Eec-iv Issues,92 5.0 No Start Looking For Assistance

Discussion in '1979 - 1995 (Fox, SN95.0, & 2.3L) -General/Talk-' started by oxide80, Jul 29, 2014.

  1. Ok , I'm new to this site, the members here seems to be a wealth of knowledge and hopefully I can find my issue.

    1992 mustang 5.0 hb, 5spd, bone stock.

    Here's a run down of what's going on and what I've done.

    Car ran fine on a day it was pouring out.... shut it off, went to restart it 5 minutes later and it wouldn't start. Fuel pump wasn't on I assumed blown pump. I was late for work so I just parked it and took my truck.

    2 weeks later I have a few hours to tinker... tested the pump with no power, tested the fp relay power source, 12 volts at 2 wires, nothing going to pump. Replaced both FP and EEC relays. EEC relay clicking, still nothing from FP relay. Tested FP jumper at OBD1 port. FP turned on. Tried to run codes with jumper wire and KOEO test.... CEL was on dim.... No flashes. After 6-7 tries got coded for 40,60, and tps. Nothing else. CEL stays on dim except when cranking .

    I pulled the ECU, no obvious damage or corrosion. Plugged in my breakout box. Tested pin 1 had 12 v. Decided to do the grounds next. 40,60 both ground out fine.... pin 20 has no continuity to ground. Is it supposed to? I checked from pin 20 on the harness to the batter neg and it had continuity. 're- connected the ECU to the breakout box, checked continuity from pin 40 to 60, good, 20 to 40 no good, 20 to 60 no good. Jumped 20 to 60..... CEO came on bright, key on, fp kicked on. Car started....

    1week later went to tinker some more, another down pour.... found right kick panel wet..... pulled the the ECU..... it was wet inside.... so obviously I have a leak..... but with pin 20 not having ground should I assume bad ecu?
  2. Take that jumper off.

    Pin 20 goes to ground. Trace it out, its probably broken or corroded where it bolts to the car.

    20 should have cont. To 40 and 60. Nice troubleshooting.
    oxide80 likes this.
  3. Thanks for the reply! I'm going to double check the ground for Pin 20. It's not long, It's right under the ECU, prob 12" of wire. It answers my question about pin 20, being tied 40 and 60. I had good continuity from the ECU harness pins 20,40,60 to the neg battery but nothing from the breakout box on 20... this week I will double check everything.
  4. Yeah, if it got wet in there, I bet that ground corroded out, either at the connector or internal to the plastic near the connector.

    Working in the phone industry for a dozen years, I've seen the damage caused by ground + water + dissimilar metals + dc voltage.
  5. Pin 20 is the TFI shield wire ground. It does not necessarily connect to pins 40 and 60.

    Diagram courtesy of Tmoss & Stang&2birds

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

    See the following website for some help from Tmoss (diagram designer) & Stang&2Birds (website host) for help on 88-95 wiring 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 PowerWindowWiring.gif

    93 power window wiring
  6. Thanks for the diagram, question I have is I have continuity from the ECU pin 20 to bat neg. With the breakout hooked up I have no continuity to ground. It only was after I used a jumper from breakout box pin 20 to 40/60 the CEL came on bright, FP cycled and the car started. Coincident? So pin 20 only grounds the TFI?
  7. I have another diagram that shows the path, I 'll see if I can find it. Pins 40 & 60 are the main computer power ground.


    Revised 28-Oct-2012 to add signal ground description & possible problems if it is bad

    Grounds are important to any electrical system, and especially to computer controlled engines. In an automobile, the ground is the return path for power to get back to the alternator and battery.

    1.) The main power ground is from engine block to battery: it is the power ground for the starter & alternator.

    2.) The secondary power ground is between the back of the intake manifold and the driver's side firewall. It is often missing or loose. It supplies ground for the alternator, A/C compressor clutch and other electrical accessories such as the gauges. The clue to a bad ground here is that the temp gauge goes up as you add electrical load such as heater, lights and A/C.

    Any car that has a 3G or high output current alternator needs a 4 gauge ground wire running from the block to the chassis ground where the battery pigtail ground connects. The 3G has a 130 amp capacity, so you wire the power side with 4 gauge wire. It stands to reason that the ground side handles just as much current, so it needs to be 4 gauge too.

    The picture shows the common ground point for the battery , computer, & extra 3G alternator ground wire as described above in paragraph 2. A screwdriver points to the bolt that is the common ground point.

