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How to check sensors and valves that may be causing issues.
Note: This is not inclusive, but can be a good guide for beginners.
Before I begin to explain any of this, it is important to understand how your internal combustion engine works. Your 97-03 is not as simply as a 70 model carburetor intake and engine. The PCM nearly controls and regulates every sensor your engine runs off of. Sensors are a huge part of why your engine runs smoothly and operates the way it does. If you are having any symptoms of Poor idle, Stalling, Poor drive-ability, ETC. Then the culprit could be a simple sensor, valve (controllable), or hose. Your PCM (power-train control module) monitors and reacts to each of these sensors, they are constantly sending information to the PCM and the PCM sends back a reaction. The concept seems simple, but the logic of how everything works in harmony can be very complex. This is why I wanted to go over how to check a few of the sensors your engine depends on to run in its maximum effective range (good drivability, good idle, no stalling, proper mpg, etc). I know a lot of you guys have acess to scan tools etc, but you can actually accomplish a lot with a little deductive reasoning and a simply $20 DMM (aka voltage and ohm meter). I highly suggest anyone to purchase a DMM, it is a valuable tool and can save you a lot of headaches.
I’m not going to go into detail with a( how-to) to replace all of these, but I will point out in each sensor what you need to look for when it is functioning properly. I will also denote the location and what voltage or resistance to look for when checking sensors and or controllable valves.
1. Exhaust Gas Recirculation (EGR) and Control Systems
2. Evaporative Emissions Control System (EVAP)
3. Positive Crankcase Ventilation (PCV) system
4. Engine Coolant Temperature (ECT) sensor
5. Mass Airflow Sensor (MAF)
6. Intake Air Temperature (IAT) sensor
7. Throttle Position Sensor (TPS)
8. Oxygen sensor
9. Brake On/Off (BOO) switch
There are other sensors such as the VSS, Fuel Pressure, etc. that are better left to a scan tool if you aren’t sure what to look for. I think the Index above should cover a lot of the general asked questions for “why is my truck surging” , “bad idle”, etc.
EGR- Exhaust Gas Recirculation (EGR) system
1) EGR – If a code is displayed there are several possibilities for EGR failure. Engine coolant temperature (ECT) sensor, TPS, MAF sensor, TCC system and the engine rpm govern the parameters the EGR system use for distinguishing the correct ON time.
Symptoms and codes of a BAD EGR Valve:
Car or truck runs fine, but the Check Engine Light is on
EGR Valve Fault Code:
P0401 EGR System Flow Insufficient.
P01406 EGR Valve Pintle Position.
Really BAD gas mileage.
Lack of power as you accelerate the vehicle down the road.
Truck seems to run okay above 30 miles and hour but once you come to an idle, the engine barely stays running and/or idles rough. Once you take off again, it runs OK.
Exhaust Gas Recirculation (EGR) System
The purpose of the Exhaust Gas Recirculation (EGR) system on your engine is to limit oxides of nitrogen (NOx) emissions. This is done by redirecting small amounts of exhaust gas back through the upper intake to get mixed in with the air/fuel charge. The resultant diluted air/fuel mixture (i.e., less oxygen per cubic foot) burns cooler and combustion chamber temperatures are reduced accordingly, thereby lowering NOx emissions.
The Ford DPFE/EGR system is controlled by the Powertrain Control Module (PCM). The system employs an EGR Valve, an Electronic Vacuum Regulator (EVR), and a Delta Pressure Feedback EGR sensor (DPFE).
- The EGR valve is mounted on (or very close) to the upper intake and is connected to both the intake and the exhaust system by virtue of a special EGR Tube. The valve has a vacuum port that allows it to be controlled (opened and closed) by the EVR. When the valve is open, exhaust gas flows into the upper intake and mixes with the a/f charge.
**Very obvious, it is the piece with the large circular top** All parts enclosed inside it.
- The DPFE Sensor measures EGR flow across an orifice located inside the special EGR Tube. The orifice is positioned between two hose ports coming off the tube which are connected to the DPFE sensor with special heat-resistant hoses. When the EGR Valve is open, a pressure differential is created across the orifice (intake manifold pressure vs. exhaust pressure). By design, this difference in pressure is measured by the DPFE sensor in terms of voltage. The DPFE voltage signal output to the PCM is directly proportional to the flow of exhaust gas entering the intake manifold.
