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Signal Return Circuit issue. EFI EXPERTS HELP!

20K views 17 replies 2 participants last post by  wbrockstar 
#1 · (Edited)
hey guys, Mook again. let me bring you up to speed, doing a 2.3 to 5.0 swap in my 93 fox. everything is in and the engine runs, but not well. the 5.0 originally came with a SD ecm/harness. I have since bought a 93 MAF harness and ecm to match my 93 chassis. with the SD ecm connected to the MAF harness, with no maf connected and a vacuum line run from the intake to the map sensor the car would run but would not idle well as my camshaft (e303) does not play well with SD. BUT all sensor reading were with in normal limits.

Since replacing the SD ecm with the 93 MAF ecm (remanufactured unit bought from Oreilly's) the TPS voltage shows a constant 4.98v closed and 5.01 @WOT with koeo. I thought maybe I missed a ground on the harness or possibly had something grounded to the chassis that needed powered. I have been chasing this for about 2 weeks. still no progress. As far as I can tell from diagrams and my memory of removing the wires the first time around I have not missed any grounds. Though there is one ring terminal on the harness that goes inside to the dash that I am not 100% sure if it's a ground or not, but i have been grounding it. I'll attach a picture. It is on the same harness as the yellow fuse link with the ring terminal that is connected to starting solenoid. and i noticed it has continuity with the yellow fuse link when the doors are open and the door switches arent pressed. making me think its a ground for the lights inside.

I have the injector harness ground connected to the back of the cylinder head to the same bolt that has the ground strap that goes to the firewall. Do i need to make it a separate ground?

I disconnected the ecm and tested pin 46(signal return) at the self test connector by connecting one lead on DVOM to it and the other to chassis ground. koeo voltage shows 00 which means its an open circuit and functioning normally correct?

Afterwards i tested pin 1 on MAF connector (ecm still disconnected from harness) along with pin 37 and 57 on the ecm connector all had 12v.

I then reconnected the ecm and did the koeo test again with pin 46 at the self test connector and BAM! 4.98v constant. which from my understanding it need to be around 0.5v with ecm connected and koeo.

Also I've been getting mixed information on whether or not pin 46 on the ecm itself should have continuity with pins 40 and 60. I tested mine none the less, and there is no continuity between 46 and 40 or 60. but there is continuity between 40 and 60 which is to be expected.

I would like to think its not the ecm that's bad since it is a brand new (reman) unit. And if it is bad, was it something with my harness ie a short somewhere that could have fried something inside? I have talked to orielly's and they have ordered me a new one but I'd hate to plug that in aswell and fry that one too. if it were infact something wrong with my harness.

Any thought's at all would be greatly greatly appreciated. this has been driving me nuts as it seems to be the only thing keeping me from driving this car, which has been sort of a dream project for the last 5 years.
 

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#2 · (Edited)
Do you have an extra wire attached to the right hand side starter solenoid post?? That post should only have the starter cable attached to it,but I dont think the other wire belongs there?? Look at the picture below which shows a guy pointing to the small post on the top of the solenoid then look at all of the wires attached to the left hand post on the solenoid.Do you see how that large cable,which has another wire forked off of it,is attached to the left hand post and the forked off wire is also attached to the left hand post?? In your picture,youve got the large cable attached to the left hand post,but its forked off wire is attached to the right hand post.
The post on the right only gets power when the key is turned to start while the post on the left gets power at all times.You might wanna check that to see what that wire feeds.
Do you have a 3g alternator installed?? If yes,maybe thats why youve got that wire on the other post???The last two pictures shown at the bottom are of the starter solenoid wiring & fusible links diagram.

The injector harness ground youre referring to is called the hego ground.Its an orange wire with a ring terminal on it.Its the ground for the O2 sensor's.When its loose,corroded or disconnected,it will cause o2 lean codes 41 & 91.Yes youve got it connected to a good ground.Its normally bolted to one of the rear lower intake studded bolts,but the cylinder head is fine too.

