Rich At Idle, Maybe Cruise Too
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JRICHKER and any other geniuses,
I found this in an old post after I took the car into Ford to get the secondary fuel system code checked out and most likely the pump relay replaced. (I can't lift the seat right now and the relay is under the replaced carpet.) I know it is a dealership, but they were the first ones who said "No Problem" to the mods on the car. This post was talking about the fuel relay circut.
"Computer: If you got this far and everything else checked out good, the computer is suspect.
Remove the test jumper from the ECC test connector located under the hood. Probe computer pin 22 with a safety pin and ground it to chassis. Make sure the computer and everything else is connected. Turn the ignition switch to the Run position and observe the fuel pressure. The pump should run at full pressure.
If it doesn't, the wiring between pin 22 on the computer and the fuel pump relay is bad.
If it does run at full pressure, the computer may have failed. "
http://www.stangnet.com/mustang-forums/threads/wont-start-bad-ecm.853975/#post-8596183
I did not run the test before dropping off the car, but it ran up to full pressure every time it was turned on.
So the new assistant service person at Ford (who is nice, but does not know a chrome bumper from a facia), relays (from the mechanic in the background) that the ecm is bad and they have located a used one with 90 day warranty for me. It is on the way. (I would have preferred rebuilt and tested.) She had no idea what he and I were trying to talk about and she did not put him on. So I will ask in person. AND I am having them save my old one just in case.
Besides using the same code tool or paperclip I have, I ask the following so I know they did not just jump to a conclusion. What other tests could he have run to determine the computer was bad?
I found this in an old post after I took the car into Ford to get the secondary fuel system code checked out and most likely the pump relay replaced. (I can't lift the seat right now and the relay is under the replaced carpet.) I know it is a dealership, but they were the first ones who said "No Problem" to the mods on the car. This post was talking about the fuel relay circut.
"Computer: If you got this far and everything else checked out good, the computer is suspect.
Remove the test jumper from the ECC test connector located under the hood. Probe computer pin 22 with a safety pin and ground it to chassis. Make sure the computer and everything else is connected. Turn the ignition switch to the Run position and observe the fuel pressure. The pump should run at full pressure.
If it doesn't, the wiring between pin 22 on the computer and the fuel pump relay is bad.
If it does run at full pressure, the computer may have failed. "
http://www.stangnet.com/mustang-forums/threads/wont-start-bad-ecm.853975/#post-8596183
I did not run the test before dropping off the car, but it ran up to full pressure every time it was turned on.
So the new assistant service person at Ford (who is nice, but does not know a chrome bumper from a facia), relays (from the mechanic in the background) that the ecm is bad and they have located a used one with 90 day warranty for me. It is on the way. (I would have preferred rebuilt and tested.) She had no idea what he and I were trying to talk about and she did not put him on. So I will ask in person. AND I am having them save my old one just in case.
Besides using the same code tool or paperclip I have, I ask the following so I know they did not just jump to a conclusion. What other tests could he have run to determine the computer was bad?
FYI: I will be without computer or Internet for the next 5-6 days, so if you don't get follow up help message from me for a while, don't fret.
If all the sensors and wiring are working correctly, you get a code 11 and nothing else without the engine running.
The engine running codes will be 11, and then 4. The engine will then test the EGR by opening the EGR valve , causing the engine to stumble if it is working correctly. At that point pushing the throttle above the 2500 RPM point will start the cylinder balance test.
If the computer has internal problems, there are only 2 codes it will set, code 15 and code 19. If the computer has serious problems, the codes won't dump and/or the engine won't run.
Code 15 - No Keep Alive Memory power to PCM pin 1 or bad PCM (Memory Test
Failure). The voltage to the Keep Alive Memory (KAM) is missing (wiring problem)
or the KAM is bad. The KAM holds all of the settings that the computer "learns" as
it operates and all the stored error codes that are generated as a result of
something malfunctioning while the engine is running. Use a voltmeter to check
the voltage to the pin 1 on the computer - you should always have 12 volts. No
constant 12 volts = bad wiring. If you do always have the 12 volts, then the KAM is
bad and the computer is faulty.
