Mustang throttle position sensor

Mustang throttle position sensor

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12/15/2009 small
revisions in wording

7/15/2011 expanded slightly. Added geometric mean of idle

Throttle Position Sensor TPS


Engineering specifications:


The Mustang Fox body throttle position sensor (TPS) uses a
floating ground-return to
the computer. The only throttle position sensor ground is at the computer. (DO NOT ground the TPS on the
engine or anywhere else!) Because the TPS ground floats from the engine, the proper place to measure TPS
voltages are between the TPS black ground wire and the other TPS wires. We
should not
measure between some engine
part and the TPS wires, or between the vehicle chassis and the TPS wires.

If a TPS wire or connection is open, or if the TPS is defective, it will show
by measuring between TPS wires.

The throttle position sensor has three wires, voltage source,
sensor output, and ground return. The floating ground return keeps “ground loop”
voltages, like unwanted small voltages from engine-to-chassis, from influencing
the TPS reading.

If you are going to read TPS voltage, it makes sense to read
exactly what the
computer should be seeing. Do
not read what the manufacturer went
out of his way to avoid, by measuring to some metal engine part. Measure
throttle position
sensor voltage by
connecting your
meter between the
black and green
leads. Never measure
from the green lead
to an engine
component or bolt.
This is because the
computer reference
is to the car
chassis inside the
compartment, and the
black throttle
position sensor wire
is referenced to
that ground point and that point alone!

It is actually OK to poke
through the wire insulation with a sharp meter probe. Don’t force
through wire insulation with a dull probe, or a wide probe tip. If you
have to, sharpen the meter probes to a needle-like point so they easily
poke through the wire insulation, and so they only leave a tiny hole.


Use a good meter


Voltages you should
see at the TPS
(throttle position
sensor), with
ignition key on and the engine not running, are:

Orange to
black without the TPS connected…..5.0 volts
nominal. This is a computer-supplied, regulated, reference voltage. The TPS must
have 5.0 volts nominal between orange and black to work. 4.9 volts is OK, as is
5.1 volts, if the voltage is steady.

Green to black
without the TPS
from 0 to 5 volts.
The green wire voltage, without the TPS connected, is unpredictable. This is
because the green wire connects to the computer’s TPS input
pin. The voltage on this wire depends on the sensor dividing voltage down from the 5 volt
Orange line.
The green wire is very sensitive
to leakage resistances when the
TPS is unplugged, so it can give an erratic reading. If the unplugged reading is
outside the range of 0-5 volts, you might have an electrical connector moisture
problem. It should be somewhere between zero and 5 volts.

Black to
vehicle chassis,
timing chain cover,
bell housing, or
engine block….less
than 0.5 volts under
any operating


The stock computer allows this idle voltage range for the EEC-IV A9L:

Because of meter and component tolerances, you never want to go outside
the range. Anything inside the range above will run exactly the same.


With the TPS
connected, idle-position voltages
should be….

Orange to
black……5.0 volts
NOMINAL depending on
your meter. This is the supply voltage from a voltage
regulator inside the EEC.

Green to
black……..absolute maximum range of 0.6
volts to 1.15 volts,
0.8 to 0.85 volts is very
good at idle.
This is sensor output.

Green to black
with accelerator
floored….at least
3 volts higher than
the green to black
idle voltage!

Black to
vehicle chassis,
timing chain cover,
bell housing, or
engine block….less
than 0.5 volts under
any operating
condition. This is sensor ground.

The voltage from the green wire to the black wire should smoothly vary
from idle voltage (0.6-1.1 volts) to over 4 volts at WOT. The voltage change as
the throttle is opened or closed should be smooth. The voltage should follow the throttle position. If
the voltage jumps around at some throttle
positions, the TPS is probably bad. If the voltage on the green wire is stuck
high or low and the rest are normal, it is probably a bad sensor.

Optimum TPS Voltage Setting

The stock computer reads the TPS voltage, and remembers it, when the key
is initially turned on. Any green to black wire voltage between 0.6 and 1.15
volts is considered an acceptable idle (closed throttle) voltage. Ford intentionally
allowed a fairly wide range of voltages. After all, each car in production will
be slightly different.

With a wide range of voltages, the factory and dealerships did not have to worry about setting TPS
voltage for each individual car….and nether do we.

There is some crazy idea we have to set the TPS to some specific voltage,
or that a different TPS idle voltage will change engine response, idle, or
power. It makes no difference at all what the voltage is, so long as it is
within the allowable range of voltages. Performance, idle quality, and throttle
response will not change with any setting within the allowed range.

In some cases, especially if it makes us feel good (the car won’t care one
bit), we might want to
center the TPS voltage. When we want to center something between an upper and lower limit, the ideal
center point choice is not
the simple average (although average is close) or middle of the range. Centered is not an exactly equal amount above and below
the extremes. The perfect or ideal
center point is actually the
geometric mean
. The geometric mean creates the same
proportion or percentage of voltage tolerance above and below some point.
This is the real center or middle of a range.

