No CEL

Tancalhan

Member
Apr 11, 2020
30
2
18
19
Frederic WI
So I recently pulled the upper and lower intake off my 89 Mustang GT. 62K original miles, 5.0 5 speed. It was leaking oil out of the back China wall so I fixed that. While I had everything apart, I cleaned everything - IAC, EGR, TB, fuel rail, rebuilt the injectors, ETC. After reinstalling everything, I have a high rpm misfire but it’s not a mis, it’s like the engine completely dies. Before I did all this, I replaced the plugs, wires, cap, and rotor with motor craft parts. Except the wires, but those are ford racing. I also took the TFI off and replaced the thermal compound with new, arctic silver compound. I was trying to read the codes to see if there was any issue, and I have no check engine light. Yes it’s an 89 so it’s a mass air car, I’ve seen the CEL on before, with the key set to the On position, but now it doesn’t even turn off. Could the bulb have burnt out or am I missing something? Thanks!
 
  • Sponsors(?)


Tancalhan

Member
Apr 11, 2020
30
2
18
19
Frederic WI
*edit* I should add that when I add the jumper wire from the STI to the black/white wire, the car starts, idles to 1500 rpms and then dies. Exhaust smells rich when that happens as well.
 

HemiRick

I'd be looking at jacking under the house
Jun 28, 2020
764
288
73
58
Memphis TN
Does the CEL come on w the key on-no start ? Then go out after start? Or stay on after start? your post is not clear.
 

Tancalhan

Member
Apr 11, 2020
30
2
18
19
Frederic WI
Sorry, I forgot to mention that clearly. I have no check engine light. Doesn’t come on at all, it used to though. Not sure when it stopped.
 

Mustang5L5

Put lubricant all over the balls
Mod Dude
Feb 18, 2001
36,431
12,338
224
Massachusetts
Sorry, I forgot to mention that clearly. I have no check engine light. Doesn’t come on at all, it used to though. Not sure when it stopped.


Most likely the bulb is burned out. You can still check the codes using a code reader or a test bulb. You just won't be able to use the dash bulb to count blinks until you replace the bulb.

This is a good reader to use that many of us have.
Amazon productView: https://www.amazon.com/INNOVA-3145-Ford-Digital-Reader/dp/B000EW0KHW/ref=asc_df_B000EW0KHW?tag=bingshoppinga-20&linkCode=df0&hvadid=80195681205772&hvnetw=o&hvqmt=e&hvbmt=be&hvdev=c&hvlocint=&hvlocphy=&hvtargid=pla-4583795269552441&psc=1
 

General karthief

wonder how much it would cost to ship you a pair
Mod Dude
Aug 25, 2016
20,171
6,819
203
polk county florida
Ran fine before you tore it apart?
Go back over your work, focus on electrical connections.
Make sure the grounds are all attached, there are about 4 or 5
 

Tancalhan

Member
Apr 11, 2020
30
2
18
19
Frederic WI
Ran fine before you tore it apart?
Go back over your work, focus on electrical connections.
Make sure the grounds are all attached, there are about 4 or 5
Yes it ran great. I double checked everything. Every ground, every vacuum line, everything is good. I bought that code reader and I had no codes. It just drives differently. Random misfire, random stumble, almost like it loses fuel pressure sporadically. I checked the regulator, and that was good. Atleast, with it running I pulled the vacuum line off and no fuel came out so I’m assuming it’s good. I checked the timing and it’s at 12°. I’m at a loss.
 

jrichker

StangNet's favorite TOOL
SN Certified Technician
Mar 10, 2000
27,441
2,772
234
75
Dublin GA
lowendmac.com
Yes it ran great. I double checked everything. Every ground, every vacuum line, everything is good. I bought that code reader and I had no codes. It just drives differently. Random misfire, random stumble, almost like it loses fuel pressure sporadically. I checked the regulator, and that was good. Atleast, with it running I pulled the vacuum line off and no fuel came out so I’m assuming it’s good. I checked the timing and it’s at 12°. I’m at a loss.
Computer will not go into diagnostic mode on 86-90 models 5.0 Mustangs .

Revised 23 May 2020 to include warning about jumpering the dark brown connector with a black/orange wire.(12 volts) to the test connector.

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 any signal ground on the engine mounted fuel injector harness with the battery pigtail ground as the other reference point for the ohmmeter probe.

Engine mounted fuel injector wiring harness sensors for a 5.0 mustang
63347.gif


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.

WARNING!!! There is a single dark brown connector with a black/orange wire. It is the 12 volt power to the under the hood light. Do not jumper it to the computer test connector. If you do, you will damage the computer.

OR

If an O2 sensor harness from an automatic transmission Mustang is used with an A9L manual shift transmission computer. The 12 volts from the automatic transmission starter circuit will damage the A9L computer.

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.

attachments\58312


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 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.

While you have the computer connector disconnected from the computer, turn the ignition switch to the Start position and look for 12 volts on pin 46 of the computer wiring harness. If you see 12 volts then you have an automatic transmission O2 sensor harness. That will damage the A9L manual shift transmission 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.

20160325_234845-jpg.553347


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 1.5 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.
eec-iv-computer-connector-for-5-0-mustang-gif.gif


a9x-series-computer-connector-wire-side-view-gif.gif


Diagram courtesy of Tmoss & Stang&2birds

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

88-91_5.0_EEC_Wiring_Diagram.gif


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

See the graphic for the 10 pin connector circuit layout.
?temp_hash=3ef2497fff29a7a9daee955cf93e5805.jpg