It sounds like youre using the Vref wire to test the tps instead of the Signal wire?? If youre using the Signal wire and getting 5.0 volts,the tps is bad or the Vref wire is shorted to the Signal wire.If youre testing the Vref wire and getting 5.0 volts,that indicates youre getting the correct Vref voltage,which is 5.0 volts.If youre getting 5.0 volts at the Signal Return wire,the Vref wire might be shorted to the Signal Return wire.
The VREF wire is the 5.0 volt supply wire from the ecm that feeds power to the evp,tps & bp sensors.
How it works= the ecm sends 5.0 volts to the sensor through the vref wire,then the sensor outputs a certain amount of that voltage as its signal down the signal wire,then whatever voltage is left over gets sent down the signal return wire.
((( Example=tps receives 5.0 volts from Vref,then outputs 3.0 volts as its signal voltage down the Signal wire,then the remaining 2.0 volts is sent down the Signal Return wire )))
VREF is the orange/white wire shown in the 91-93 wiring diagram at the bottom.The wire you need to test is the SIGNAL wire (grey/white)
Touch the red meter lead to it & the black lead to the SigRtn wire (black/white) Tps closed throttle operational range is between .49-1.20 volts,but anything between .70-1.00 is best.WOT voltage should be your closed throttle voltage + 2.71 volts.So if closed throttle is .80 volts,wot will be .80 + 2.71= 3.51 volts
Closed Throttle= .70-1.00 volts
WOT= 3.41-4.84 volts
Lightly tap on the tps while youre opening & closing the throttle blade,to make sure no voltage spikes or dead spots pop up on the meter.If they do,replace the tps.(((The 1st pic at the bottom= 91-93 wiring diagram)))

When you test pin 46 (Signal Return) wire for continuity between the self test connector & pin 46 @ the ecm (ecm disconnected) you need to test it for ohms,not voltage,since youve got the wire isolated from power & ground due to the fact youve got the wire disconnected at both ends (ecm disconnected & self test terminal is not connected to anything at its end) To test pin 46 wire for continuity,set the meter to read ohms,touch the meter leads to the wire at the self test connector and pin 46 at the ecm (ecm disconnected) You should get 5.0 ohms or less.
While youre there,test between the eec ground wire @ the fender apron and pin 40 then pin 60.You should also get 5.0 ohms or less.If all 3 wires pass,the external wiring for pins 40,46 & 60 are good.If you get more than 5.0 ohms,that indicates the wire is open between its two end points.
Pin 46 is the internal ecm Signal Return ground for the sensors and clutch safety switch + the ground used to trigger the koeo/koer tests.Pins 40 & 60 are also internal grounds inside the ecm.All 3 of these grounds are tied together inside the ecm.Pin 46 runs from the self test connector to the ecm then the act,ect,evp,tps & bp sensors SIGRTN wire splices into pin 46 wire.The SIGRTN wire also runs from the clutch safety switch to the SIGRTN wire.The eec ground @ the fender apron,converts from a single wire into two wires once it plugs into the main ecm harness and these are the wires that run to pin 40 & 60 @ the ecm.So yes,pin 40,46 & 60 are tied together inside the ecm.If the eec ground wire is loose/corroded or disconnected @ the fender apron,it will cause the engine to stall and the fuel pump will only run when the key is in the start position,but it'll stop running once the key is released to the run position.
((( The ground wire screwed to the passenger floorboard that runs directly to the ecm case is the ground for pin 20 )))
If you ever have a scenerio where a scanner wont trigger the code tests and a jumper wire between the STI & SIGRTN terminal also wont trigger the code tests,thats an indication the SIGRTN wire is open between the self test connector & pin 46 @ the ecm or the SIGRTN trace(pin 46) on the ecm printed circuit board is burnt/open.If its open on the ecm board,you can simply solder a piece of 32ga wire across the open SIGRTN trace and the ecm will be working like new again.BTW- when most people develop this issue they immediately go out and buy a new ecm and trade theres in as a core,not knowing that all they had to do to repair the ecm was solder in one small piece of wire and whadda you know a working ecm again!!!
If this trace is open on the board or the wire is open between where the evp and tps wire splices into the main SIGRTN wire & the ecm,the code tests wont activate.You can still trigger the tests though by running a jumper wire between the STI terminal & the neg battery post.If the SIGRTN wire or trace is open,like mentioned earlier,youll also get codes for the act,tps,bp,evp & ect sensors.
The trace is usually burnt by either using the wrong O2 sensor harness, accidentally using the 12volt hood light terminal to trigger the code test instead of using the STI terminal or somehow shorting 12volts to the signal return wire by accident or due to a chafed wire.

