn Assy. and watch Mustangs being built...Foxbodys! Remember the 1986 GTs? They had a flat black hood that was actually a decal. There was one lady, probably in her late 50's, who wet down the hoods and slid those decals on like she could do it in her sleep! The other job that was amazing was leading the roofs to the sides on coupes and hatchbacks (the last facility at Ford that still leaded bodies). They had a couple of guys that could take paddles and with about 2 strokes, the joint was perfect...didn't need any finishing at all. Unfortunately, their life expectancy probably wasn't very long. That's how I knew that college had done me well! Kind of deja-vu: In management, I used to have company lease cars and I remember in 1989 I leased a Mustang LX 5.0 hatchback. We abused company cars like mad. I remember, living in Michigan, loading a full half-cord of firewood in that car. Road like a Lincoln! My son was young and we were practicing baseball in the front yard. He threw one over my head and it hit near the top of the front fender...left a dent so defined that you could see the stitches of the baseball in the fender...no hiding that from the company garage! Fun times...

I assume the wire that you are mentioning that I might try grounding is the single ground wire that would normally be plugged into the OBD 1 tester? I thought about it, but didn't want to try it unless it was suggested. I realize its a hillbilly fix, but what the hay....I was born in WV! :D Thanks again for everyone's help and...yes...I am a coward around electronics!

I know this is opinion for another thread, but my biggest mod was going from a 2.73 to a 3.73 rear axle. Great ratio for the AOD, but the T-5....wish I had done a 3.55 as the car now feels like it could use an extra gear or two...:D
Yes, the one the tester grounds. I work for Chrysler, I'm in Auburn Hills occasionally. I'm a field service engineer. I fix stuff like this all the time. But since covid we are on lots of restrictions. Miss getting out and about. Replace the computer or send it out and get it repaired.
 

jglass3

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Well, I tried jumpering the OBD 1 ground lead to the negative battery post. Car started as usual and there was no CEL...for about 15 seconds. Then the CEL illuminated and stayed illuminated for the entire 1/2 hour test drive. The car drives perfectly and everything works. The only nuisance is that hot restarts are a little tricky (engine cranks like the spark plugs are removed). The other thing I noticed is that after the engine is turned off, their is no residual vacuum for the brakes, etc. So if you're on a hill and depress the clutch pedal before starting the car, there are no brakes. Not a biggie as I live in a very flat area of VA. Anyway, I think I'm going to pull the CEL bulb as I'm a little leery of having my ECU repaired. After all, it's originally an AOD car that came with a A9P ECU and when I went to pull the ECU, found that it was an A9L. On top of that, the warranty seal was broken on the current A9L which means someone was inside poking around. Funny that it worked just fine with no CEL while running the AOD before I swapped to a T-5. Just too much bad vibes with the electrics in this car.

By the way, good to see an "East-Sider" driving a Mustang. Chrysler makes some fantastic engines! Used to meet with some of Chrysler's engineers at the old Highland Park facility. Thanks for your advice.
 

General karthief

wonder how much it would cost to ship you a pair
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Just for fun, if you still have the egr valve plug it in (you don't have to attach it to the intake) to the harness and see if the cel still comes on.
 

jglass3

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I unplugged the resistor pack that I purchased from LMR and plugged in my EGR valve, started the car, and no change....the CEL as bright as ever. I then ran a jumper from the OBD 1 ground to the negative battery post. The engine started and ran kinda herky-jerky, but no CEL. Then it stalled. When I restarted the engine it went back to the old smooth idle with the CEL glowing bright. The CEL has been on for quite a while now...maybe it will just burn out! BTW, I did have the EGR resistor pack installed with the AOD and never a CEL. Good thought though... thanks.
 

Mustang5L5

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Your realize you are running in basically limp mode, and probsbly not making full power potential.

don’t just brush it off. Fix this the right way.