Engine Codes

Code 13 - Key on Engine off - ISC did not respond properly (extends to touch throttle then retracts for KOEO) – ISC

Key on Engine running - Idle Speed Control motor or Air Bypass not controlling idle properly (generally idle too high)

If your idle is above 725 RPM, the computer will set this code. Normal idle speed is 650-725 RPM.
Higher than that means that someone has mechanically set the idle speed by use of the idle speed screw,
and has effectively disabled to computer’s ability to control idle speed.


Code 21 – ECT sensor out of range. Broken or damaged wiring, bad ECT sensor.
Note that that if the outside air temp is below 50 degrees F that the test for the
ECT can be in error.

The ECT sensor has absolutely nothing to do with the temperature gauge. They are
different animals. The ECT sensor is normally located it the RH front of the engine in
the water feed tubes for the heater.

The ACT & ECT have the same thermistor, so the table values are the same

ACT & ECT test data:

Use Pin 46 on the computer for ground for both ECT & ACT to get most accurate
readings.

Pin 7 on the computer - ECT signal in. at 176 degrees F it should be .80 volts

Pin 25 on the computer - ACT signal in. at 50 degrees F it should be 3.5 volts. It is
a good number if the ACT is mounted in the inlet airbox. If it is mounted in the lower
intake manifold, the voltage readings will be lower because of the heat transfer.

Voltages may be measured across the ECT/ACT by probing the connector from
the rear. A pair of safety pins may be helpful in doing this. Use care in doing it
so that you don't damage the wiring or connector.

Here's the table :

50 degrees F = 3.52 v
68 degrees F = 3.02 v
86 degrees F = 2.62 v
104 degrees F = 2.16 v
122 degrees F = 1.72 v
140 degrees F = 1.35 v
158 degrees F = 1.04 v
176 degrees F = .80 v
194 degrees F = .61
212 degrees F = .47 v
230 degrees F = .36 v
248 degrees F = .28 v

Ohms measures at the computer with the computer disconnected, or at the sensor with the sensor disconnected.

50 degrees F = 58.75 K ohms
68 degrees F = 37.30 K ohms
86 degrees F = 27.27 K ohms
104 degrees F = 16.15 K ohms
122 degrees F = 10.97 K ohms
140 degrees F = 7.60 K ohms
158 degrees F = 5.37 K ohms
176 degrees F = 3.84 K ohms
194 degrees F = 2.80 K ohms
212 degrees F = 2.07 K ohms
230 degrees F = 1.55 K ohms
248 degrees F = 1.18 k ohms

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 Mustang FAQ - Engine Information

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

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.
Measuring the voltage at the computer helps you spot broken wiring and intermittent connections.


Codes 44 & 94 - AIR system inoperative - Air Injection. Check vacuum lines for leaks, & cracks.
Disconnect the big hose from smog pump: with the engine running you should feel air output.
Reconnect the smog pump hose & apply vacuum to the first vacuum controlled valve: Its purpose is to
either dump the pump's output to the atmosphere or pass it to the next valve. The next vacuum controlled
valve directs the air to either the cylinder heads when the engine is cold or to the catalytic converter when
the engine is warm. Disconnect the big hoses from the back side of the vacuum controlled valve and start
the engine. Apply vacuum to the valve and see if the airflow changes from one hose to the next.
The two electrical controlled vacuum valves mounted on the rear of the passenger side wheelwell turn the
vacuum on & off under computer control. Check to see that both valves have +12 volts on the red wire.
Then ground the white/red wire and the first solenoid should open and pass vacuum. Do the same thing to
the light green/black wire on the second solenoid and it should open and pass vacuum.