If the computer has to "relearn" all the optimum settings every time it powers up,
the initial 5-30 minutes of operation may exhibit surges, poor low speed performance,
and rough idle.
Note that some aftermarket chips will cause code 15 to set. Remove the chip,
clear the codes and retest.
Before replacing the computer, remove the battery ground cable for about 20
minutes. This will clear all the codes. Retest after several days of running. If the 15
code is gone, then don't worry about it. If it is still there, then you get to do some
troubleshooting.
See the following website for some help from Tmoss (diagram designer) & Stang&2
Birds (website host) for help on 88-95 wiring http://www.veryuseful.com/mustang/tech/engine/
Diagram courtesy of Tmoss & Stang&2birds
Code 19 - Engine off - No Vehicle Power (pins 37 + 57) or bad PCM VPWR Diagnosis. This is a wiring problem that is from a bad ECC power relay, bad connection, bad fuse link, bad ignition switch or a bad computer. The ECC relay is located on top of the computer under the passenger side kick panel. Pull the connector off any fuel injector and measure the voltage on the red wire: if its 12 volts or better, the ECC relay is OK. If the ECC relay is OK, pull the kick panel off and measure the voltage at pins 37 & 57. If it is 12 volts or more, then the computer's diagnostic firmware has taken a dump and is defective.
Engine running - Erratic idle during test (reset throttle & retest) - Idle Set Procedures .
See http://www.stangnet.com/mustang-forums/698148-help-me-create-surging-idle-checklist.html#post6855020 for the best way to set the mechanical base idle and cleaning procedure for the IAC/IAB.
http://www.veryuseful.com/mustang/tech/engine/images/IgnitionSwitchWiring.gif
http://www.veryuseful.com/mustang/tech/engine/images/fuel-alt-links-ign-ac.gif
http://www.veryuseful.com/mustang/tech/engine/images/88-91_5.0_EEC_Wiring_Diagram.gif
If all the sensors and wiring are working correctly, you get a code 11 and nothing else without the engine running.
The engine running codes will be 11, and then 4. The engine will then test the EGR by opening the EGR valve , causing the engine to stumble if it is working correctly. At that point pushing the throttle above the 2500 RPM point will start the cylinder balance test.
If the computer has internal problems, there are only 2 codes it will set, code 15 and code 19. If the computer has serious problems, the codes won't dump and/or the engine won't run.
Code 15 - No Keep Alive Memory power to PCM pin 1 or bad PCM (Memory Test
Failure). The voltage to the Keep Alive Memory (KAM) is missing (wiring problem)
or the KAM is bad. The KAM holds all of the settings that the computer "learns" as
it operates and all the stored error codes that are generated as a result of
something malfunctioning while the engine is running. Use a voltmeter to check
the voltage to the pin 1 on the computer - you should always have 12 volts. No
constant 12 volts = bad wiring. If you do always have the 12 volts, then the KAM is
bad and the computer is faulty.
If the computer has to "relearn" all the optimum settings every time it powers up,
the initial 5-30 minutes of operation may exhibit surges, poor low speed performance,
and rough idle.
Note that some aftermarket chips will cause code 15 to set. Remove the chip,
clear the codes and retest.
Before replacing the computer, remove the battery ground cable for about 20
minutes. This will clear all the codes. Retest after several days of running. If the 15
code is gone, then don't worry about it. If it is still there, then you get to do some
troubleshooting.
