Help Reading CEL Light

Mhalc1 · May 27, 2025

Greetings everyone, my 1990 foxbody has been giving a CEL while accelerating around 1500-2500 rpm only. every time this has happened before i would get a tps code, i would adjust my tps and the code would go away. Now i cant seem to pull the code. I did check the tps which was showing around .80v i adjusted it to .98 and still the cel comes on while accelerating.

my KOEO codes looks like "11" and "15" my engine has always shown a "15" for as long as i have had the car.
my KOER code looks like just "4" am i reading these correctly.

I have explorer upper and lower intake as well as explorer heads, 70mm throttle body and spacer. and stock ecu with no chips. the car was swapped to 5speed but all these mods were done at least 3 years ago and the car has ran fine CEL free for quit sometime.

Can anyone ensure i am reading these codes correctly?

EDIT here are some youtube links to the videos

View: https://youtube.com/shorts/FZ9CxVxj364?feature=share

View: https://youtube.com/shorts/jI5CXd4SA20

View: https://drive.google.com/file/d/1puswBLjnopCjV3t8TMJO7bCf9LUEX3hO/view?usp=sharing

View: https://drive.google.com/file/d/1fuM1fgGcydeuna117VcIAZdkUZs4oMd8/view?usp=sharing

gkomo · May 27, 2025

Pictures don't seem to be showing correctly.

Mhalc1 · May 27, 2025

gkomo said:
Pictures don't seem to be showing correctly.

They should be video links to videos uploaded to google drive. EDIT: i just uploaded some youtube clips.

Mustang5L5 · May 27, 2025

0.80 won’t cause the ECU to throw a code. The acceptable range is 0.5-1.25volts.

You can ignore the “4 code.

Code 15 is typical of a ECU that is losing power. Are you disconnecting the battery at all? If so, you need to do a few engine cycles and then pull the codes. You have code 11 and 15 which tells me the ECU has been reset by a power cycle, or you have an old school chip on the ECU .

Mhalc1 · May 28, 2025

Mustang5L5 said:
0.80 won’t cause the ECU to throw a code. The acceptable range is 0.5-1.25volts.

You can ignore the “4 code.

Code 15 is typical of a ECU that is losing power. Are you disconnecting the battery at all? If so, you need to do a few engine cycles and then pull the codes. You have code 11 and 15 which tells me the ECU has been reset by a power cycle, or you have an old school chip on the ECU .

I am not disconnecting from the battery at all, to my knowledge I do not have a chip… the code 15 has always shown up ever since I bought the car 5 years ago.

Still wondering what this intermittent code is while accelerating… maybe the ecu can’t save it due to code 15

manicmechanic007 · May 28, 2025

.8 to 1.3v is what they always said in Ford tech school
I'd have the codes read by an actual scan tool (latch, unlatch clear codes and all that)
Paperclip does work
A 1990 does not have an adjustable TP sensor, so what have you been doing, drilling bolt holes? to adjust it?
Are you having a drivability problem or just the light?
I would plug a Ford star tester or a super star2 or a NGS into and do a running test
You'll get the key on engine off (KOEO) codes on the way there, if there is a hard fault or memory code

manicmechanic007 · May 28, 2025

I'd go through that processor connector and verify power on all the power pins
Sounds like you are losing KA power on pin1 for the code 15
That will make it try to re learn idle speed each time the power interruption happens

Mustang5L5 · May 28, 2025

Mhalc1 said:
I am not disconnecting from the battery at all, to my knowledge I do not have a chip… the code 15 has always shown up ever since I bought the car 5 years ago.

Still wondering what this intermittent code is while accelerating… maybe the ecu can’t save it due to code 15

Code 15 is no Keep-Alive Memory power to pin 1. Any time you shut the car down it's essentially wiping the ECU's memory. So you won't get that code that keeps coming on while driving because it gets stored in the Continuous Memory, and that's being wiped for some reason.

Check voltage to PIN 1 on the ECU. It should always have power.

Pin 1 is fed from a fusibile link that supplies the EEC relay. It doesn't go through the relay. Really the only way this fails is if the wire goes bad or is cut or pulls out of the connector on the ECU relay.

I wouldn't rule out an ECU issue as well.

AeroCoupe · May 28, 2025

Probably should pull the ECU out and check for the “old school chip” which will be plugged into the ECU on the opposite side of where the harness plugs into it.

Mhalc1 · May 28, 2025

manicmechanic007 said:
A 1990 does not have an adjustable TP sensor, so what have you been doing, drilling bolt holes? to adjust it?
Are you having a drivability problem or just the light?
I would plug a Ford star tester or a super star2 or a NGS into and do a running test
You'll get the key on engine off (KOEO) codes on the way there, if there is a hard fault or memory code

I have been doing the paper clip method, my TPS is an aftermarket model with a little bit of adjustment. There is no noticeable drivability issues and the CEL is intermittent. I see it only sometimes while other times i can accelerate with no CEL. i want to say it happens before the engine warms up but I keep an eye on my coolant temps and it happens around the same temp. If i cant figure this out soon i will see if there is a tech near me who has one of the diagnostic tools you mentioned.

manicmechanic007 said:
I'd go through that processor connector and verify power on all the power pins
Sounds like you are losing KA power on pin1 for the code 15
That will make it try to re learn idle speed each time the power interruption happens

Is the processor connector the same as the test port under the hood, or are you referring to a connection at the ECU?