The geometric mean of .6 and 1.15 is the square root of .6*1.15.
This is 0.83 volts.     0.83 volts is the same percentage
below 1.15 volts as it is above .6 volts. 0.83 volts is the geometric middle of the numbers
when we look at possible voltage errors or “drift”. 

The average would be .6+1.15 /2 = 0.875 volts, but this would be off (a little above) the
geometric center. In this case, voltage is closer to the upper limit than the
lower limit, as a tolerance percentage error.  When we go 0.9 volts or higher, it
gets even worse.  No matter what advice you read, do not intentionally set your TPS at some voltage near a limit!
If you do that, if your meter is off, or if the resistor in the TPS ages or gets a little dirty, or
if some connection gets just a little bad, the TPS can move out of idle range
more easily. The best idea is the center for a target.

Part Throttle

Part throttle is any voltage between wide open throttle and idle voltage.
There is some hysteresis designed in so we do not get false changes or bouncing
between the three throttle states.

Wide Open Throttle

stock computer is programmed internally to “think” WOT
(wide open throttle) is 2.71 volts higher than the
idle TPS voltage. This means if idle voltage is .75 volts, the computer
assumes any voltage between the green wire and black wire of 3.46 volts
or more
is wide open throttle. (This is 2.71+.75 = 3.46
volts in this example of .75 volts idle voltage.)

Let’s say this again, because WOT is the most critical threshold we want
to reach. Your car’s idle voltage plus 2.71 volts is what the early Mustang computer thinks is WOT
voltage. WOT is where the computer immediately transfers to open loop operation and
where the computer ignores narrow-band factory oxygen sensors.

For the computer to be fully “safe and solid” into WOT, not teetering near
the edge where electrical fluctuations might make the computer drop out of WOT
mode, the TPS voltage, with wide open
throttle, should  be at least 0.25 volts above threshold voltage. This
means we want: idle voltage+2.71+.25 , or you car’s idle voltage plus 2.96, as
the very minimum WOT voltage.  When you hold the “pedal to the metal”, you
must have more than 3 volts between the green and black wires!

Testing WOT
voltage is probably more
important than testing idle voltage. The computer really does not care
about idle voltage, so long as  idle voltage is somewhere between 0.6 to 1.15 volts.
This allows for errors and tolerances, the actual limit is 0.5 to 1.25
If you are drilling TPS holes and setting TPS for an
exact voltage like .97 volts, you are pretty much just wasting your time. The computer cares
less if that voltage is .6 volts, 1.15 volts, or anything in between.
The computer
considers all voltages within that range
the same.
What you should be doing is making sure the idle TPS voltage is stable,
and somewhere around 0.8 volts, and that the TPS varies smoothly from
idle to WOT voltages that are above 3 volts. 

Be absolutely sure to check WOT voltage. My 1989 LX initially was OK at
idle, but because of a defective throttle sensor, never went into WOT open loop mode!

The most important

As the
accelerator is
slowly moved there
should be a smooth
steady change on the
green without any
sudden jumps in
voltage. This is
best seen on an
analog meter (one
with a moving
needle)! This
is important for the
enrichment to work,
and for functions
that depend on the
relative throttle

Remember, the computer considers a range centered on 0.812 volts as “idle”.
The computer also considers anything more than 2.71 volts above actual idle voltage as
wide open throttle, but we should add a little extra voltage to WOT position to
be safe. 

Contrary to claims or rumors, it doesn’t matter what the TPS is set at for
idle so long as it is between .6 volts and 1.1 volts. The computer will not
produce better idle, better response, more stable idle, or better fuel economy if you set at any
specific voltage in the acceptable range.     

My Background

I was a Ham radio operator when I was 12 years old. I built my own
transmitters and receivers from old junk radio and television carcasses I
brought home from the local dump. Reading books and building things taught me
how electronics systems worked. 

I built my first tachometer from scratch when I was 13 years old. This was
about 1964. I put this in my dad’s ’57 Ford. My hand-made tachometer used brand new devices called
“transistors” (most stuff was still vacuum tubes back then) and a meter from a WW II surplus radio set. At that time I was
also repairing car radio’s for the teenagers next door to me, including hand-making
solid-state tubes to replace the troublesome 0Z4  gas rectifier tubes used in car radios.

The older teenagers next door, especially Tom Hilding, got me interested in
cars. Tom had a ’62 Chevy II with six-cylinder flags, but it had a dual quad
small block and four speed. It would go sideways in the first three gears
(mostly due to the bad tires back then). This started me out in life thinking sleeper
cars were the best cars to own.

Like most red-blooded American’s who grew up in the 60’s and 70’s, I love
American muscle cars of all types. I have owned American Motors, Chevy,
Studebaker, and Ford muscle cars, but mostly when they were available used and

I have primarily design analog systems for the past 40 years, along with some
digital systems. I have
designed commercially manufactured after market solid-state ignition systems, as well as
metering, instrumentation, and automotive test equipment. I managed engineering
in the precision meter manufacturing division of a large aftermarket gauge and
test equipment supplier.
We supplied test equipment to Sun, Sears, Mac Tools, Snap-On, and many others. I
still design manufactured products, including studio audio, medical, and
consumer communications equipment.


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