See the following website for some help from Tmoss (diagram designer) & Stang&2
Birds (website host) for help on 88-95 wiring http://www.veryuseful.com/mustang/tech/engine/
Diagram courtesy of Tmoss & Stang&2birds
Code 19 - Engine off - No Vehicle Power (pins 37 + 57) or bad PCM VPWR Diagnosis. This is a wiring problem that is from a bad ECC power relay, bad connection, bad fuse link, bad ignition switch or a bad computer. The ECC relay is located on top of the computer under the passenger side kick panel. Pull the connector off any fuel injector and measure the voltage on the red wire: if its 12 volts or better, the ECC relay is OK. If the ECC relay is OK, pull the kick panel off and measure the voltage at pins 37 & 57. If it is 12 volts or more, then the computer's diagnostic firmware has taken a dump and is defective.
Engine running - Erratic idle during test (reset throttle & retest) - Idle Set Procedures .
See http://www.stangnet.com/mustang-forums/698148-help-me-create-surging-idle-checklist.html#post6855020 for the best way to set the mechanical base idle and cleaning procedure for the IAC/IAB.
http://www.veryuseful.com/mustang/tech/engine/images/IgnitionSwitchWiring.gif
http://www.veryuseful.com/mustang/tech/engine/images/fuel-alt-links-ign-ac.gif
http://www.veryuseful.com/mustang/tech/engine/images/88-91_5.0_EEC_Wiring_Diagram.gif
Thanks, now I know what to listen for. I've never seen either of those codes, but 11 was not always there. I hope your computer is better than my car's.
I found earlier message on my cell phone, and the service rep said the tech determined my computer (PCM this time) was bad while testing my Oxygen Sensors.
I just hope he really knows what he is doing, and that me erasing the codes while recent testing did not confuse him. It runs, dumps codes, but as the title says, it runs so rich to choke a herd of Prius at idle.
If it is not the computer, the wasted time waiting for a computer and on another tech (if I can not get it with help) will be as upsetting as the wasted money. I just want to enjoy my car while the weather is nice. I'll follow up after I get it back.
I found earlier message on my cell phone, and the service rep said the tech determined my computer (PCM this time) was bad while testing my Oxygen Sensors.
I just hope he really knows what he is doing, and that me erasing the codes while recent testing did not confuse him. It runs, dumps codes, but as the title says, it runs so rich to choke a herd of Prius at idle.If it is not the computer, the wasted time waiting for a computer and on another tech (if I can not get it with help) will be as upsetting as the wasted money. I just want to enjoy my car while the weather is nice. I'll follow up after I get it back.
OK, Here is as brief as summary as I can get. I took it to the dealer to get the code for the secondary fuel pump circut checked out. I TOLD them to Please replace the relay for the fuel pump and check out the circut for thet code 96 that kept coming back.
I also said it was running rich and to please replace the O2 sensors while it was in the air. I did not know the miles on the sensors and it would be cheaper than a separate trip. So he started with the O2 sensors, determined there was no power to the bank 2 heated wire, and after more testing, that lead to a bad computer determination. ( I hope not just a bad wire.) it had no codes on the o2 sensor, just more random goofy ones that did not make the ticket. There was NO work done on the relay or fuel pump circut since the code did not appear with the new/used computer with very little driving.
That made me mad. I do not want to end up stranded if there is a code 96 intermittent problem.
So we are back to codes 94, 44 (air pump system and cats that I would like to replace with hight flow nes) and code 34. He assumed I had blocked off the EGR, which I had not. I had not put that on the note with the list of modifications and he did not call.
I will try the EGR flow test and then shortening up the EGR sensor pintle and see if that eliminates the code 34.
(Test procedure and and lots of pretty diagrams on the EGR system are at this link.) http://www.stangnet.com/mustang-forums/threads/how-to-test-if-egr-blocked.860939/#post-8656265)
Why do I care? I want the car working just right before I step up to a bigger motor or more power.
Guru jrichker, I regret not testing the O2 sensors first instead of going top down. Do you have any suggestions on any next step on the EGR if it flows and shortening does not do it? Also, would you replace the fuel pump relay at 153K or 253K miles anyway? ( Carfax has both mileages on it, so I do not know the real miles). I'll find someone to help move the seat if I need to.
