CODE: 31 (KOEO) - EVP circuit below minimum voltage. Vref (5 volt reference
voltage supplied by the computer) missing or broken wire or bad connection in circuit.
Use a DVM to check for 5 volts on the orange/white wire. If it is missing, look for +5
volts at the orange/white wire on the TPS or MAP sensor located on the firewall near the
center of the car. Use the black/white wire for the ground for the DVM.
With the sensor removed from the EGR and still connected, press the plunger and watch
the voltage change on the brown/lt green wire. Pull the passenger side kick panel and
measure the voltage at the computer. You will need to remove the plastic cover over the
wires and probe them from the backside. A safety pin may prove very useful for this task.
Use pin 27, EVR input (brown/lt green wire) and pin 46, signal ground (black/white wire)
to measure the voltage. The orange/white wire is Vref and should always be 5 volts -/+
.25 volt. Be sure to measure Vref at the EGR sensor to rule out any broken wires or bad
connections.
Code 66 MAF below minimum test voltage.
Insufficient or no voltage from MAF. Dirty MAF element, bad MAF, bad MAF wiring,
missing power to MAF. Check for missing +12 volts on this circuit. Check the two links
for a wiring diagram to help you find the red wire for computer power relay switched +12
volts. Check for 12 volts between the red and black wires on the MAF heater (usually
pins A & B). while the connector is plugged into the MAF. This may require the use of a
couple of safety pins to probe the MAF connector from the back side of it.
There are three parts in a MAF: the heater, the sensor element and the amplifier. The
heater heats the MAF sensor element causing the resistance to increase. The amplifier
buffers the MAF output signal and has a resistor that is laser trimmed to provide an
output range compatible with the computer's load tables.
The MAF element is secured by 2 screws & has 1 wiring connector. To clean the
element, remove it from the MAF housing and spray it down with electronic parts cleaner
or non-inflammable brake parts cleaner (same stuff in a bigger can and cheaper too).
The MAF output varies with RPM which causes the airflow to increase or decease. The
increase of air across the MAF sensor element causes it to cool, allowing more voltage to
pass and telling the computer to increase the fuel flow. A decrease in airflow causes the
MAF sensor element to get warmer, decreasing the voltage and reducing the fuel flow.
Measure the MAF output at pins C & D on the MAF connector (dark blue/orange and
tan/light blue) or at pins 50 & 9 on the computer. Be sure to measure the sensor output by
measuring across the pins and not between the pins and ground.
At idle = approximately .6 volt
20 MPH = approximately 1.10 volt
40 MPH = approximately 1.70 volt
60 MPH = approximately 2.10 volt
Check the resistance of the MAF signal wiring. Pin D on the MAF and pin 50 on the
computer (dark blue/orange wire) should be less than 2 ohms. Pin C on the MAF and pin
9 on the computer (tan/light blue wire) should be less than 2 ohms.
There should be a minimum of 10K ohms between either pin C or D on the MAF wiring
connector and ground. Make your measurement with the MAF disconnected from the
wiring harness.
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/
Ignition switch wiring
http://www.veryuseful.com/mustang/tech/engine/images/IgnitionSwitchWiring.gif
Fuel pump, alternator, ignition & A/C wiring
http://www.veryuseful.com/mustang/tech/engine/images/fuel-alt-links-ign-ac.gif
Computer,. actuator & sensor wiring
http://www.veryuseful.com/mustang/tech/engine/images/88-91_5.0_EEC_Wiring_Diagram.gif
Fuse panel layout
http://www.veryuseful.com/mustang/tech/engine/images/MustangFuseBox.gif
Vacuum routing
http://www.veryuseful.com/mustang/tech/engine/images/mustangFoxFordVacuumDiagram.jpg
Code 67 - clutch not depressed (5 speed) or car not in neutral or park (auto) or A/C
in On position when codes where dumped. Possible neutral safety switch or wiring
problem. This code may prevent you from running the Key On Engine On tests. You can
generally ignore this code, since it has no effect on engine performance.
The computer wants to make sure the A/C is off due to the added load on the engine for
the engine running tests. It also checks to see that the transmission is in Neutral and the
clutch depressed (T5, T56, Tremec 3550 & TKO)). This prevents the car from being
driven while the computer is in test mode. Key On Engine Running test mode takes the
throttle control away from the driver for several tests. This could prove hazardous if the
computer was jumpered into test mode and then driven.
The NSS code 67 can be bypassed for testing. You will need to temporarily ground
computer pin 30 to the chassis. Computer pin 30 uses a Lt blue/yellow wire. Remove the
passenger side kick panel and then remove the plastic cover from the computer wiring
connector. Use a safety pin to probe the connector from the rear. Jumper the safety pin to
the ground near the computer.
Be sure to remove the jumper BEFORE attempting to drive the car!!!
Code 85 - CANP solenoid - The Carbon Canister solenoid is inoperative. Check
vacuum lines for leaks and cracks. Check electrical wiring for loose connections,
damaged wiring and insulation. Check solenoid valve operation by grounding the
gray/yellow wire to the solenoid and blowing through it.
The computer provides the ground for the solenoid. The red wire to the solenoid is
always energized any time the ignition switch is in the run position.
