The throttle positioning sensor on a Mercury is a small electronic sensing device located on the throttle body, which is near the base of the carburetor. The sensor tells the car's computer how much to accelerate at any given speed and load. The TPS affects fuel ratios and timing, which govern fuel economy. When the TPS malfunctions, it can cause a variety of problems with overall performance, including stalling and no-start conditions.
Open the hood and locate the throttle position sensor on the engine. Because the throttle position sensor reads electronic signals associated with engine speed and performance, the most notable effects of a bad sensor will involve erratic engine behavior. If the adjustment signal is too far out of specifications, the vehicle might not even start, provided all starting circuit areas are functioning properly. If the vehicle stalls when approaching or stopped at a light, the TPS can be the root cause. If the engine sputters at low or high speed, the sensor could be the problem. Hesitation, when applying the accelerator, or complete cutout can also be attributed to TPS failure or poor adjustment. The TPS specifications can raise or lower the engine performance if the idle stop screw has been recently adjusted. Binding carburetor linkage can give false sensor readings since the cable operates the opening and closing of the throttle plate.
Locate the TPS on the throttle body of the engine. A small harness connects it to the car's computer or ECU. The ECU and the TPS communicate with each other. Make sure the plug connection is tight and clean. Spray pressurized contact cleaner on the TPS connectors. Make certain that the throttle body linkage is moving smoothly and that the levers and cams are rotating freely. Manually open and close the throttle body plate by moving the lever that has the cable attached to it. It should travel up and down without binding. Inspect the throttle body for any debris, such as leaves, clods of dirt, twigs, leftover tools and, in some cases, dead rodents and nests. Examine the bolts that hold the sensor in place--they should be tight. Make sure the air cleaner has a new element and is not clogged by shining a flashlight beam through the filter element. You should see light through the element. Examine all vacuum lines for cracks or disconnections. Disconnected vacuum lines will change the RPMs, throwing the TPS reading off.
Use an ohmmeter to determine the exact voltage range of the device. Disconnect the TPS at its source. Turn the ignition key to the "On" position. Set the voltmeter to the 0 to 15 volt range. Connect one of the ohmmeter's test leads to the center connector of the TPS. Attach the other lead to either the 12-volt connector that leads to the computer or the ground (negative side). It does not matter which one. Very slowly move the throttle cam from its stop position all the way to its wide open position. While moving it, look at the read-out on the gauge. It should go up or down with a steady arc, matching the speed-pace of the manual movement. If it dives or climbs suddenly, it means the TPS is defective. If it shows full resistance, that means a break has occurred in the resistance element inside the TPS, indicating a complete failure.
For an approximate range, use these specification numbers:
•Closed throttle--between 0.6 and 1.0 volts
•Part throttle--triggered at 0.04 volts above closed throttle
•Full throttle--triggered at 2.71 volts above closed throttle