I also said it was running rich and to please replace the O2 sensors while it was in the air. I did not know the miles on the sensors and it would be cheaper than a separate trip. So he started with the O2 sensors, determined there was no power to the bank 2 heated wire, and after more testing, that lead to a bad computer determination. ( I hope not just a bad wire.) it had no codes on the o2 sensor, just more random goofy ones that did not make the ticket. There was NO work done on the relay or fuel pump circut since the code did not appear with the new/used computer with very little driving.
That made me mad. I do not want to end up stranded if there is a code 96 intermittent problem.
So we are back to codes 94, 44 (air pump system and cats that I would like to replace with hight flow nes) and code 34. He assumed I had blocked off the EGR, which I had not. I had not put that on the note with the list of modifications and he did not call.
I will try the EGR flow test and then shortening up the EGR sensor pintle and see if that eliminates the code 34.
(Test procedure and and lots of pretty diagrams on the EGR system are at this link.) http://www.stangnet.com/mustang-forums/threads/how-to-test-if-egr-blocked.860939/#post-8656265)
Why do I care? I want the car working just right before I step up to a bigger motor or more power.
Guru jrichker, I regret not testing the O2 sensors first instead of going top down. Do you have any suggestions on any next step on the EGR if it flows and shortening does not do it? Also, would you replace the fuel pump relay at 153K or 253K miles anyway? ( Carfax has both mileages on it, so I do not know the real miles). I'll find someone to help move the seat if I need to.
If the steps I outlined for the EGR do not work, the only thing left is an intermittent bad connection or broken wire in the EGR sensor ground circut.
The fuel pump relay is less than $15 at AutoZone:.On 87-91 cars, it is under the seat, on 92-93 cars it is under the MAF by the A/C WOT cutoff relay. it takes about 3 minutes to replace it. Your just push the seat all the way back and you can reach it. There may be a bolt with a 10MM or 11MM head that bolts the relay mount bracket to the floorpan seat mount. Code 96 can also be caused by an intermittent bad connection or broken wire.
The fuel pump relay is less than $15 at AutoZone:.On 87-91 cars, it is under the seat, on 92-93 cars it is under the MAF by the A/C WOT cutoff relay. it takes about 3 minutes to replace it. Your just push the seat all the way back and you can reach it. There may be a bolt with a 10MM or 11MM head that bolts the relay mount bracket to the floorpan seat mount. Code 96 can also be caused by an intermittent bad connection or broken wire.
jcgafford
10 Year Member
code 94 and 44 are all to familiar... post #52 on this thread. http://www.stangnet.com/mustang-forums/threads/high-idle-rough-start.860113/page-3 jrichker led me by the nose to the problem and a little junkyard scavange for a usable tad and tab and it was all set.
Hello jcgafford,
The salvage yard here (not yardS) is poorly stocked or badly inventoried, and the folks are not helpful or friendly here. I see the car from the road, but supposedly it is a 4 cylinder convert. (with a 5.0 emblem and a roof?) Gosh knows why they are busy and in business. I have to wait until I am going to the other end of the state and call ahead. Or I can pay shipping and hope they get me all the missing parts. Thanks for the link when I do. The missing parts were on my 79 too, so it is not like they should be this hard to get at a good place.
Guru jrichker,
There's the's this titanium colored fuzzy stuff (replacement carpet) over the relay. But it will get replaced with a little help. The floor looks nice this way, but is not as helpful.
I'll let you know how the EGR adjusting goes. Thanks again.
NEWS FLASH that may help 91 up owners - The wiring diagram WITH (closer to) the right colors is in the back of the HAYNES manual, page 12-48. I figured the manual would be too basic and generic to help, but this is good news. For example, the wires attached to the TPS and EGR valve are closer to what are actually on my car. Gosh knows why Ford switched a few wires, but there is help for newer ones too.