    The battery common ground is a 10 gauge pigtail with the computer ground attached to it.
    Picture courtesy timewarped1972

    Correct negative battery ground cable.

    3.) The computer's main power ground (the one that comes from the battery ground wire) uses pins 40 & 60 for all the things it controls internally: it comes off the ground pigtail on the battery ground wire. Due to its proximity to the battery, it may become corroded by acid fumes from the battery.
    In 86-90 model cars, it is a black cylinder about 2 1/2" long by 1" diameter with a black/lt green wire.
    In 91-95 model cars it is a black cylinder about 2 1/2" long by 1" diameter with a black/white wire.
    You'll find it up next to the starter solenoid where the wire goes into the wiring harness.

    All the grounds listed in items 1,2 & 3 need to bolt to clean, shiny bare metal. A wire brush or some fine sandpaper is the best thing to use to clean the ground connections.

    4.) All the sensors have a common separate signal ground. This includes the TPS, ACT, EGR, BAP, & VSS sensors. This ground is inside the computer and connects pin 46 to pins 40 & 60, which are the main computer grounds. If this internal computer ground gets damaged, you won't be able to dump codes and the car will have idle/stall/ performance problems

    5.) The O2 sensor heaters have their own ground (HEGO ground) coming from the computer. This is different and separate from the O2 sensor ground. It is an orange wire with a ring terminal on it. It is located in the fuel injector wiring harness and comes out under the throttle body. It gets connected to a manifold or bolt on back of the cylinder head.

    6.) The TFI module has 2 grounds: one for the foil shield around the wires and another for the module itself. The TFI module ground terminates inside the computer.

    7.) The computer takes the shield ground for the TFI module and runs it from pin 20 to the chassis near the computer.

    See for help for help troubleshooting voltage drops across connections and components. Be sure to have the maximum load on a circuit when testing voltage drops across connections. As current across a defective or weak connection, increases so does the voltage drop. A circuit or connection may check out good with no load or minimal load, but show up bad under maximum load conditions. .

    Voltage drops should not exceed the following:
    200 mV Wire or cable
    300 mV Switch
    100 mV Ground
    0 mV to <50 mV Sensor Connections
    0.0V bolt together connections


    Extra grounds are like the reserve parachute for a sky diver. If the main one fails, there is always your reserve.

    The best plan is to have all the grounds meet at one central spot and connect together there. That eliminates any voltage drops from grounds connected at different places. A voltage drop between the computer ground and the alternator power ground will effectively reduce the voltage available to the computer by the amount of the drop.
  8. b]Diagrams courtesy of Tmoss & Stang&2birds[/b]

    The diagram below shows pin 20 as case and TFI shield wiring ground. Its primary purpose is the shield the computer and TFI wiring from electromagnetic noise & RFI.

    Complete computer, actuator & sensor wiring diagram for 88-91 Mass Air Mustangs
  9. Thank jrichker for the diagrams, a lot of help. Obviously the ecu is sensitive to power input and grounds in order for everything to work right. I seem to have a ground issue, now its just determining if its internal grounding or external wiring harness ground. KOEO I get no FP circuit to trigger for a few seconds. But if I jump it on the OBD1 connection it will run. Is it common for the FP trigger circuit to burn up or short out? I did find a little water in the ecu case this last bout of rain... should I assume ecu and try a new one?
  10. I respectfully disagree, it should have a continuous path due to both being grounded. All three pins should have 0 ohms between them.
  11. I dont think so, because you basically created a ground for pin 20 when you jumped it to ping 40 (or was it 60) and all functions, by your account, restored.
  12. @modulistic

    There is no physical connection inside the computer for pins 20 and the connected pair of pins 40 & 60. Pin 20 is a shield/case RFI ground and does not carry power.

    Once you bolt the braid (pin 20) on the wiring harness to body ground and connect pins 40 & 60 (the quick connect to the battery negative cable), there will be a common point which is ground and should show about 0 ohms on a common ohmmeter/multimeter.

    If pins 40 & 60 are open circuited due to bad or missing connection (the quick connect on the battery cable corroded, damaged or missing), and you jumper pin 20 to pins 40 or 60, you now have a power ground for the computer. It will be an electrically inefficient one, but a ground never the less.

