Check Engine Light??

Foxfirst846 · Jan 24, 2017

Hello Everyone!! New to the forums here but have been getting advice for past month or so since I bought my 93 fox body. I have a question about my check engine light.. I know that it's the source to check codes through the computer. My problem is it's staying on constantly.. I've tried using a jumper paper clip under hood, disconnecting the battery for half hour... Still nothing flashing. It just is solid from time key comes on. Any advice is appreciated as I'm trying to fix some backyard mechanics mistakes where they obviously cut corners and I wanna do it the right way!! Thanks in advance!!

Mustang5L5 · Jan 24, 2017

My preferred method for reading codes on these cars.
Amazon product ASIN B000EW0KHWView: https://www.amazon.com/INNOVA-3145-Ford-Digital-Reader/dp/B000EW0KHW

What sort of modifications does the car have? Anything such as modified or non-stock wiring? Are you probing the correct pins with the paper clip to trigger the ECU to begin dumping codes?

General karthief · Jan 24, 2017

There is a sticky thing in the tech section for dumping codes, let us know what you get. read it carefully there is a connection you don't want to use.

DSB88GT · Jan 24, 2017

karthief said:
There is a sticky thing in the tech section for dumping codes, let us know what you get. read it carefully there is a connection you don't want to use.

Yes...be VERY careful. I think it's the one with the brown wire, but check to make sure.

Foxfirst846 · Jan 24, 2017

Thanks for advice! I am probing the right pins. I don't know what all modifications they've done. I believe it has aftermarket cam I know it has edelbrock intake BBk headers and h pipe and a larger throttle body.. I was going to buy the scanner. But will it work if the lights just staying on? I'll look for sticky to read.

Foxfirst846 · Jan 24, 2017

Thanks for advice! I am probing the right pins. I don't know what all modifications they've done. I believe it has aftermarket cam I know it has edelbrock intake BBk headers and h pipe and a larger throttle body.. I was going to buy the scanner. But will it work if the lights just staying on? I'll look for sticky to read.

Mustang5L5 · Jan 24, 2017

i believe @jrichker has a tech memo for this

JD1964 · Jan 24, 2017

Mustang5L5 said:
My preferred method for reading codes on these cars.
Amazon product ASIN B000EW0KHWView: https://www.amazon.com/INNOVA-3145-Ford-Digital-Reader/dp/B000EW0KHW

I have this scanner. Worth every penny and not even expensive.

General karthief · Jan 24, 2017

I'm thinking you went through the procedure out lined in the dump the codes sticky. I believe when the check engine light is on steady it is in limp mode, but not sure.
Questions:
Is this an original V8 car, your title my show this or check the door jam for the specification sticker and decode it. You may be able to go to decodethis.com and enter your vin
Is it a trans swap, stick to auto or auto to stick
List known mods or alterations by you or previous owner

jrichker · Jan 24, 2017

Where you are at now is a two step process....

Step 1.) Dump codes sticky

Look at the top of the 5.0 Tech forum where the sticky threads are posted. One of them is how to dump the computer codes. Codes may be present even if the CEL (Check Engine Light) isn’t on. You don’t need a code reader or scanner – all you need is a paper clip, or if your lady friend has a hair pin, that will do the job.
I highly suggest that you read it and follow the instructions to dump the codes. http://www.stangnet.com/mustang-forums/threads/how-to-pull-codes-from-eec4.889006/

Step 2.) Either post the codes you got and I will try to give you the 5.0 Fox Mustang specific definitions and fixes

OR

Go to step 3.)
Computer will not go into diagnostic mode on 91-95 model 5.0 Mustangs

Revised 7-June-2014 to change resistance figures to wiring checks

How it is supposed to work:
The grey/red 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. As long as you are successful dumping the codes by using the gray/red wire on the diagnostic connector for the ground when dumping, the computer’s internal ground on pin 46 is good.

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 ohm 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 grey/red 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 (grey/red wire) on the self- test connector and battery ground. You should see less than 1 ohm.

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 grey/red wire and pin 46 on the computer wiring connector: it should be less than 1 ohm. More than 1 ohm is a wiring problem. If it reads 1 ohm 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 ohm. More than that and the computer’s internal ground has failed, and the computer needs to be repaired or replaced.

See http://www.stangnet.com/mustang-forums/749974-computer-issue.html#post7490537 for Joel5.0’s fix for the computer internal signal ground.

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 than 1 ohms is a wiring problem

The following is a view from the computer side of the computer wiring connector: it is for an A9L, A9P computer.

Diagram courtesy of Tmoss & Stang&2birds

Check out the diagram and notice all the places the grey/red wire goes. Almost every sensor on the engine except the MAF is connected to it.

91-93 5.0 Mustangs

Complete computer, actuator & sensor wiring diagram for 94-95 Mass Air Mustangs

See the graphic for the 10 pin connector circuit layout.

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

Foxfirst846 · Jan 24, 2017

karthief said:
Is this an original V8 car, your title my show this or check the door jam for the specification sticker and decode it. You may be able to go to decodethis.com and enter your vin
Is it a trans swap, stick to auto or auto to stick
List known mods or alterations by you or previous owner

This is an original GT V-8 car, it's still the AOD transmission... I haven't done any modifications yet.. Trying to troubleshoot the check engine light first before doing any modifications... But I can't seem to figure it out.. Someone said I'll have to have a shop RESET the check engine light. Then it'll give me the codes.. Is there another way to reset the light? I've tried pulling the battery for 15 minutes... Soon as I turn in key the light comes on...

General karthief · Jan 24, 2017

Your 15 minute disconnected battery thing should have reset the light, was the light off for a second when you restarted the car?
First, check your grounds. There is one on the drivers side firewall that connects to the back of the head on that side and there are a couple ground wires that are attached to the fender apron by the battery.
You tried pulling the codes per the instuctions and the cel does not blink, right?