The salvage yard here (not yardS) is poorly stocked or badly inventoried, and the folks are not helpful or friendly here. I see the car from the road, but supposedly it is a 4 cylinder convert. (with a 5.0 emblem and a roof?) Gosh knows why they are busy and in business. I have to wait until I am going to the other end of the state and call ahead. Or I can pay shipping and hope they get me all the missing parts. Thanks for the link when I do. The missing parts were on my 79 too, so it is not like they should be this hard to get at a good place.
Guru jrichker,
There's the's this titanium colored fuzzy stuff (replacement carpet) over the relay. But it will get replaced with a little help. The floor looks nice this way, but is not as helpful.
I'll let you know how the EGR adjusting goes. Thanks again.
NEWS FLASH that may help 91 up owners - The wiring diagram WITH (closer to) the right colors is in the back of the HAYNES manual, page 12-48. I figured the manual would be too basic and generic to help, but this is good news. For example, the wires attached to the TPS and EGR valve are closer to what are actually on my car. Gosh knows why Ford switched a few wires, but there is help for newer ones too.
Ok, the relay is replaced, the ERG position sensor is shortened, the EGR is flowing, and the codes are reset. We'll see what happens after some driving.
I am still chasing down wiring as time allows (I will pull codes again soon when it is a nice day for a drive), and I NEED to confirm the answer to this question.
Are the gauge temp sensor and the computer temp sensor on COMPLETELY different circuts????
My gauge went from reading low since I got the car, to spiking on and off, to not reading at all. I cleaned the terminal, dielectric greased it and the gauge is now good. I want to make sure that would have nothing to do with an over voltage in the computer sensors I was getting. The only way I see this being a possibility is if it tried to ground back through the harness to elsewhere.
THANK YOU for the help again!
Are the gauge temp sensor and the computer temp sensor on COMPLETELY different circuts????
My gauge went from reading low since I got the car, to spiking on and off, to not reading at all. I cleaned the terminal, dielectric greased it and the gauge is now good. I want to make sure that would have nothing to do with an over voltage in the computer sensors I was getting. The only way I see this being a possibility is if it tried to ground back through the harness to elsewhere.
THANK YOU for the help again!
ratio411
Founding Member
New codes. koeo, 11 and 10. So that is good. Running codes are 91 and still 34. I have the air pump stuff sitting there on the card board. So do I check for VAC leaks for the code 91 and ignore the 34 EGR for now?
I've been away to Florida cleaning up my mother-in-law's house. She's 94 and can't see all the dirt and isn't mobile to bend and reach even if she could. Here's my best answer:
Testing the O2 sensors 87-93 5.0 Mustangs
Measuring the O2 sensor voltage at the computer will give you a good idea of how well they are working. You'll have to pull the passenger side kick panel off to gain access to the computer connector. Remove the plastic wiring cover to get to the back side of the wiring. Use a safety pin or paper clip to probe the connections from the rear.
Disconnect the O2 sensor from the harness and use the body side O2 sensor harness as the starting point for testing. Do not measure the resistance of the O2 sensor , you may damage it. Resistance measurements for the O2 sensor harness are made with one meter lead on the O2 sensor harness and the other meter lead on the computer wire or pin for the O2 sensor.
Backside view of the computer wiring connector:
87-90 5.0 Mustangs:
Computer pin 43 Dark blue/Lt green – LH O2 sensor
Computer pin 29 Dark Green/Pink – RH O2 sensor
The computer pins are 29 (LH O2 with a dark green/pink wire) and 43 (RH O2 with a dark blue/pink wire). Use the ground next to the computer to ground the voltmeter. The O2 sensor voltage should switch between .2-.9 volt at idle.
