WHAT IS THE TPS?
The Throttle Position Sensor is located (on the Mark
VIIs) on top of the air intake. It is black, mostly round,
has 2 screws holding it in place, and a several wires going
down to a plug on the back side of the air intake.
The TPS is designed to keep track of the position of
the throttle plate, which is located inside the air intake.
When you step on the gas, the throttle opens part way gulping
large amounts of air to mix with the fuel. How much it
opens, depends on how hard you step on it. It’s a direct
relation to your foot.
When idling, the throttle plate is generally closed
and a limited amount of air is being funneled around the
throttle plate through the EGR check valve (on the front
of the air intake).
The TPS should be recording a voltage of .98 to .99
with the ignition on, engine running or not. If it’s out
of range, the computer will try to figure out what’s going
on and adjust for the problem.
When the TPS is bad or badly out of adjustment, you
will generally notice a “surging” (idle going up and down)
while the computer tries to “seek” the right setting.
There are other things that can contribute to this surging
but one of them is the TPS.
BEFORE YOU ADJUST THE TPS
You should set your "base" idle speed before you finalize
your TPS adjustment.
The engine should be warm. Disconnect the Intake Air
Control - IAC, the 5-inch long cylinder on the front of
the air intake.
Below the air intake, you will find the idle stop and
screw. Using a short flat bladed screwdriver, adjust the
screw to get about 500-550 RPM. If you can't get it below
600, then get it as low as you can. Some people recommend
that you make sure the stop screw is actually stopping
the throttle plate from totally closing. A closed plate
may stick or bind.
After adjusting the stop screw, reconnect the IAC.
Restart the car 4-5 times or take it for a drive. The
computer will re-establish the idle speed and should keep
it around 725, depending on what the spec is for your car.
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CHECKING THE VOLTAGE
You need a digital voltage multi-meter.
To give you something to pick up the signal with,
you need to stick a pin, or something slim, down
into the Green wire or better still, the connector
of the Green wire from the TPS. This is the Positive
wire.
Stick another pin into the Black wire or connector.
This is the negative wire.
With the meter turned on and ignition on, make
contact with the positive pin with your positive
probe from the meter. Do the same with the negative.
Your multi-meter should now be registering something
above .93. If it doesn't settle down, you don't have
good contact.
ADJUSTING THE TPS
Adjusting the TPS is a bit tricky because you
don’t necessarily move the TPS all the way one way
or the other.
With the ignition switch on, loosen the 2 screws
on the TPS only to the point that you can barely
move the TPS with pressure from a screwdriver or
by tapping lightly on it.
Once you have a stable reading,
move the TPS around "a
hair at a time" until you attain between .98 and
.99. That’s good. You may have to move the TPS
in all four directions to get the voltage right.
Tighten the 2 screws.
The computer will take a few minutes of driving
to reset itself. After driving the car a day or two,
check the voltage again to make sure the TPS hasn't
worked loose again. It shouldn't.
MODIFYING THE TPS
If you can't move the TPS enough to get the right
voltage, as stated, you can enlarge the holes a bit
with a “rat-tail” file, electric drill, or something
similar to give you more movement. Before you bother
with this though, make sure you have moved the TPS
in every direction to try to find the perfect spot.
When you have completed to job to your satisfaction,
it is recommended to put a little silicone into any
punctures you may have made, to prevent oxidation.
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