89 Fox Wiring Issue (help)

Discussion in 'Fox 5.0 Mustang Tech' started by Thomas Presley, Aug 23, 2013.

  1. Hi fellow stang lovers, Im new to this whole forum thing and trying my best to navigate through site. Hope in in the correct area for my issue. So I've been having some nightmarish issues with my wiring for a few weeks now and I'm counting more grey hairs on my head every day trying to figure this out on my own and would greatly appreciate some guidance with my crisis.

    Ok so I've got an 89 gt with unaltered wiring harness, and I believe Ive narrowed it down to wiring in the steering collumn by accident. Whats happening is when I try to pull codes with koeo from the self test connector(while engine is having idle issues) I have no problem doing that. My test light is green when probed to the check enging light wire, and gator clip connected to positive lead on battery, and off we go with codes.

    Well all of the sudden after having only tinkered with my guage cluster and surrounding trim around steering collumn, my car runs and sounds perfect. The only problem is that now I am unable to pull codes using exact technique as before. As soon as I turn key my green light goes out and needless to say again, but no codes. Could I have a bad printed circuit board on guage cluster?

    If anyone can take this info and point me in the right direction(wires to inspect first), I would be happier than an oppossum eating s**t on a frosty morning. Im not exactly a wiring troubleshooting fan but I'm learning against my will. So any help that may prevent me from spending days tracking down what is probably a simple fix, would work for me.

    I can give more info if needed, but figured I would start here. Thanks in advance.
    #1 Thomas Presley, Aug 23, 2013
    Last edited: Aug 23, 2013
  2. Are you using the check engine light to dump codes or a test lamp? Does the Check Engine Light (CEL) come on when you first turn the ignition switch to run when you start the car?

    At the test connector is a Tan wire that connects to the check engine light and the computer. You use it with the dump codes for the 86-88 test description. That gets you past having a burned out CEL bulb or bad wiring in the cluster. See below for more help.

    Dump the codes: Codes may be present even if the Check Engine Light (CEL) isn't on.

    Dumping the computer diagnostic codes on 86-95 Mustangs

    Revised 26-July-2011. Added need to make sure the clutch is pressed when dumping codes.

    Codes may be present even if the check engine light hasn’t come on, so be sure to check for them.

    Here's the way to dump the computer codes with only a jumper wire or paper clip and the check engine light, or test light or voltmeter. I’ve used it for years, and it works great. You watch the flashing test lamp or Check Engine Light and count the flashes.

    Post the codes you get and I will post 86-93 model 5.0 Mustang specific code definitions and fixes. I do not have a complete listing for 94-95 model 5.0 Mustangs at this time.

    Be sure to turn off the A/C, and put the transmission in neutral when dumping the codes. On a manual transmission car, be sure to press the clutch to the floor.
    Fail to do this and you will generate a code 67 and not be able to dump the Engine Running codes.



    If your car is an 86-88 stang, you'll have to use the test lamp or voltmeter method. There is no functional check engine light on the 86-88's except possibly the Cali Mass Air cars.


    The STI has a gray connector shell and a white/red wire. It comes from the same bundle of wires as the self test connector.

    89 through 95 cars have a working Check Engine light. Watch it instead of using a test lamp.


    The STI has a gray connector shell and a white/red wire. It comes from the same bundle of wires as the self test connector.

    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.

    What to expect:
    You should get a code 11 (two single flashes in succession). This says that the computer's internal workings are OK, and that the wiring to put the computer into diagnostic mode is good. No code 11 and you have some wiring problems. This is crucial: the same wire that provides the ground to dump the codes provides signal ground for the TPS, EGR, ACT and Map/Baro sensors. If it fails, you will have poor performance, economy and driveablity problems

    Some codes have different answers if the engine is running from the answers that it has when the engine isn't running. It helps a lot to know if you had the engine running when you ran the test.

    Dumping the Engine Running codes: The procedure is the same, you start the engine with the test jumper in place. Be sure the A/C is off, and clutch (if present) is pressed to the floor, and the transmission is in neutral. You'll get an 11, then a 4 and the engine will speed up to do the EGR test. After the engine speed decreases back to idle, it will dump the engine running codes.

    Trouble codes are either 2 digit or 3 digit, there are no cars that use both 2 digit codes and 3 digit codes.

    Your 86-88 5.0 won't have a working Check Engine Light, so you'll need a test light.
    See AutoZone Part Number: 25886 , $10

    Alternate methods:
    For those who are intimidated by all the wires & connections, see Actron® for what a typical hand scanner looks like. Normal retail price is about $30 or so at AutoZone or Wal-Mart.