91-93 5.0 Mustangs:
Computer pin 43 Red/Black – LH O2 sensor
Computer pin 29 Gray/Lt blue – RH O2 sensor
The computer pins are 29 (LH O2 with a Gray/Lt blue wire) and 43 (RH O2 with a Red/Black wire). Use the ground next to the computer to ground the voltmeter. The O2 sensor voltage should switch between .2-.9 volt at idle.
Testing the O2 sensors 94-95 5.0 Mustangs
Measuring the O2 sensor voltage at the computer will give you a good idea of how well they are working. You'll have to pull the passenger side kick panel off to gain access to the computer connector. Remove the plastic wiring cover to get to the back side of the wiring. Use a safety pin or paper clip to probe the connections from the rear. The computer pins are 29 (LH O2 with a red/black wire) and 27 (RH O2 with a gray/lt blue wire). Use pin 32 (gray/red wire) to ground the voltmeter. The O2 sensor voltage should switch between .2-.9 volt at idle.
Note that all resistance tests must be done with power off. Measuring resistance with a circuit powered on will give false readings and possibly damage the meter. Do not attempt to measure the resistance of the O2 sensors, it may damage them.
Testing the O2 sensor wiring harness
Most of the common multimeters have a resistance scale. Be sure the O2 sensors are disconnected and measure the resistance from the O2 sensor body harness to the pins on the computer. Using the Low Ohms range (usually 200 Ohms) you should see less than 1.5 Ohms.
87-90 5.0 Mustangs:
Computer pin 43 Dark blue/Lt green – LH O2 sensor
Computer pin 29 Dark Green/Pink – RH O2 sensor
Disconnect the connector from the O2 sensor and measure the resistance:
From the Dark blue/Lt green wire in the LH O2 sensor harness and the Dark blue/Lt green wire on the computer pin 43
From the Dark Green/Pink wire on the RH Os sensor harness and the Dark Green/Pink wire on the computer pin 43
91-93 5.0 Mustangs:
Computer pin 43 Red/Black – LH O2 sensor
Computer pin 29 Gray/Lt blue – RH O2 sensor
Disconnect the connector from the O2 sensor and measure the resistance:
From the Red/Black wire in the LH O2 sensor harness and the Red/Black wire on the computer pin 43
From the Dark Green/Pink Gray/Lt blue wire on the RH Os sensor harness and the Gray/Lt blue wire on the computer pin 29
94-95 5.0 Mustangs:
Computer pin 29 Red/Black – LH O2 sensor
Computer pin 27 Gray/Lt blue – RH O2 sensor
From the Red/Black wire in the LH O2 sensor harness and the Red/Black wire on the computer pin 29
From the Dark Green/Pink Gray/Lt blue wire on the RH Os sensor harness and the Gray/Lt blue wire on the computer pin 27
There is a connector between the body harness and the O2 sensor harness. Make sure the connectors are mated together, the contacts and wiring are not damaged and the contacts are clean and not coated with oil.
The O2 sensor ground (orange wire with a ring terminal on it) is in the wiring harness for the fuel injection wiring. I grounded mine to one of the intake manifold bolts
Make sure you have the proper 3 wire O2 sensors. Only the 4 cylinder cars used a 4 wire sensor, which is not compatible with the V8 wiring harness.
Replace the O2 sensors in pairs if replacement is indicated. If one is weak or bad, the other one probably isn't far behind.
Testing the O2 sensors 87-93 5.0 Mustangs
Measuring the O2 sensor voltage at the computer will give you a good idea of how well they are working. You'll have to pull the passenger side kick panel off to gain access to the computer connector. Remove the plastic wiring cover to get to the back side of the wiring. Use a safety pin or paper clip to probe the connections from the rear.
Disconnect the O2 sensor from the harness and use the body side O2 sensor harness as the starting point for testing. Do not measure the resistance of the O2 sensor , you may damage it. Resistance measurements for the O2 sensor harness are made with one meter lead on the O2 sensor harness and the other meter lead on the computer wire or pin for the O2 sensor.