    Good engineering dictates that you never connect a shield ground to a power ground that has any length of wire. The voltage drop across the length of the wire and connectors used for the power ground decreases the efficiency of the shield ground.

    See http://atmelcorporation.wordpress.c...ground-common-shield-and-power-supply-return/ for more explanation on the types of grounds
    #12 jrichker, Jul 31, 2014
    Last edited: Jul 31, 2014
  13. roger that. I was thinking he was testing through the harness, which should have a path to ground through the chassis. with computer on a bench, there should be no continuity through pins 20 to 40 or 60
  14. The computer is highly suspect at this point . Let's try the fuel pump troubleshooter test path to be sure.

    Fuel Pump Troubleshooting for 91-93 Mustangs

    Revised 08-Dec-2012 to add check for 12 volts at ignition coil to prove the ignition switch is good.

    Clue – listen for the fuel pump to prime when you first turn the ignition switch on.
    It should run for 2-5 seconds and shut off. To trick the fuel pump into running, find the ECC test connector and jump the connector in the upper RH corner to ground.




    If the fuse links are OK, you will have power to the pump. Check fuel pressure – remove the cap from the Schrader valve behind the alternator and depress the core. Fuel should squirt out, catch it in a rag. A tire pressure gauge can also be used if you have one - look for 37-40 PSI. Beware of fire hazard when you do this.

    No fuel pressure, possible failed items in order of their probability:
    A.) Tripped inertia switch – press reset button on the inertia switch. The hatch cars hide it under the plastic trim covering the driver's side taillight. Use the voltmeter or test light to make sure you have power to both sides of the switch

    B.) Fuel pump Relay:
    On 91 cars, it is located under the driver's seat.
    On 92 and 93 cars it is located under the MAF. Be careful not to confuse it with the A/C WOT cutoff relay which is in the same area. See the diagram to help identify the fuel pump relay wiring colors.
    Be sure to closely check the condition of the relay, wiring & socket for corrosion and damage.
    C.) Clogged fuel filter
    D.) Failed fuel pump
    E.) Blown fuse link in wiring harness.
    F.) Fuel pressure regulator failed. Remove vacuum line from regulator and inspect
    for fuel escaping while pump is running.

    Theory of operation:
    Read this section through several times. If you understand the theory of operation, this will be much easier to troubleshoot. Refer to the diagram below frequently.

    Diagram of the fuel pump wiring for 91-93 cars.

    The electrical circuit for the fuel pump has two paths, a control path and a power

    Remember that the computer does not source any power to actuators, relays or injectors, but provides the ground necessary to complete the circuit. That means one side of the circuit will always be hot, and the other side will go to ground or below 1 volt as the computer switches on that circuit.

    The control path consists of the computer, and the fuel pump relay coil. It turns the fuel pump relay on or off under computer control. The switched power (red wire) from the ECC relay goes to the relay coil and then from the relay coil to the computer (light blue\orange wire). The computer provides the ground path to
    complete the circuit. This ground causes the relay coil to energize and close the contacts for the power path. Keep in mind that you can have voltage to all the right places, but the computer must provide a ground. If there is no ground, the relay will not close the power contacts.

    The power path picks up from a fuse link near the starter relay. Fuse links are like fuses, except they are pieces of wire and are made right into the wiring harness. The feed wire from the fuse link (pink/black wire) goes to the fuel pump relay contacts. When the contacts close because the relay energizes, the power flows
    through the pink/black wire to the contacts and through the dark green\yellow wire to the inertia switch. The other side of the inertia switch with the brown\pink wire joins the pink/black wire that connects to the fuel pump. The fuel pump has a black wire that supplies the ground to complete the circuit.

    Power path:
    Power feed: Look for 12 volts at the pink/black wire (power source for fuel pump relay).
    No voltage or low voltage, bad fuse link, bad wiring, or connections. Remember that on 92 or later models the fuel pump relay is located under the Mass Air meter. Watch out for the WOT A/C control relay on these cars, as it is located in the same place and can easily be mistaken for the fuel pump relay.

    Relay: Turn on the key and jumper the ECC test connector as previously described. Look for 12 volts at the dark green\yellow wire (relay controlled power for the fuel pump). No voltage there means that the relay has failed, or there is a broken wire in the relay control circuit.

