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

Mustang5L5

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The possible cause for an open circuit and the burned signal ground trace in the computer is a mismatch between the O2 sensor wiring harness and the computer.

O2 Sensor harness interchange and modification

Originally Posted by 302EFI


Revised 16-Oct-2011 to add O2 sensor harness warnings
The wires for the 02's and low oil did not change throughout the years, they are all in the same place.
The main ones you need to worry about are (on the harness end (ECU) that plugs into the 02 plug) is:
\- 1. Lightblue / yellow
- 2. White / Purple
- 3. Purple / Yellow
The White/Purple & Purple/Yellow gets looped for a automatic ECU
The Purple/Yellow & Lightblue/Yellow for a manual ECU

601141.jpg

Not all wires are shown for clarity and simplicity

See http://forums.corral.net/forums/gen...manual-auto-differences-year-differences.html for more O2 sensor wiring harness info

Basic premise to use with transmission swaps:
Only run a 5 speed trans O2 harness with an A9L. Do not run an Auto O2 sensor harness with an A9L. Doing so will damage the computer’s internal signal ground.
Only run an Auto trans O2 sensor harness with an A9P in a car that has an Auto trans. Using a 5 speed trans O2 sensor harness with an Auto trans will cause no crank problems.
See http://www.stangnet.com/mustang-forums/749974-computer-issue.html#post7490537 for Joel5.0’s fix to the computer internal signal ground.
The 4 cylinder O2 harness uses 4 wire O2 sensors. It probably won’t work correctly without modifying it.

@Mustang5L5 please provide a sanity/clarity check to insure that I didn't miss anything in the O2 sensor harness modification


Your diagram needs to be modified. What you show for 88-93 manual trans really only applies to 88-90

1987 to early 1988 harness for both 5spd and AOD (no jumper at all)
later 1988-1990 5-spd harness (jumper from pin 1 to pin 5)
91-93 5-spd harness (jumper from pin 1 to pin 6)
later 88-93 AOD harness (jumper from pin 5 to pin 6)
 
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jglass3

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I checked all the ground wires I could find and all were good...even cleaned up each and reattached...no resistance between grounds and negative battery cable. I started with Code 22 which should be the MAP/BAP sensor. In checking my connector, the wires I have are Black/White, Light Green/Black and Gray/Red. I'm assuming that the Gray/Red may actually be my Orange/White. Anyway, I checked resistance between the Black/White and the negative battery cable and came up with a repeatable 287 ohms. Sounds like I may have a ground issue as you suggested. I assume my next step is to turn on the ignition and check voltage between the Gray/Red (Orange/White) and Black/White, looking for 5 volts? I do not have an oscilloscope, but it looks like I may be heading toward the source of my problem with a digital VOM. Just thinking, can I simply add a jumper wire and ground the Black/White wire to the car to make a successful ground?
 

jrichker

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I checked all the ground wires I could find and all were good...even cleaned up each and reattached...no resistance between grounds and negative battery cable. I started with Code 22 which should be the MAP/BAP sensor. In checking my connector, the wires I have are Black/White, Light Green/Black and Gray/Red. I'm assuming that the Gray/Red may actually be my Orange/White. Anyway, I checked resistance between the Black/White and the negative battery cable and came up with a repeatable 287 ohms. Sounds like I may have a ground issue as you suggested. I assume my next step is to turn on the ignition and check voltage between the Gray/Red (Orange/White) and Black/White, looking for 5 volts? I do not have an oscilloscope, but it looks like I may be heading toward the source of my problem with a digital VOM. Just thinking, can I simply add a jumper wire and ground the Black/White wire to the car to make a successful ground?
No...
Here's why....

Why do the engine control sensors have a separate ground?
The computer pin 46 signal ground is a critical component: it provides a signal ground for the Map/Baro, TPS, ECT, EGR position and ACT sensors. Every sensor has to have a reliable ground for it to provide accurate and useful information to the engine control computer.
The more current that flows through a conductor, whether it is a cable or the car body, the greater the voltage drop. The ground on the car body and ground cable for the engine block may have different voltage drops. Some of the voltage drops change depending on electrical loads such as lights, A/C, and heater blower settings.
Under high current loads, there can be large changes in the voltage drop across the ground wiring that goes back to the negative battery cable. If the engine sensors ground is combined with power ground, the computer will see these changes in the ground as a change in sensor input. This voltage change is very undesirable for reliable computer operation, so the engine sensors all have a separate shared ground.
The engine mounted sensors all have outputs of 5 volts or less. Therefore, any small changes in voltage seen by the computer analog signal inputs can have a large effect in engine performance. Signal ground is used in many circuits that have analog inputs to eliminate the voltage drop across the engine block and body grounds when current flows through them. There is very little current flowing through the signal ground and the voltage drop across the signal ground is so small that it can be ignored by the computer.
The voltage output of any sensor must always use the signal ground for the sensor under test in order to eliminate any errors or variances. That way, what the voltmeter sees is what the computer sees unless there is a problem with the sensor wiring.