Mustang5L5 said:
Check voltage to PIN 1 on the ECU. It should always have power.

Pin 1 is fed from a fusibile link that supplies the EEC relay. It doesn't go through the relay. Really the only way this fails is if the wire goes bad or is cut or pulls out of the connector on the ECU relay.

I wouldn't rule out an ECU issue as well.

I had a few minutes before work and checked the fuseable link G at the starter solenoid to ensure it was getting 12v there. i went to look for the EEC relay near the ECU but could not find it easily and didnt have time to really look.

AeroCoupe said:
Probably should pull the ECU out and check for the “old school chip” which will be plugged into the ECU on the opposite side of where the harness plugs into it.

Based on the replies i think your rite i should pull the ECU to check for a chip, to probe pin 1 and to locate my EEC relay.

gkomo · May 28, 2025

Mhalc1 said:
I have been doing the paper clip method, my TPS is an aftermarket model with a little bit of adjustment. There is no noticeable drivability issues and the CEL is intermittent. I see it only sometimes while other times i can accelerate with no CEL. i want to say it happens before the engine warms up but I keep an eye on my coolant temps and it happens around the same temp. If i cant figure this out soon i will see if there is a tech near me who has one of the diagnostic tools you mentioned.

Is the processor connector the same as the test port under the hood, or are you referring to a connection at the ECU?

I had a few minutes before work and checked the fuseable link G at the starter solenoid to ensure it was getting 12v there. i went to look for the EEC relay near the ECU but could not find it easily and didnt have time to really look.

Based on the replies i think your rite i should pull the ECU to check for a chip, to probe pin 1 and to locate my EEC relay.

The processor connector i believe being discussed is at or within the ECU itself behind the passenger side kick panel. There is one giant connector on the bottom of the ECU with tons of wires coming out... they are referring to if there is a chip coming off the side of the ECU.

The ECU relay (if i'm not mistaken) is right above the ECU. Everything is tight in that space. Once you remove the kick panel you may have some padding or something covering the ECU itself, move that out of the way then you'll see the ECU there, above it should be a white? relay.

Might be worth your while just to send the ECU into ECUExchange and have them go through it anyways.

AeroCoupe · May 29, 2025

This may help with figuring out what wires to back probe on the ECU harness. I stole this from one of jrichker’s posts.

Diagram courtesy of Tmoss & Stang&2birds

Computer wiring harness connector, wire side

Computer wiring harness connector, computer side

AeroCoupe · May 29, 2025

Also, picture of the ECU bracket and the ECU relay. I snagged this out of one of 95steedamustang’s posts.

Mhalc1 · May 29, 2025

AeroCoupe said:
Also, picture of the ECU bracket and the ECU relay. I snagged this out of one of 95steedamustang’s posts.

Wow thanks for both posts! I will pull the ecu out this weekend and do some tests.

Mhalc1 · Jun 8, 2025

As an update I’m working on the mustang today, and ensured that the ecu does not have a chip… it in fact still has the warranty sticker still in tact. I am getting 12v to pin 1 from the connector. I believe the ecu needs servicing, it looks pretty bad, I think I will open it up and see if it’s something I can fix as the closest ecu dude is asking $350. I also need the foxbody for my daily driving during the week as my other car is down.

Noobz347 · Jun 8, 2025

Home - ECU Exchange

ECU Exchange specializes in the re-manufacturing and repair of engine control modules.

www.theecuexchange.com

gkomo · Jun 8, 2025

Ya ECU exchange does great work and should be cheaper. However I don’t think they will turn it around quickly. They had mine for like 3 weeks or more.

Mustang5L5 · Jun 8, 2025

5-spd car with an auto A9P? You should take a peek at the o2 harness jumper and make sure it’s configured properly to match the auto ECU.

Mhalc1 · Jun 8, 2025

I just picked up an a3m ecu hopefully it will work plug and play… I will attempt to repair mine. It’s an auto ecu because I 5spd swapped my fox some years ago

Mustang5L5 · Jun 8, 2025

If you go with the A3M you will definitely need ot check the O2 harness jumper and repin it.

Help Reading CEL Light

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now i can hopefully expect to receive the shaft

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That is…until I whipped out my Bissell

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That is…until I whipped out my Bissell

lube between the nut and the face. I know my lubes

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now i can hopefully expect to receive the shaft

lube between the nut and the face. I know my lubes

lube between the nut and the face. I know my lubes

Member

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Stangnet Facilities Maint Tech... Er... Janitor

now i can hopefully expect to receive the shaft

That is…until I whipped out my Bissell

Member

That is…until I whipped out my Bissell

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