    Inertia switch:
    The location for the inertia switch is under the plastic for the driver's side taillight.
    There should be a round plastic pop out cover over it, remove it to access the switch button.
    With the test connection jumpered and ignition switch in The Run position as described above, check the brown/pink wire. It should have 12 volts. No 12 volts there, either the inertia switch is open or has no power to it. Check both sides of the inertia switch: there should be power on the dark green\yellow (inertia switch input) and brown/pink wire (inertia switch output). Power on the dark green\yellow wire and not on the brown/pink wire means the inertia switch is open.
    Press on the red plunger to reset it to the closed position. Sometimes the inertia switch will be intermittent or will not pass full power. Be sure that there is 12 volts on both sides of the switch with the pump running and that the voltage drop measured across the switch is less than .75 volts.

    Pump wiring: Anytime the ignition switch is in the Run position and the test point is jumpered to ground, there should be at least 12 volts present on the black/pink wire. With power off, check the pump ground: you should see less than 1 ohm between the black wire and chassis ground.

    Make sure that the power is off the circuit before making any resistance checks.
    If the circuit is powered up, your resistance measurements will be inaccurate.


    Control path:
    Relay: The red wire for the fuel pump relay coil gets its power feed from the ECC relay.
    No 12 volts here, and the ECC relay has failed or there is bad wiring or bad connections coming from it. The ECC relay is located on top of the computer, which is under the passenger’s side kick panel. It is not easy to get to, you must have small hands or pull the passenger side dash speaker out to access it.

    Another possibility is that the ignition switch is faulty: look for 12 volts on the red/green wire on the ignition coil. No 12 volts with the ignition switch in the Run position and the ignition switch is faulty or the fuse link in the ignition wiring is blown.

    Relay: The light blue/orange wire provides a ground path for the relay power. With the test connector jumpered according to the previous instructions, there should be less than .75 volts.
    Use a test lamp with one side connected to battery power and the other side to the light blue/orange wire on the fuel pump relay. The test light should glow brightly. No glow and you have a broken wire or bad connection between the test connector and the relay. To test the wiring from the computer, remove the passenger side kick panel and disconnect the computer connector. It has a 10 MM bolt that holds it in place. Remove the test jumper from the ECC test connector.
    With the test lamp connected to power, jumper pin 22 to ground and the test lamp should glow.
    No glow and the wiring between the computer and the fuel pump relay is bad.

    Computer: If you got this far and everything else checked out good, the computer is suspect.
    Remove the test jumper from the ECC test connector located under the hood. Probe computer pin 22 with a safety pin and ground it to chassis. Make sure the computer and everything else is connected. Turn the ignition switch to the Run position and observe the fuel pressure. The pump should run at full pressure.
    If it doesn't, the wiring between pin 22 on the computer and the fuel pump relay is bad.
    If it does run at full pressure, the computer may have failed.

    Keep in mind that the computer only runs the fuel pump for about 2-3 seconds when you turn the key to the Run position. This can sometimes fool you into thinking the computer has died.
    Connect one lead of the test light to power and the other lead to computer pin 22 with a safety pin.
    With the ignition switch Off, jumper the computer into self test mode like you are going to dump the codes. Turn the ignition switch to the Run position. The light will flicker when the computer does the self test routine. A flickering light is a good computer. No flickering light is a bad computer. Remove the test jumper from the ECC test connector located under the hood.

    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

    Fuel pump runs continuously:
    The fuel pump relay contacts are stuck together or the light blue/orange wire has shorted to ground. Remove the fuel pump relay from its socket. Then disconnect the computer and use an ohmmeter to check out the resistance between the light blue/orange wire and ground. You should see more than 10 K Ohms (10,000 ohms) or an infinite open circuit. Be sure that the test connector isn’t jumpered to ground.
    If the wiring checks out good, then the computer is the likely culprit.

    Prior to replacing the computer, check the computer power ground. The computer has its own dedicated power ground that comes off the ground pigtail on the battery ground wire. Due to it's proximity to the battery, it may become corroded by acid fumes from the battery. It is a black cylinder about 2 1/2" long by 1" diameter with a black/lt green wire. You'll find it up next to the starter solenoid where the wire goes into the wiring harness.

  15. Thanks both of you for the help, the pin 20 tied to 40/60 answered my question. Last thing to check will be ecu ground and power.. just clean them up I think I had .3-6 ohms but can't remember at this point. Only takes a few minutes anyhow. Its going to be a few days before I get back to the car but I will post what else I find. Thanks again
  16. thanks for the help, ended up being a bad ecu