Here's a repeat of the testing process for signal ground problems in case you missed something....

The 91 and later Mustangs will have a different color signal ground wire in the body side of the circuit . The wire colors for the engine mounted fuel injector harness are the same.. See the diagrams below for more help

Disconnect the positive battery cable to insure correct results when measuring the resistance of grounds. The small voltage drop that is often in a circuit can cause erroneous readings. Since the computer and several other things still draw current even with the ignition switch in the Off position, this is a necessary step.

Check the black/white wire resistance. Connect one ohmmeter lead to the black/white wire on the TPS and one lead to the negative post on the battery. You should see less than 1.5 Ω; more than that indicates a problem. Always take resistance measurements with the circuit powered off.

Check the resistance of the black/white signal ground on the MAP/BARO sensor on the firewall behind the upper intake manifold. The resistance should be less than 1.5 Ω If the resistance value of the black/white signal ground on the MAP/BARO is1.5 Ω and the TPS is higher or doesn't read at all, then there is a problem in the engine mounted fuel injection harness or the 10 pin connector


Clean the 10 pin salt & pepper shaker connectors.
Diagram courtesy of Tmoss & Stang&2birds
TPS_IAB_Pic.jpg


See the graphic for the 10 pin connector circuit layout.
68512.jpg

The injector power pin is the VPWR pin in the black 10 pin connector. [/b]


How it is supposed to work:
The black/white wire (computer 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. 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 Ω 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 it 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, which are power ground for the computer.

See the pictures below for help finding and fixing the burnt computer trace.


hash-146a243133771eeba54f17b17d721b1f-jpg.jpg


The fix is some careful soldering of a small jumper wire across the burnt section of copper trace.




How to test the wiring:

Disconnect the positive battery cable to insure correct results when measuring the resistance of grounds. The small voltage drop that often in a circuit can cause erroneous readings. Since the computer and several other things still draw current even with the ignition switch in the Off position, this is a necessary step.

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 Ω



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: 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.

Measure the resistance between the black/white wire on each of the following sensors: TPS, ECT, ACT and EGR. If you find one that is greater than 1.5 Ω, measure between that sensor and pin #1 of the white 10 pin connectors. Pin #1 is the center pin and is labeled sig-rtrn on the diagram

The 91 and later Mustangs will have a different color signal ground wire in the body side of the circuit . The wire colors for the engine mounted fuel injector harness are the same..

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/ Everyone should bookmark this site.


Complete computer, actuator & sensor wiring diagram for 88-91 Mass Air Mustangs
88-91_5.0_EEC_Wiring_Diagram.gif


Complete computer, actuator & sensor wiring diagram for 91-93 Mass Air Mustangs
91-93_5.0_EEC_Wiring_Diagram.gif
 
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jglass3

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Well, I was afraid of that. :( I have done a little more checking this evening. I tested for resistance between the Black/White and the negative battery terminal on the BAP, TPS and EGR...and coincidentally, all are 287 ohms. I did an ignition on voltage check from the Black/White to the Gray/Red (Orange/White) and instead of 5volts, I got -460mv? Not sure how that's possible, but that's what my VOM read for each of the three locations. Seems like a number of posters are mentioning the O2 sensors. When I received the T-5 conversion kit from NEO Mustangs, it had a new harness that connected right up to my existing O2 sensors. Others are talking about jumpering leads. any thoughts on next steps...other than popping out the CEL? One other thought is that after the trans. swap, my idle was a little on the low side such that if you quickly let off the accelerator at idle while parking, it has a tendency to stall. I greatly appreciate your help to-date. Seems like I'm finding problems, but not sure of a cure...
 

jrichker

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Well, I was afraid of that. :( I have done a little more checking this evening. I tested for resistance between the Black/White and the negative battery terminal on the BAP, TPS and EGR...and coincidentally, all are 287 ohms. I did an ignition on voltage check from the Black/White to the Gray/Red (Orange/White) and instead of 5volts, I got -460mv? Not sure how that's possible, but that's what my VOM read for each of the three locations. Seems like a number of posters are mentioning the O2 sensors. When I received the T-5 conversion kit from NEO Mustangs, it had a new harness that connected right up to my existing O2 sensors. Others are talking about jumpering leads. any thoughts on next steps...other than popping out the CEL? One other thought is that after the trans. swap, my idle was a little on the low side such that if you quickly let off the accelerator at idle while parking, it has a tendency to stall. I greatly appreciate your help to-date. Seems like I'm finding problems, but not sure of a cure...
Follow the instructions that I have posted and use Mustang5L5's O2 sensor wiring information.