*RESULTS* No code test activation with a scanner or a jumper wire connected between the STI & SIGRTN terminal + codes for the sensors listed above= open SIGRTN wire or open SIGRTN ecm trace.
(((The 2nd pic= pin 40,46 & 60 wiring diagram)))

The ring terminal youre not 100% sure about,does it match wire G201 in the picture below?? If yes,thats indeed a ground.
(((The 3rd pic= starter solenoid wiring diagram)))

Yes the MAF pin 1 & pins 37 & 57 should all read 12 volts.These pins are powered by the eec relay,when the key is turned on.The red wire is referred to as the VPWR wire.This wire is shared by every component you see in the wiring diagram below, that has a red wire connected to it and every one of those components should have 12 volts during key on/engine off and key on/engine running conditions.If this wire doesnt show 12 volts,the eec relay or fusible link G (yellow) is normally to blame.


NOTE: I may come back to this and add more information later,so if you see the word "EDIT" added,I'll post the additional information below it....



**** EDIT ****

Its extremely important to run the correct O2 sensor harness with your application.Im no expert on this topic and it can get very confusing,so Im posting a very,very,very detailed link below since it will get explained in detail by a few very educated members.
You definitely dont wanna run a 5speed ecm + AOD o2 harness together.That will cause the ecm to get fried as soon as you push the clutch pedal down then turn the key to start.12 volts get sent through the signal return trace (pin 46) when using this combination & that burns the trace open on the board.You can check your ecm for this condition by removing the panel off of it and inspecting the signal return trace.The last 3 pictures shown below details an ecm with a burnt signal return trace + the same ecm after the trace has been repaired with a soldered in piece of wire + the 5speed & AOD O2 sensor harness difference.
To remedy the thing that caused the trace to burn,you can buy the correct o2 harness or simply repin the looped wire to match the application.This will be described in the link below.BTW- the thread in the link is very long,so get ready to read,but it'll explain the issue & its fix by the time youre done reading.
The fact that youve got voltage to your signal return wire may be an issue related to the thread.I dont know if 5 volts will damage the trace or if its gotta be 12 volts before it does damage,but the thread should talk about that too.I dont wanna confuse you by trying to explain the problem in how I word it,so I'll stop here so you can start reading the thread.
Dont forget to check the last 3 pictures below,if youve still got your ecm.



Oxygen sensor Harnesses - Manual/Auto differences and year differences - Ford Mustang Forums : Corral.net Mustang Forum
 

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#7 ·
To test pin 46 wire for continuity,set the meter to read ohms,touch the meter leads to the wire at the self test connector and pin 46 at the ecm (ecm disconnected) You should get 5.0 ohms or less.
While youre there,test between the eec ground wire @ the fender apron and pin 40 then pin 60.You should also get 5.0 ohms or less.
Alright, I got a chance to run by and poke around at the car after i got off work today. I set the multimeter to ohms and got a reading of 0.2 ohms on pin 46, testing from the ecm connector to the self test connector. Also connected to the main ground @ the fender apron and test pins 40 and 60 got a reading of 0.3 ohms on each one.

Now I'm not sure if this next bit of information is going to be relevant but I figured while i had the leads connected to pins 40 and 60 I'd test with the key in the on position and got a reading of -1.6ohms on pin 60 and -00.0 ohms on pin 40. I was going to check to see if there was any voltage on pins 40 and 60 while the key was turned (also not sure if that would be of any use) but i forgot.

All readings were taken with the ecm disconnected.
 
#3 ·
I do not have a 3g alternator installed, just the stock unit. There is only 1 wire connected to the right hand side of the solenoid. and it goes to the solenoid on the starter itself.

I was aware of the O2 harness compatibility issues, and have not hooked an O2 harness to this car since i've put it together, especially not since I have put the new ecm in. The O2 harness I have is from an auto car and the ecm i bought is for a manual car, so I made sure not to connect the O2 harness to the main engine harness. This ecm has never been started or ran with any O2 harness connected.

