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 :
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
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.
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
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/
http://www.veryuseful.com/mustang/tech/engine/images/IgnitionSwitchWiring.gif
http://www.veryuseful.com/mustang/tech/engine/images/fuel-alt-links-ign-ac.gif
http://www.veryuseful.com/mustang/tech/engine/images/88-91_5.0_EEC_Wiring_Diagram.gif
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 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 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.
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.
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.
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.