At this point it is highly likely that you have burnt signal ground trace inside the computer. The stalling Idle is another symptom of the brunt trace inside the computer. You should see less than 1.5Ω between any of the sensor white/black wires and the battery ground terminal.
 

jglass3

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Follow the instructions that I have posted and use Mustang5L5's O2 sensor wiring information.

At this point it is highly likely that you have burnt signal ground trace inside the computer. The stalling Idle is another symptom of the brunt trace inside the computer. You should see less than 1.5Ω between any of the sensor white/black wires and the battery ground terminal.
Thanks Jrichker, I really appreciate your patience with me. As a retired engineer, I would normally not be satisfied until I had fully corrected a problem. In this case, however, the car is running fine and everything works. To have to pull and either repair or replace a PCM as well as to possibly have to modify wiring to the O2 sensors is just too much on a 30 year old car that could experience more sensor/wiring failures at any moment. Unfortunately, I'm going to take the coward's way out and pull the CEL since the car is running perfectly and seems to have no other issues. I would like to ask you a couple of questions before I pull the CEL:

1) Is there any value to grounding a black/white wire from one of the sensors or am I looking to complicate my issue?
2) Would you pull the CEL bulb completely or replace it with a burnt-out bulb? My guess is that it doesn't matter since a failed filament breaks the circuit anyway.

Sorry I am taking the sissy's way out but I just can't seem to equate the value of the result to the amount of work/expense it will require to make a proper repair. Thanks again for taking the time to educate me. I've definitely learned a lot about EFI controls...much more than I learned during my 30 years working at Ford!
 

Mustang5L5

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There really isn't any reason to change the o2 wiring harness around unless you've swapped to a manual trans ECU. I forget if you said you did do this.
 
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jglass3

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If that is the case, any idea why my ECU got fried? If I simply replace my ECU with a known working ECU, will it fry again? What's your recommendation? The reason I ask is that if I don't see opportunity for a little light at the end of the tunnel, the CEL is coming out today. I really didn't know about the O2 harness, but I had seen some posts that talked about jumpering wires to prevent frying one of the printed circuits on the ECU. Thanks again for your thoughts...everyone has been very helpful, kind and supportive.
 

Mustang5L5

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Best way to check is to remove the ECU, and probe pin 46to ground. AOD-wired cars will have voltage. Manual wired cars won't. Running a manual ecu on an AOD-wired car will fry it due to the voltage on this pin during crank.

ALso, what's the 3-digit ECU code on your ecu?

You really don't want to just move on here because although the car may run good today, no guarantee it will continue to do so if the pin 46 trace did get burned up (partially or completely)
 
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jglass3

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As a retired engineer, I hate to give up this easily, however, this problem seems insurmountable. I checked and the ECU in my car (which I'm sure is original) is an A9L. Any next thoughts, or pull the CEL bulb and let it go at that?
 

Mustang5L5

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Has it always been the A9L or was that part of the kit with the manual trans?

you do need to verify the o2 harness jumper. Do this ASAP as it may need to be repinned before you fry the ECU.

A9L into a former AOD car can fry it unless you swap the o2 sensor jumper. It’s easy to do.

this is 99% likely the source of your issue. It’s possible the ECU has not burned the trace up yet, but it will one day. So not cycle the key to on again until you confirm/swap the jumper

 

jglass3

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Yeah, unfortunately I moved into a management position within Marketing, Sales and Service and pushed paper. Only in product engineering for about my first 6 years and worked as a lead engineer on the Lima 2.3L engine. I guess my problem is not insurmountable but seems like a lot of work just to turn off the CEL knowing that on a 30 year old car it's just a matter of time before the next sensor or connector fails...like tracking down the burnt out bulb on a string of Christmas tree lights...life's too short...:)
 

General karthief

wonder how much it would cost to ship you a pair
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I've hammered on my mustang for almost 50k miles for the last 15 years and no problem finding the source the few times my cel came on and I'm a high school dropout,
You are obviously missing something basic when you did the drive train swap, like the o2 re-pin like mentioned above.
 

jglass3

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I'll be the first to say that college did not make me smart. The older I get, the more I look up to people like yourself who learned life lessons and aren't afraid to tackle any problem...much smarter than myself. After dinner, I'm going to pull my O2 harness. Looking at your feed for repinning it, I'm not sure which pins I'm supposed to jumper. Any additional directions would be appreciated.
 

jglass3

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Also, do I jumper both connectors at the O2 sensors themselves or the connector that joins the two wires to the O2 sensors?
 

General karthief

wonder how much it would cost to ship you a pair
Mod Dude
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This:
will show you exactly how/where/and what to do,
And I'm scared to death to do things to my car, with the exception of driving the snot out of it, I just know that most mechanics now don't know nothin 'bout working on these cars and given the info here I can fix most anything on this rag.
 

Mustang5L5

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Correct. A single 8-pin plug thst will have 7 wires on the body side, and 5 wires plus a jumper loop on the harness side