Backside view of the computer wiring connector:
87-90 5.0 Mustangs:
Computer pin 43 Dark blue/Lt green – LH O2 sensor
Computer pin 29 Dark Green/Pink – RH O2 sensor
The computer pins are 29 (LH O2 with a dark green/pink wire) and 43 (RH O2 with a dark blue/pink wire). Use the ground next to the computer to ground the voltmeter. The O2 sensor voltage should switch between .2-.9 volt at idle.
91-93 5.0 Mustangs:
Computer pin 43 Red/Black – LH O2 sensor
Computer pin 29 Gray/Lt blue – RH O2 sensor
The computer pins are 29 (LH O2 with a Gray/Lt blue wire) and 43 (RH O2 with a Red/Black wire). Use the ground next to the computer to ground the voltmeter. The O2 sensor voltage should switch between .2-.9 volt at idle.
Testing the O2 sensors 94-95 5.0 Mustangs
Measuring the O2 sensor voltage at the computer will give you a good idea of how well they are working. You'll have to pull the passenger side kick panel off to gain access to the computer connector. Remove the plastic wiring cover to get to the back side of the wiring. Use a safety pin or paper clip to probe the connections from the rear. The computer pins are 29 (LH O2 with a red/black wire) and 27 (RH O2 with a gray/lt blue wire). Use pin 32 (gray/red wire) to ground the voltmeter. The O2 sensor voltage should switch between .2-.9 volt at idle.
Note that all resistance tests must be done with power off. Measuring resistance with a circuit powered on will give false readings and possibly damage the meter. Do not attempt to measure the resistance of the O2 sensors, it may damage them.
Testing the O2 sensor wiring harness
Most of the common multimeters have a resistance scale. Be sure the O2 sensors are disconnected and measure the resistance from the O2 sensor body harness to the pins on the computer. Using the Low Ohms range (usually 200 Ohms) you should see less than 1.5 Ohms.
87-90 5.0 Mustangs:
Computer pin 43 Dark blue/Lt green – LH O2 sensor
Computer pin 29 Dark Green/Pink – RH O2 sensor
Disconnect the connector from the O2 sensor and measure the resistance:
From the Dark blue/Lt green wire in the LH O2 sensor harness and the Dark blue/Lt green wire on the computer pin 43
From the Dark Green/Pink wire on the RH Os sensor harness and the Dark Green/Pink wire on the computer pin 43
91-93 5.0 Mustangs:
Computer pin 43 Red/Black – LH O2 sensor
Computer pin 29 Gray/Lt blue – RH O2 sensor
Disconnect the connector from the O2 sensor and measure the resistance:
From the Red/Black wire in the LH O2 sensor harness and the Red/Black wire on the computer pin 43
From the Dark Green/Pink Gray/Lt blue wire on the RH Os sensor harness and the Gray/Lt blue wire on the computer pin 29
94-95 5.0 Mustangs:
Computer pin 29 Red/Black – LH O2 sensor
Computer pin 27 Gray/Lt blue – RH O2 sensor
From the Red/Black wire in the LH O2 sensor harness and the Red/Black wire on the computer pin 29
From the Dark Green/Pink Gray/Lt blue wire on the RH Os sensor harness and the Gray/Lt blue wire on the computer pin 27
There is a connector between the body harness and the O2 sensor harness. Make sure the connectors are mated together, the contacts and wiring are not damaged and the contacts are clean and not coated with oil.
The O2 sensor ground (orange wire with a ring terminal on it) is in the wiring harness for the fuel injection wiring. I grounded mine to one of the intake manifold bolts
Make sure you have the proper 3 wire O2 sensors. Only the 4 cylinder cars used a 4 wire sensor, which is not compatible with the V8 wiring harness.
Replace the O2 sensors in pairs if replacement is indicated. If one is weak or bad, the other one probably isn't far behind.