When testing the TPS I made sure not to test at the VREF wire and tested between the singal wire (green wire on stock tps), at the TPS pigtail, and chassis ground. Got 4.98v with the throttle closed
Also I may have used the wrong term in my original post, so I want to clarify. When I said I tested for continuity between pins 46, 40, and 60 on ecm itself (not the harness) what I meant was I switch the dvom to the setting that beeps if there is connectivity between the two leads. When i connected the leads to pins 40, and 60 there was a beep as expected. but when i connected the leads to pins 46 and 40 or 60 there was nothing, no beep.

I had read on a different forum that I should test the SIGRTN at the self test connector with ecm connected and koeo and i should get a reading of around 0.5v. when i did this test i got 4.98 same as when checking the signal wire on the TPS.

I will definitely try the steps you have discussed using ohms not volts, maybe the post i read the user mistyped. I'm not sure when exactly ill be able to check, as the car is not being stored at my house so it makes popping in to check a few little things a bit inconvenient since its a 30 min drive one way lol
 
#4 ·
I do not have a 3g alternator installed, just the stock unit. There is only 1 wire connected to the right hand side of the solenoid. and it goes to the solenoid on the starter itself.
You must have a mini starter?? That must be why youve got it wired that way?? I forgot about the mini starter being on the 93 model.
 
#6 ·
I just thought it was something I should point out since Im used to seeing only one large cable running from the right post to the starter then the forked off cable/wire being run to the same left hand post,like the picture I posted.Guess it makes no difference as long as it works.
I'll try to get to the other things you listed in your previous
post here shortly.
 
#8 ·
SOURCE Jrichker @ Stangnet


Disconnect the battery positive terminal before making any resistance checks.
The voltage drop in the ground cable will cause incorrect resistance readings.

How it is supposed to work:
The black/white wire (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. If this ground is bad, none of the sensors mentioned will work properly. That will severely affect the car's performance. You will have hard starting, low power and drivability problems. 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 ohms 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 the test connector black/white wire 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.

The STI (Self Test Input ) is jumpered to ground to put the computer into test mode. Jumpering it to power can produce unknown results, including damage to the computer. The ohm test simply verifies that there are no breaks in the wiring between the test connector and the computer input.

How to test the wiring :
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 ohms.

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 on the computer wiring connector: it should be less than 1.5 ohms. More that 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 that that and the computer’s internal ground has failed, and the computer needs to be repaired or replaced.

If the first ground check was good, there are other wires to check. Measure the resistance between the STI computer self test connector (red/white wire) and pin 48 on the computer main connector: it should be less than 1.5 ohms. More that 1.5 ohms is a wiring problem
 
#9 ·
You wont do any damage to the ecm as long as you dont:
1) connect jumper cables to the battery posts in reverse polarity
2) connect the signal return terminal to the 12v hood light terminal accidentally instead of the sti terminal
3) have a short between the Vpwr & Signal Return wires
4) use the wrong O2 sensor harness

I would try doing the same tests you done on the tps,to the bp sensor,since they share the vref & signal return wire.You can also do an ohms test (ecm disconnected) between the vref (pin 26) & signal return (pin 46) wire.You should get 10,000+ ohms.If you get a low reading,that indicates those pins are shorted to each other on the ecm board.

The last thing I would do is plug the ecm in and run a koeo test by connecting a jumper wire between the signal return & sti terminal.If the test wont activate,run the wire between the sti terminal & neg battery terminal then attempt to run the test again.Post the codes you get,if any.
 
#10 ·
I had another chance today to break out the multi meter and again, following the procedure you've posted, with the ecm disconnected, i got 0.2 ohms on pin 46 and 0.3 ohms on both pin 40 and 60. i then tested pin 48 at the ecm side and at the self test connector. I got a reading of 0.8 ohms. I then tested the ecm itself and got the same reading of 0.3 ohms between pins 40 and 60 but got no reading at all (dvom indicated open circuit) between pins 46, and 40 or 60.

Hopefully tomorrow I will have a chance to get back over there and test pins 26 and 46 as you've suggested.
 
#11 ·
If youre getting an open circuit reading between 46 & 40/60,that ecm needs to be inspected for an open signal return trace.If you run a koeo test before you remove the panels off the ecm,that should id whether the trace is bad or not.Codes should pop up for those sensors (EVP,BP,TPS,ACT,ECT)
If those codes are present,remove the ecm panels & start looking.