Charcoal canister plumbing - one 3/8" tube from the bottom of the upper manifold to the
rubber hose. Rubber hose connects to one side of the canister solenoid valve. Other side
of the solenoid valve connects to one side of the canister. The other side of the canister
connects to a rubber hose that connects to a line that goes all the way back to the gas
tank. There is an electrical connector coming from the passenger side injector harness
near #1 injector that plugs into the canister solenoid valve. It's purpose is to vent the gas
tank. The solenoid valve opens at cruse to provide some extra fuel.
It does not weigh but a pound or so and helps richen up the cruse mixture. It draws no HP
& keeps the car from smelling like gasoline in a closed garage. So with all these good
things and no bad ones, why not hook it up & use it?
Code 41 or 91 - O2 indicates system lean. Look for a vacuum leak or failing O2
sensor.
The computer sees a lean mixture signal coming from the O2 sensors and tries to
compensate by adding more fuel. Many times the end result is an engine that runs pig
rich and stinks of unburned fuel.
The following is a Quote from Charles O. Probst, Ford fuel Injection & Electronic Engine
control:
"When the mixture is lean, the exhaust gas has oxygen, about the same amount as the
ambient air. So the sensor will generate less than 400 Millivolts. Remember lean = less
voltage.
When the mixture is rich, there's less oxygen in the exhaust than in the ambient air , so
voltage is generated between the two sides of the tip. The voltage is greater than 600
millivolts. Remember rich = more voltage.
Here's a tip: the newer the sensor, the more the voltage changes, swinging from as low as
0.1 volt to as much as 0.9 volt. As an oxygen sensor ages, the voltage changes get smaller
0.2 and slower - the voltage change lags behind the change in exhaust gas oxygen.
Because the oxygen sensor generates its own voltage, never apply voltage and never
measure resistance of the sensor circuit. To measure voltage signals, use an analog
voltmeter with a high input impedance, at least 10 megohms. Remember, a digital
voltmeter will average a changing voltage." End Quote
Testing the O2 sensors
Measuring the O2 sensor voltage at the computer will give you a good idea of how well
they are working. You'll have to pull the passenger side kick panel off to gain access to
the computer connector. Remove the plastic wiring cover to get to the back side of the
wiring. Use a safety pin or paper clip to probe the connections from the rear. The
computer pins are 29 (LH O2 with a dark green/pink wire) and 43 (RH O2 with a dark
blue/pink wire). Use the ground next to the computer to ground the voltmeter.
Note that all resistance tests must be done with power off. Measuring resistance with a
circuit powered on will give false readings and possibly damage the meter. Do not
attempt to measure the resistance of the O2 sensors, it may damage them.
Testing the O2 sensor wiring harness
Most of the common multimeters have a resistance scale. Be sure the O2 sensors are
disconnected and measure the resistance from the O2 sensor body harness to the pins on
the computer.
The O2 sensor ground (orange wire with a ring terminal on it) is in the wiring harness for
the fuel injection wiring. I grounded mine to one of the intake manifold bolts.
Replace the O2 sensors in pairs if replacement is indicated. If one is weak or bad, the
other one probably isn't far behind.
Code 96 – KOEO- Fuel pump monitor circuit shows no power - Fuel pump relay
or battery power feed was open - Power / Fuel Pump Circuits. The fuel pump lost power
at some time while the ignition switch was in the run position.
Look for a failing fuel pump relay, bad connections or broken wiring. The fuel pump
relay is located under the Mass Air Meter on Fox bodied stangs built after 91. On earlier
model cars is under the passenger seat. On Mass Air Conversions, the signal lead that
tells the computer that the fuel pump has power may not have been wired correctly. See
http://www.stangnet.com/tech/maf/massairconversion.html
Diagram courtesy of Tmoss & Stang&2birds
Look for power at the fuel pump - the fuel pump has a connector at the rear of the car
with a pink/black wire and a black wire that goes to the fuel pump. The pink/black wire should
be hot when the test connector is jumpered to the test position. .
To trick the fuel pump into running, find the ECC test connector and jump the connector in the
lower RH corner to ground.
86-90 Models:
Using the diagram, check the red/black wire from the fuel pump relay: you should see 12
volts or so. If not, check the inertia switch: on a hatch it is on the drivers side by the
taillight. Look for a black rubber plug that pops out: if you don't find it, then loosen up
the plastic trim. Check for voltage on both sides of the switch. If there is voltage on both
sides, then check the Pink/black wire on the fuel pump relay: it is the power feed to the
fuel pump. Good voltage there, then the fuel pump is the likely culprit since it is getting
power. No voltage there, check the Orange/Lt blue wire, it is the power feed to the fuel
pump relay & has a fuse link in it. Good voltage there & at the Pink/black wire, swap the
relay.
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.
Ignition switch wiring
http://www.veryuseful.com/mustang/tech/engine/images/IgnitionSwitchWiring.gif
Fuel, alternator, A/C and ignition wiring
http://www.veryuseful.com/mustang/tech/engine/images/fuel-alt-links-ign-ac.gif
Complete computer, actuator & sensor wiring diagram for 88-91 Mass Air Mustangs
http://www.veryuseful.com/mustang/tech/engine/images/88-91_5.0_EEC_Wiring_Diagram.gif
Vacuum diagram 89-93 Mustangs
http://www.veryuseful.com/mustang/tech/engine/images/mustangFoxFordVacuumDiagram.jpg
HVAC vacuum diagram
http://www.veryuseful.com/mustang/tech/engine/images/Mustang_AC_heat_vacuum_controls.gif
TFI module differences & pinout
http://www.veryuseful.com/mustang/tech/engine/images/TFI_5.0_comparison.gif