The PCM determines optimal conditions for EGR flow and then, based on the DPFE voltage signal and some other sensor data, activates the EVR to open and close the EGR valve as necessary.
- The EVR is a solenoid with two vacuum ports. One port is connected to a vacuum source/supply, and the other is connected to the EGR valve. There is also a passage that vents vacuum to the atmosphere. A disc inside the solenoid is moved by electro-magnetic force, as directed by the PCM. If more EGR flow is required, the PCM increases the duty-cycle to the EVR, moving the disc to close off the atmospheric vent, which in turn increases the amount of vacuum flow to the EGR valve. If less EGR flow is desired, the PCM decreases the duty-cycle to the EVR, allowing for more atmospheric venting and hence less vacuum flow to the EGR valve.
The EVR is a "normally closed" solenoid, which means that when it is de-energized, the position of the disc allows for maximum venting to the atmosphere (resulting in negligible vacuum flow to the EGR valve).
EVR is on the right with the Circular top. It is next to the DPFE.
Note that Ford EGR systems DO NOT engage when:
1) The engine is cold ;
2 ) The engine is at idle;
3 ) The engine is at WOT;
4) At low ambient temps (water vapor from the exhaust can freeze on the throttle plate).
How to check:
To check the EGR vacuum regulator, disconnect the electrical connector to the EGR vacuum regulator, turn the ignition key ON and check for battery voltage to the solenoid (see picture below) . Battery voltage should be present 12V+.
The Right terminal should be Red/Yellow for +12V, left terminal Pink/Black is GRD
Next, use an ohmmeter and check the resistance of the EGR vacuum regulator. It should anywhere from 30-70 ohms
Unplug the two prong sensor as in the previous step and use your ohmeter to prob the two terminals.
When doing this, make sure the prob the right terminal (Red/Yellow with the red prob) The other terminal is for the Black probe.
Check for reference voltage to the DPFE sensor. With the ignition key on (engine not running), check for voltage on the harness side of the electrical connector on terminal VREF. It should be between 4.0 and 6.0 volts. If the test results are incorrect, replace the DPFE sensor.
This is a 3 prong connector. Top Prong is Brown/Green, Middle Prong is Grey/Red, Bottom Prong is Brown/White.
Top= Brown/Green= Signal to ECM
Middle = Grey/Red = GND
Bottom = Brown/White = REF
With the engine cold there should be NO EGR therefore the voltage should be approximately 0.20 to 0.70 volts. As the engine starts to warm and EGR is signaled by the computer, voltage values should increase to approximately 4.0 to 6.0 volts.
Last edited by ibd2328; 01-20-2014 at 08:48 PM.
The Following User Says Thank You to ibd2328 For This Useful Post:
2) EVAP- Evaporative Emissions Control System
How it works;
The Evaporative System (EVAP) prevents the fuel vapors from the fuel tank from escaping into the atmosphere. The fuel vapors from the fuel tank get trapped in the charcoal canister; the charcoal canister is filled with small pellets of activated charcoal that absorb and store vapors.
When the engine is running and other conditions allow, the fuel vapors are purged from the charcoal canister into the engine air intake to be burned (see the diagram). The vent control valve is usually installed at the back of the truck; near or at the charcoal canister. The vent control valve is an electrical solenoid/valve that is controlled by the engine computer (ECM). The vent control valve is normally open; it controls the air supply into the canister. The vent control valve closes when the ECM performs the leak test of the EVAP system.
During the leak test, the ECM seals off the EVAP system by closing both, the vent control valve and the purge control valve (solenoid). Once the EVAP system is sealed, the ECM monitors the vacuum or pressure in the EVAP system. If the EVAP system doesn't hold the vacuum or pressure, the ECM recognizes an EVAP leak.
(EVAP) consists of a charcoal-filled canister and the hoses connecting the canister to the fuel tank, fuel vapor management valve (VMV), a fuel tank pressure sensor, fuel filler cap, fuel vapor valve, ported vacuum and intake manifold vacuum.
