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

General karthief · Oct 26, 2020

Did you reset the computer when you took the resister pack out and plug in the egr valve?
Look, you need to send that computer to the exchange to get it fixed, stop thinking that it's gonna malfunction again.
Or just pull the bulb and get on with life driving in limp mode,

jrichker · Oct 26, 2020

jglass3 said:
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?

Find a TV repair shop and ask them to use the instructions that I posted to repair the computer's burnt trace. While they are at it ask them how much will they charge to replace the electrolytic caps if you supply the parts.

The higher temp rating and longer lifecycle ratings increase the cost of a capacitor. In automotive circuits, those are important factors, and the highest rating stands the best chance of lasting the longest and working the best. Most capacitors used in automotive applications are rated at 105° C The typical capacitor used in most automotive electronics is less than $1.50 each.

jglass3 · Oct 26, 2020

I'm with you guys...I should be fixing this the right way. If I was confident that repairing or replacing the ECU would solve my problem, I would do it in a flash. I thought this car was unmolested, but when I discovered that a previous owner replaced the ECU (with the wrong ECU) it makes me wonder what other concerns are lurking beyond the ECU. BTW, does a 5.0L Foxbody have any type of vacuum reservoir other than the PB booster? Haven't seen anything in the shop manual. Just curious...

Mustang5L5 · Oct 26, 2020

There’s a small one inside the pass fender and another for cruise control inside the drivers fender.

Decipha · Oct 26, 2020

If you burnt the trace for sig rtn take pin 46 signal return wire, tap it and go to ground. See if that fixes you up.

Otherwise your best bet is to just upgrade to a 94-95 mustang v8 automatic ecu. They are a dime a dozen, much CahEAPER and easier to find than the fox ecus, they dont have any of those problems and they are an easy upgrade.

Its literally only 3 wires that need to be moved. You will need to buy a $250 moates quarterhorse though. The custom tuning software for all 1983-2020 fords is free i the downloads on my website. I have a free base tune file as well that you can simply load in and go with just following basic instructions.

CBAZA Swap / EFIDynoTuning

General karthief · Oct 27, 2020

See post #81
My computer is a replacement from many years ago and about 35k miles, the original lasted 115k, the Exchange will fix it up, test and warranty it, stop wringing your hands worrying about how long it will last or if it will fix the problem, get it done. Get it right. on down the road.

jglass3 · Oct 27, 2020

I must admit that I don't think I reset the computer after removing the resistor pack and reinstalling the EGR, I assume the reset is simply disconnecting negative battery terminal, turn on headlights, turn off headlights and reconnecting negative battery terminal? I can give that a try this morning.

Mustang5L5 · Oct 27, 2020

Disconnect batt for 30 mins to erase the computer memory.

That code 34 (EGR) is likely your CEL culprit.

Just soder a small jumper to bridge the burned out trace. 10 mins of work and done. Test for continity on pin 40/46/60 and call it a day

jglass3 · Oct 27, 2020

The good news is that I left the negative battery cable disconnected overnight so the memory in my computer must have been erased. I removed the EGR resistor pack and replaced it with the original EGR valve. Just for grins, I ran a jumper from the OBD ground to the negative post of the battery. When I started the engine, it ran a little rough at first but no CEL for about 15 seconds. Then the idle smoothed and the CEL illuminated. So I turned off the ignition, removed the negative battery cable, removed the jumper from the OBD ground and let the car sit for 30 minutes (even turned on the headlights). When I reconnected the negative battery cable and started the car, smooth idle and CEL illuminated in all it's glory. Needless to say, I disconnected the EGR valve and reinstalled the EGR resistor pack.

When I had my ECU open, I saw what I believe to be, the burnt trace and noticed in the photo where I need to install a jumper. I just don't think I'm that good of a solderer.... I suspect having the ECU refurbished might be just the beginning of a wild ride through Foxbody electronics.... The CEL bulb comes out today, a fitting fix for a car that appears to have a questionable past...why not just booger it up a little more?!

Mustang5L5 · Oct 27, 2020

Don't be so self-deprecating on the car. You did the T5 swap correctly by changing out the proper harness and repining the O2 harness. Regardless of the car's past, it's supposed to have an A9L in there now, and the wiring changes are correct for that.

You know your A9L is damaged. Pulling the CEL bulb doesn't make the problem go away, it kicks the can down the road.

jglass3 · Oct 27, 2020

One last question: If I remove my ECU and send it out to be repaired, what does everyone think the likelihood is of the repaired ECU failing as well...as soon as I install it in the car? I'm mean, are they really touchy and super=prone to failure?

Mustang5L5 · Oct 27, 2020

Yours failed because of a condition where someone put voltage to a pin that shouldn't have had voltage. It wasn't necessarily the ECU's fault. The AOD cars are wired differently, so what happened to your car was a condition that ANY ecu would have been damaged. I could have plugged my A9L in and it would have been damaged as well.

You've corrected that, and to double check, you can test for voltage on pin 46 while cranking with the ECU removed. No voltage, you can rest easy.

The 2 most common reasons these ECU's fail 99% of the time is due to bad capacitors which leak onto the board and corrode their terminals (which the ECU rebuilder would replace) and burning out the pin 46 trace when people mix and match wiring in cars they frankenstien together. The vast majority of people cannot do such repairs, so this is why the A9L ecu market is overpriced.

Fixing the ECU should give you many many many more years of use without worry.

jglass3 · Oct 27, 2020

I know I'm going to hate myself, but I'm gonna give it a try....thanks for all of your advice and support!

Mustang5L5 · Oct 27, 2020

Can you send a close up pic of the burned trace?

jglass3 · Oct 27, 2020

Here are a couple of photos of (what I think) is the burnt trace. It is precisely where the burnt trace was located in the photos in an earlier post.

jglass3 · Oct 27, 2020

Sorry, just one photo...the other came out blurry...

Mustang5L5 · Oct 27, 2020

Easy jumper then. You just need a wire from here to here.

It does look like there is a resistor that ties into that trace as well. You'll want to test for continuity from your repair to the resistor, but it looks like the burn is after the intersection point.

jglass3 · Oct 27, 2020

Well, I see the trace that needs to be jumpered, and if it was household plumbing, I'd have a go at it. But I'm not near as steady as I used to be, plus if I send it in, they will go through it and test all the circuits. They pretty much guarantee 24-48 hour turnaround, so out it goes today!

Mustang5L5 · Oct 27, 2020

Did you soecifically mention the burned trace to them?

jglass3 · Oct 27, 2020

Yes, I did...wasn't sure how to describe it, however, I did tell them that I was having internal ground concerns due to a burnt trace. Supposedly, they check all the circuits.....

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

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