When you do test pin 26 & 46,test at both locations.Between the pins on the ecm wiring harness & the pins on the ecm itself.
 
#14 · (Edited)
I unplugged the salt/pepper connectors and the resistance between pins 26 and 46 went from 1200 to 3000ohms but never any higher. So i pulled the entire engine harness out of the car and cut all the factory tape off and went thought the dang thing. I did not find any breaks or exposed wires touching anywhere on the wires that run from pins 26 and 46. So as far as i could tell there was no short anywhere in the main harness. but according to the DVOM there must be. :(

I also tried to pull codes but it would not activate. which i didnt think it would as we have deduced there is no continuity between pins 46 and 40/60 on the ecm itself. I have not taken the ecm apart to check for burnt traces yet because it's still under warranty and I'm waiting on the new ecm to arrive. once it does, the part store will warranty it out and give me the new one.

I did notice however, on the male side of the salt/pepper connectors on the white one the pin that corresponds to V Ref (pin number 2) looks blackened/burnt compared to all the rest of the pins. I'll attach pictures.
I have not taken the injector harness off the car to check all the wires on it just yet. I am desperately hoping it's just the ecm and nothing more as originally when i had the SD ecm in the car all sensor voltages etc. came back normal.

but there is always the possibility that something to do with the harness did in fact short out the pin 46 trace when i started it up the first time. still though im hoping i just got unlucky with this reman unit. It's not at all uncommon at least in home PC electornics to get a bunk remaned unit and have to send it right back.

EDIT: I just noticed in the picture it looks like pin #9 is all blackened as well
 

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#15 · (Edited)
Yeah if sigrtn is blown,you have to install the jumper wire between the STI terminal & the neg battery post to trigger the test.If you dont have a functional check engine light,you can run a test light between the STO terminal & the pos battery post plus the jumper wire between STI & the neg battery post and the code test should begin,eventhough the signal return is blown inside the ecm.

If the test wont activate using the process I just listed,the STI (pin 48) wire is open between the test connector & ecm or the STI circuit inside the ecm is suspect.
If you can hear the ecm activating the test (relays clicking,etc) but your test light isnt blinking (to flash the codes) the STO (pin 17) wire is open between the test connector and ecm or the STO circuit inside the ecm is suspect.
STI= triggers the code test
STO= cel/mil circuit

You can also try disconnecting the sensors and testing between the pins at the white connector.Pin 9 is a ground,which is for the O2 sensor heaters.If it was to come in contact with a power wire,that could have caused it to short and turn black.

I was doing some reading and found out that the reason the tps was showing 4.98volts on the signal return wire is because the signal return voltage doesnt have anywhere to return to,if the trace is open on the board or the wire is open in the harness.



Which O2 sensor harness are you running ???

1a= SD
1b= MAF

1a= AOD
1b= T5

1b + 1a= MAF/AOD harness



*** EDIT ***
Read the following thread.It details the various O2 sensor harnesses and shows pictures of the various harness plugs and their pin locations and explains very well which one you need to run.


http://www.allfordmustangs.com/foru...blues-those-swapped-t5s-4cyl-conversions.html
 
#16 ·
Which O2 sensor harness are you running ???

1a= SD
1b= MAF

1a= AOD
1b= T5

1b + 1a= MAF/AOD harness
I am not currently running any O2 sensors or harness and have not since the engine was put in the car along with the new ecm. I was just trying to get everything buttoned up and make sure it at least ran somewhat correctly. Wasnt worried about AFR so i never hooked it up. Plus its an AOD Speed density O2 harness and my ecm is manual MAF so i never hooked them together. I am going to wait until the warrantied ecm comes in before i do any further testing. but i will update when i have the new ecm.
 
#18 ·
What a relief right?? Me included,theres no telling how many guys have went out and spent hundreds of dollars to replace the ecm,when all that was needed for repair is a piece of wire soldered across that trace.I wish I wouldve known about this fix back in the day when I had to replace my own ecm.Ive learned many things on here and sites like sbftech since becoming a member and this knowledge has definitely saved me tons of time,money and head scratching.
Glad to see you finally got it figured out. thumbsup3.gif
 
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