    Or for a nicer scanner see Equus - Digital Ford Code Reader (3145) – It has a 3 digit LCD display so that you don’t have to count flashes or beeps.. Cost is $30-$36.
    #2 jrichker, Aug 23, 2013
    Last edited: Aug 23, 2013
  3. I am not using the check engine light to dump codes as it will not flash codes on the instrument panel anymore. It does however come on momentarily when the key is turned to the on position. I am using a test light to get codes at this time due to both of my code readers having been fried diagnosing this problem. Yesterday I was capable of pulling codes in which I received 11 with koeo, and 94 and 44 with koer which is emissions related and I am aware this will not affect drivability so not too concerned with these particular codes. It appears that my IAC, and timing advance are not responding 100% of the time in which I experience an erratic idle. I do know that I have a burnt trace on ecu to my check engine light, but I tested for continuity past the burnt spot and all seems fine so I looked past this considering I can dump codes with my light at self test connector. I did blow fuse 18 in the beginning of all of this but replaced fuse and I cannot seem to replicate this. I have taken the entire dash harness and engine harness apart and inspected the wires for damage such as burnt wiring(short), only to find nothing of that sort, so I wrapped it up and moved on. I just cannot seem to get a consistant fault, im chasing my tail here. It could just be that me and my car have been together so long that now she feels that she can screw with me, but has taken this way too far. Ive been reading literature for two months now and cannot pinpoint this problem. I have fixed numerous issues in wiring in which I felt could pose a potential issue, and the car would perform properly and leave me believing I solved the issue, only to shortly go back to its strange behavior. I prey it is something simple that I am just overlooking, which very well may be the case considering the amount of info I have absorbed during these past two months. This car has virtually been problem free for the past 23 years till now.

    If it may help I would be willing to post a video of what is going on and how it is acting. Whatever it takes at this point.
    Sorry for any useless info here, but I just may be able to write a book on my ventures with this nightmare and just want it to be over.
  4. yes, the trace looks identical to this one, only its on the #17 pin on ecm. Could this cause issues even if multimeter shows 0.o4 during resistance check?
  5. Your computer (or you) has burned up the trace and possibly the circuitry to make the check engine light work. See the diagram below and trace pin 17 - its' a tan wire and the wire function is STO/LMIL

    Diagram courtesy of Tmoss & Stang&2birds


    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/ Everyone should bookmark this site.

    Complete computer, actuator & sensor wiring diagram for 91-93 Mass Air Mustangs

    Complete computer, actuator & sensor wiring diagram for 88-91 Mass Air Mustangs

    Ignition switch wiring

    Fuel, alternator, A/C and ignition wiring

    Vacuum diagram 89-93 Mustangs

    HVAC vacuum diagram

    TFI module differences & pin out

    Fuse box layout

    87-92 power window wiring
    http://www.veryuseful.com/mustang/tech/engine/images/mustang87-92 PowerWindowWiring.gif

    93 power window wiring
  6. Ok, I have swapped my computer for a know good computer, what ever my wiring problem is, has toasted a handful of air charge temp sensors. Does anyone know what could cause this. My engine bay seems to be a lot hotter than normal only after underdrive pully install. I am going to resort back to a 165 degree thermostat, hopefully this will keep it cooler.
  7. The underdrive pulleys don't work well on a hot environment like Ft. Worth. You better have a 3 core or super radiator to make it work. The lower temp thermostat isn't going to work well with a computer that expects to see 195 degree engine coolant temps as a regular operating mode.
  8. Ok, I will leave the 195 in there, and I do have a 3 core radiator.
    So what are your thoughts on my ACT sensor. Where should I start troubleshooting?
  9. I do now have a working check engine light with the new computer, also this wiring issue causes my IAC not to work sometimes. I have checked the circuit with no luck. Apparently it is working properly when I check it.
  10. i have replaced my IAC 3 times, BWD product, and the car achieves a steady and smooth idle for about a week or less. tonite, i didn't need to put my foot on the pedal as the RPMs were hanging up around 2200, but this morning, car ran well. i read somewhere that the Motorcraft brand may be worth the extra 30 bucks- is it possible i am 'blowing out' these IAC's? i have checked the voltage and i have no codes or engine light. thankyou.
  11. It looks like from TMoss's drawing you only have two wires to the ACT. One wire goes to pin 25 in the ECU light green \ pink. The other wire is the signal return ground. #46 black \ white. The ECT, ACT, Baro Sensor, TP sensor and egr valve position sensor all share the #46 wire.

    Its difficult for me to picture how there is a wiring problem in that circuit that would only be affecting the one sensor. I would think due to the signal return it would affect more than one. The ecu could be sending something bad out the 25 pin, but I think its unlikely. It would have to be really bad, and it would still feed back through the signal return until it physically melted the resistor inside the ACT and broke the circuit, or shorts its to the engine ground. Then you would probably see a blown fuse somewhere. I just don't see it.

    Take your time and slow down with the wiring. Maybe buy a new harness, and don't take this one apart. Be gentle with it.
  12. Its possible the light green \ pink # 25 is shorting to ground somewhere. I don't think it would fry the sensor, maybe just pull down the signal to 25.
  13. You guys with idle/stall problems could save a lot of time chasing your tails if you would go through the Surging Idle Checklist. Over 50 different people contributed information to it. The first two posts have all the fixes, and steps through the how to find and fix your idle problems without spending a lot of time and money. I continue to update it as more people post fixes or ask questions. You can post questions to that sticky and have your name and idle problem recognized. The guys with original problems and fixes get their posts added to the main fix. :D

    It's free, I don't get anything for the use of it except knowing I helped a fellow Mustang enthusiast with his car. At last check, it had more than 134,000 hits, which indicates it does help fix idle problems quickly and inexpensively.
  14. Ok, thanks. I unplugged it to drive home tonite and idle stayed where i wanted; no hangups, no stalling.