Check Engine Light, But No Codes

blykins

Founding Member
Aug 27, 2002
122
0
17
Louisville, KY
Hey guys,

I would appreciate any help with this as it's about to get me down.

I just completed a T5 to C4 conversion.

While in the process, I removed the clutch pedal, and jumpered both connectors that plug into the clutch pedal.

Under the car, there were two connectors as well, a 2 prong plug, and a 4 prong plug.

I currently have the red/blue and white/pink wires on the 4 prong plug routed through my neutral safety switch. The black wire and the purple/orange wires are routed through my reverse switch.

The 2 prong plug is hanging in the breeze.

I can start the car (but only when the shifter is in Park or Neutral as it should be) and the reverse lights come on when the shifter goes into reverse.

The only other electrical modification that I did was to pull the low-oil level switch out of the side of the oil pan (it was leaking like a sieve) and replace it with just a regular 22mm drain plug. The low-oil switch and wire are also at this point hanging in the breeze.

I got the car fired up and immediately heard the sound of a bad exhaust leak and the check engine light came on. I continued topping off the C4's fluid level, then shut the car off to fix the exhaust leak. The exhaust leak was bad and it caused the engine to run rich; I would attribute this to extra air flowing by the O2 sensors.

The exhaust leak was fixed, but the check engine light still comes on. I disconnected the battery, then reconnected it. About 5 seconds after the engine start, the light came back on.

I then went to Autozone and bought a code scanner. I hooked it up as instructed, turned the key on, and the check engine light still stayed lit. It never flashed, even once.

I ditched the code scanner and just used a paperclip to jump between the two diagnostic connectors, thinking that maybe I received a bad scanner. I got the same results.

I've heard a few say that I need to check the ground that went to the bellhousing bolt, but I didn't have one of those. I have a braided ground strap that goes from the firewall to the driver's side cylinder head. I do not remember disconnecting any ground straps when I pulled the T5 out.

The car idles fine and seems to take the gas well, but with road conditions here, I haven't had it out of the garage yet.

Does anyone have any ideas as to why the scanner won't bring the codes off? I've tried both KOEO and KOER, with the same results.

BTW, this is on a 1990 LX Hatchback V8.
 
  • Sponsors (?)


Computer will not go into diagnostic mode on 86-90 model 5.0 Mustangs

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


Computer diagnostic connector:

B.jpg


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.
There is a dark brown connector with a black/orange wire near the diagnostic connector. It is
the 12 volt power to the under the hood light. If this happens, it 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. Only an experienced electronics technician can open the computer up & repair the trace if
it burns up and creates an open circuit.

The STI (Self Test Input ) is jumpered to ground to put the computer into test mode. The STI
has a gray connector shell and a white/red wire 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.

attachment.php


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 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

The following is a view from the computer side of the computer wiring connector.
eec04.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 Mustang FAQ - Engine Information

harness02.gif
 
Thanks for your help.

Here's what I've accomplished so far.

I took my multimeter and measured resistance between the computer test ground on the diagnostic connector on the driver's side inner fender and the negative terminal on the battery. It was open.

I went ahead to the next step and removed the passenger side kick panel and the computer connector. I then ran a jumper wire all the way from the computer test ground on the connector on the driver's side inner fender to the passenger compartment. I then measured resistance between it and pin 46 on the computer connector.

It measured .6 ohms.

The positive battery terminal was disconnected as you suggested.

Here's where I'm thoroughly confused.

Do the wire colors always match between years?

Your black/white wire on the diagnostic connector is actually white with a red stripe on mine.

The white/red wire on the STI is actually orange with a red stripe on mine.

Further thoughts?
 
It seems that the wiring colors changed sometime near the end of the 1990 production year.
They started using the 91 harness which changed the wiring colors.

Here is the correct tech note for 91-93 or cars with a 91 and later wiring scheme.

Be sure to check the computer by measuring between pin 46 and either pin 40 or 60 on the computer itself. You should see less than 1 ohm.

Computer will not go into diagnostic mode on 91-93 model 5.0 Mustangs

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. 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 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. Only an experienced electronics technician can open
the computer up & repair the trace if it burns up and creates an open circuit.

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
1.5 ohms.

attachment.php


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
1.5 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 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

The following is a view from the computer side of the computer connector.
eec04.gif


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_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 Mustang FAQ - Engine Information

harness02.gif
 
Well I guess regardless of the color, I'm getting an open circuit between the computer test ground and the negative battery terminal, while I'm getting less than an ohm from pin 46 and that computer test ground connector.
 
Did you check the resistance of the computer itself? There should be less than 1.5 Ohm between pin 46 and either pin 40 or 60. More than 1.5 Ohm is usually a burnt signal trace inside the computer.
 
I have not done that part yet as I'm trying to see how to pull that puppy out.

The car was running perfectly before the swap. If the computer is bad, then I'm very leery about trying to run it in its current condition.
 
Remove the passenger side kick panel, loosen the 10 MM hex bolt until the computer harness unplugs from the computer. Remove the 10-32 machine screw from the lower RH side of the computer plastic mount strap. Lift up on the mount strap and then you can wiggle the computer out. It takes some effort, but it will come out.
 
Ok,

Got the computer out. Measured resistance between pins 46 and 40 and then between 46 and 60. I got an open circuit on both.

I've got a cheap multimeter, but I have it set to 20ohm range and it's doing nothing but showing a 1.

I suppose it's time for a new computer?

If so, I wonder what I've done in jumpering the existing connectors to fry a computer ground?