Foxfirst846 · Jan 24, 2017

karthief said:
Your 15 minute disconnected battery thing should have reset the light, was the light off for a second when you restarted the car?
First, check your grounds. There is one on the drivers side firewall that connects to the back of the head on that side and there are a couple ground wires that are attached to the fender apron by the battery.
You tried pulling the codes per the instuctions and the cel does not blink, right?

Yes I tried pulling codes with no blink.. I noticed tonight the ground strap is not hooked to back of motor. I was going to try to figure out where it's supposed to be grounded or just ground it to one of the transmission bolts on the bell housing. Does it matter where it's grounded?

General karthief · Jan 24, 2017

Well thats debatable, I would attach it to the back of the head, use a short bolt with a star washer, they have little ears that dig into the metal when tightened, or a lock washer between the head and the wire connection should work too.
If your stang has a hood light make sure its plugged onto the right connection, the gray connectpr with the red/ white wire should NOT be connected to the light for the hood.

Foxfirst846 · Jan 24, 2017

karthief said:
Well thats debatable, I would attach it to the back of the head, use a short bolt with a star washer, they have little ears that dig into the metal when tightened, or a lock washer between the head and the wire connection should work too.
If your stang has a hood light make sure its plugged onto the right connection, the gray connectpr with the red/ white wire should NOT be connected to the light for the hood.

Ok I'll try that... The connector was off of the original strap. So I purchased the copper connection from the battery area of autozone to put in there. I will try to find a spot in the back of the head to put a screw. I just didn't know if there would be a problem just screwing something into the head not knowing all about where that bolts going.. I have not noticed a hood light.. It's got a fiberglass hood that I have removed right now..

Foxfirst846 · Jan 24, 2017

karthief said:
Well thats debatable, I would attach it to the back of the head, use a short bolt with a star washer, they have little ears that dig into the metal when tightened, or a lock washer between the head and the wire connection should work too.
If your stang has a hood light make sure its plugged onto the right connection, the gray connectpr with the red/ white wire should NOT be connected to the light for the hood.

Ok I'll try that... The connector was off of the original strap. So I purchased the copper connection from the battery area of autozone to put in there. I will try to find a spot in the back of the head to put a screw. I just didn't know if there would be a problem just screwing something into the head not knowing all about where that bolts going.. I have not noticed a hood light.. It's got a fiberglass hood that I have removed right now..

Foxfirst846 · Jan 25, 2017

Does anyone have a picture of the back of heads? Or anyone that can tell me what type of bolt I should use??

General karthief · Jan 25, 2017

The back of the head is the same as the front, wait, that don't help, the fronts is covered with crap, ok follow the ground cable from the battery to the block, there should be a bolt and it should be below the oil pressure sender, unbolt it find one like it making sure its a short version, like 3/4". Clean the area around bolt hole and ground cable and reconnect. The 3/4" long bolt that matches the battery ground connection will fit a in the back of the head. Hit the zone and grab a mechanics mirror on a stick, you'll thank me later.
And one other thing, we may screw with things but we bolt stuff to an engine

Foxfirst846 · Jan 26, 2017

jrichker said:
Where you are at now is a two step process....

Step 1.) Dump codes sticky

Look at the top of the 5.0 Tech forum where the sticky threads are posted. One of them is how to dump the computer codes. Codes may be present even if the CEL (Check Engine Light) isn’t on. You don’t need a code reader or scanner – all you need is a paper clip, or if your lady friend has a hair pin, that will do the job.
I highly suggest that you read it and follow the instructions to dump the codes. http://www.stangnet.com/mustang-forums/threads/how-to-pull-codes-from-eec4.889006/

Step 2.) Either post the codes you got and I will try to give you the 5.0 Fox Mustang specific definitions and fixes

OR

Go to step 3.)
Computer will not go into diagnostic mode on 91-95 model 5.0 Mustangs

Revised 7-June-2014 to change resistance figures to wiring checks

How it is supposed to work:
The grey/red 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. As long as you are successful dumping the codes by using the gray/red wire on the diagnostic connector for the ground when dumping, the computer’s internal ground on pin 46 is good.

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 ohm 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 grey/red 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 (grey/red wire) on the self- test connector and battery ground. You should see less than 1 ohm.

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 grey/red wire and pin 46 on the computer wiring connector: it should be less than 1 ohm. More than 1 ohm is a wiring problem. If it reads 1 ohm 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 ohm. More than that and the computer’s internal ground has failed, and the computer needs to be repaired or replaced.

See http://www.stangnet.com/mustang-forums/749974-computer-issue.html#post7490537 for Joel5.0’s fix for the computer internal signal ground.

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 than 1 ohms is a wiring problem

The following is a view from the computer side of the computer wiring connector: it is for an A9L, A9P computer.

Diagram courtesy of Tmoss & Stang&2birds

Check out the diagram and notice all the places the grey/red wire goes. Almost every sensor on the engine except the MAF is connected to it.

91-93 5.0 Mustangs

Complete computer, actuator & sensor wiring diagram for 94-95 Mass Air Mustangs

See the graphic for the 10 pin connector circuit layout.

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

Hello and THANK YOU for your reply!! I've done the first ground check and it's less than 1 ohm. Now the second check I have a question about. Do I do this all inside the car on passenger floorboard? I'm not sure how else to do It other than pulling computer up to the self diagnostic area... Any advice is greatly appreciated!! I HATE electrical work...

jrichker · Jan 26, 2017

One check from the outside and you have less than 1 Ω, you are good and don't need to chase that rabbit any further.

Try dumping the codes and see what you get.

If the code dump does not work, then I would look into either borrowing a computer from a friend or acquiring another computer.

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