It's no fun when things get to that point. I hope you can get some help.
Thanks for the advice again.
Thanks for the advice again.
I want to make sure I understand this right because I am confused after reading it several times. 
Are we are looking to test two things; 1. the wiring. then 2. the O2 sensor function? I think the part that is confusing me is the following warning -
1. "Disconnect the O2 sensor from the harness and use the body side O2 sensor harness as the starting point for testing. Do not measure the resistance of the O2 sensor , you may damage it. Resistance measurements for the O2 sensor harness are made with one meter lead on the O2 sensor harness and the other meter lead on the computer wire or pin for the O2 sensor."
I understand that the power has to be off and the sensors unplugged to measure the wiring/harness resistance. But how do you measure voltage if the harness is unplugged? It is a heated sensor, right? And if it is unplugged, no signal will get to the computer? So even if I measured voltage right at the sensor instead of the computer at idle, it might not be an accurate reading, right?
So (with it unplugged, power off, battery disconnected) I can back probe the right pin at the computer connection, then measure the other end of the unplugged wire at the sensor plug under the car, right?
NEXT -So, AFTER I measure resistances of the wires, is this part done with the harness plugged in to the sensors?
"91-93 5.0 Mustangs:Computer pin 43 Red/Black – LH O2 sensor
Computer pin 29 Gray/Lt blue – RH O2 sensor
The computer pins are 29 (LH O2 with a Gray/Lt blue wire) and 43 (RH O2 with a Red/Black wire). Use the ground next to the computer to ground the voltmeter. The O2 sensor voltage should switch between .2-.9 volt at idle."
Also, did I read in another post I am supposed to use my good analog sensor for this to see if it twiches fast or slowly?
Thanks for clarifying!
1. "Disconnect the O2 sensor from the harness and use the body side O2 sensor harness as the starting point for testing. Do not measure the resistance of the O2 sensor , you may damage it. Resistance measurements for the O2 sensor harness are made with one meter lead on the O2 sensor harness and the other meter lead on the computer wire or pin for the O2 sensor."
I understand that the power has to be off and the sensors unplugged to measure the wiring/harness resistance. But how do you measure voltage if the harness is unplugged? It is a heated sensor, right? And if it is unplugged, no signal will get to the computer? So even if I measured voltage right at the sensor instead of the computer at idle, it might not be an accurate reading, right?
So (with it unplugged, power off, battery disconnected) I can back probe the right pin at the computer connection, then measure the other end of the unplugged wire at the sensor plug under the car, right?
NEXT -So, AFTER I measure resistances of the wires, is this part done with the harness plugged in to the sensors?
"91-93 5.0 Mustangs:Computer pin 43 Red/Black – LH O2 sensor
Computer pin 29 Gray/Lt blue – RH O2 sensor
The computer pins are 29 (LH O2 with a Gray/Lt blue wire) and 43 (RH O2 with a Red/Black wire). Use the ground next to the computer to ground the voltmeter. The O2 sensor voltage should switch between .2-.9 volt at idle."
Also, did I read in another post I am supposed to use my good analog sensor for this to see if it twiches fast or slowly?
Thanks for clarifying!
Measure the end to end resistance of the same O2 sensor wire. Open circuits or poor connections can cause weird problems.
Measure the O2 sensor voltage at the computer to make sure that the computer sees the O2 sensor output. Since it switches from .2 to .9 volt, an analog meter or digital meter with a bar graph scale is the best tool to observe a fluctuating voltage.
Measure the O2 sensor voltage at the computer to make sure that the computer sees the O2 sensor output. Since it switches from .2 to .9 volt, an analog meter or digital meter with a bar graph scale is the best tool to observe a fluctuating voltage.
I finally replaced the O2 sensors based on guessed miles, not the tests, and it does not stink at idle like it did. I am anticipating better mileage based on a tank so far. Here is to a cleaner rear fascia!