Remember that the computer does not source power for any actuator or relay, 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 computer provides the ground to complete the circuit to power the solenoid valve that turns the
vacuum on or off. The computer is located under the passenger side kick panel. Remove the kick panel &
the cover over the computer wiring connector pins. Check Pin 38 Solenoid valve #1 that provides vacuum
to the first Thermactor control valve for a switch from 12-14 volts to 1 volt or less. Do the same with pin
32 solenoid valve #2 that provides vacuum to the second Thermactor control valve. Starting the engine
with the computer jumpered to self test mode will cause all the actuators to toggle on and off. If after
doing this and you see no switching of the voltage on and off, you can start testing the wiring for shorts to
ground and broken wiring. An Ohm check to ground with the computer connector disconnected & the
solenoid valves disconnected should show open circuit between the pin 32 and ground and again on pin 38
and ground. In like manner, there should be less than 1 ohm between pin 32 and solenoid valve #2 and pin
38 & Solenoid valve #1.

If after checking the resistance of the wiring & you are sure that there are no wiring faults, start looking at
the solenoid valves. If you disconnect them, you can jumper power & ground to them to verify operation.
Power & ground supplied should turn on the vacuum flow, remove either one and the vacuum should stop
flowing.
See the following website for some help from Tmoss (diagram designer) & Stang&2Birds (website host)

http://www.veryuseful.com/mustang/tech/engine/images/fuel-alt-links-ign-ac.gif

http://www.veryuseful.com/mustang/tech/engine/images/88-91eecPinout.gif

See Mustang Forums at StangNet
for a very nice drawing of the Thermactor Air System (smog pump) plumbing

If you have a catalytic converter H pipe, you need to fix these codes. If you don't, then don't worry about them

Code 63 - Throttle Position Sensor (TPS) signal too low TPS . TPS out of adjustment, or 5 V VREF
missing, broken wiring or bad connections.

Wire colors & functions:
Orange/white = 5 volt VREF from the computer
Dark Green/lt green = TPS output to computer
Black/white = Signal ground from computer

Always use the Dark Green/lt green & Black/white wires to set the TPS base voltage.

Use the Orange/white & Black white wires to verify the TPS has the correct 5 volts source from the computer.

Setting the TPS: you'll need a good Digital Voltmeter (DVM) to do the job. Set the TPS voltage at .5- 1.1
range. Because of the variables involved with the tolerances of both computer and DVM, I would shoot for
somewhere between .6 and 1.0 volts. Unless you have a Fluke or other high grade DVM, the second digit
past the decimal point on cheap DVM’s is probably fantasy.

Since the computer zeros out the TPS voltage every time it powers up, playing with the settings isn't an
effective aid to performance or drivability. The main purpose of checking the TPS is to make sure it isn't
way out of range and causing problems.

The Orange/White wire is the VREF 5 volts from the computer. You use the Dark Green/Lt green wire (TPS
signal) and the Black/White wire (TPS ground) to set the TPS. Use a pair of safety pins to probe the TPS
connector from the rear of the connector. You may find it a little difficult to make a good connection, but
keep trying. Put the safety pins in the Dark Green/Lt green wire and Black/White wire. Make sure the
ignition switch is in the Run position but the engine isn't running.

Here’s a TPS tip I got from NoGo50

When you installed the sensor make sure you place it on the peg right and then tighten it down properly.
Loosen the back screw a tiny bit so the sensor can pivot and loosen the front screw enough so you can
move it just a little in very small increments. I wouldn’t try to adjust it using marks.

(copied from MustangMax, Glendale AZ)

A.) Always adjust the TPS and Idle with the engine at operating temp. Dive it around for a bit if you can and
get it nice and warm.

B.) When you probe the leads of the TPS, do not use an engine ground, put the ground probe into the lead
of the TPS. You should be connecting both meter probes to the TPS and not one to the TPS and the other
to ground.

C.) Always reset the computer whenever you adjust the TPS or clean/change any sensors. I just pull the
battery lead for 10 minutes.

D.) The key is to adjust the TPS voltage and reset the computer whenever the idle screw is changed.

Code 85 - CANP solenoid - The Carbon Canister solenoid is inoperative or missing. 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?