Fuel vapors are transferred from the fuel tank, throttle body and intake manifold to a canister where they are stored when the engine is not operating. When the engine is running, the fuel vapors are purged from the canister by a vapor management valve (VMV) which is PCM controlled, and consumed in the normal combustion process. The fuel tank pressure sensor relays the inside fuel tank pressure to the PCM which in turn regulates the EVAP system purge control system.
What can cause the code P0446:
- faulty vent control valve
- blockage at the vent hose, vent filter or vent control valve
- open or short in the vent control valve circuit
- bad charcoal canister
- EVAP system leak
- faulty EVAP system pressure sensor
- bad vacuum switching valve (VSV) (Toyota, Lexus)
- stuck open purge valve (solenoid)
Check for symptoms.
Stalling and poor driveability
Split or cracked hoses or hoses connected to the wrong tubes.
Any smell of fuel around canister, hoses, and valve.
The valve, purge valve, and canister are fairly easy to check. The canister, simply looks like a canister (located near your fuel tank). The purge canister located on the fire wall can be check using a DMM for resistance and voltage.
Ways to check the system:
Excessive pressure in fuel tank- Remove the gas cap and listen for excessive pressure. Will sound like air coming from a tire.
Check canister for cracks or leaks or any smell of fuel around it. Also check that all lines are in good condition with no kinks or cracks.
If you smell fuel in the enginge bay compartment, it could be Vapor Mangament Valve on the fire wall. This is ran by a solenoid and the PCM. So you can use a DMM to check this; See below.
Checking the purge valve wiring has voltage and ground. Also check that the Purge Valve sensor itself has resistance. It is located on your firewall on the drivers side. Cant miss it. Pull the plug off and check both.
Resistance with your ohm-meter on 200 should read around; (this is on the canister valve itself). Check the voltage with the ignition on(this is on the plug in connected to your wiring harness).
Here is a picture of the Purge Valve with the Connector Un-plugged. The red wire with a strip is the positive lead, the other is ground.
3) PCV (Positive Crankcase Ventilation system)
Why does your truck have a PCV System?
A mixture of air and gasoline is pumped through a system of tubes called the intake manifold through each cylinder's intake valve (or valves), where a spark from a spark plug causes the mixture to explode in the open space at the top of the cylinder called the combustion chamber. The pressure from this explosion drives the piston in the cylinder downward, where it causes the crankshaft to rotate. The rotation of the crankshaft not only pushes the piston back up into the cylinder so it can do all this again, but it also turns the gears within the car's transmission that eventually make the car move. Meanwhile, the rising piston pushes the air and gas left over from the explosion back out of the cylinder through an exhaust valve.
However -- and this is where crankcase ventilation comes in -- a certain amount of that mixture of air and gasoline is pulled down by the piston and slips through the piston rings into the crankcase, which is the protective cover that insulates the crankshaft. This escaping gas is called blow-by and it's unavoidable. It's also undesirable because the unburned gasoline in it can gunk up the system and produce problems in the crankcase. Until the early 1960s, these blow-by gases were removed simply by letting air circulate freely through the crankcase, wafting away the gases and venting them as emissions. Then, in the early 1960s, positive crankshaft ventilation (PCV) was invented. This is now considered the beginning of automobile emission control.
Positive crankcase ventilation involves recycling these gases through a valve (called, appropriately, the PCV valve) to the intake manifold, where they're pumped back into the cylinders for another shot at combustion. It isn't always desirable to have these gases in the cylinders because they tend to be mostly air and can make the gas-air mixture in the cylinders a little too lean -- that is, too low on gasoline -- for effective combustion. So the blow-by gases should only be recycled when the car is traveling at slow speeds or idling. Fortunately, when the engine is idling the air pressure in the intake manifold is lower than the air pressure in the crankcase, and it's this lower pressure (which sometimes approaches pure vacuum) that sucks the blow-by gases through the PCV valve and back into the intake. When the engine speeds up, the air pressure in the intake manifold increases and the suction slows down, reducing the amount of blow-by gas recycled to the cylinders. This is good, because the blow-by gases aren't needed when the engine speeds up. In fact, when the car is up to speed, the pressure in the intake manifold can actually become higher than the pressure in the crankcase, potentially forcing the blow-by gases back into the crankcase. Since the whole point of positive crankcase ventilation is to keep these gases out of the crankcase, the PCV valve is designed to close off when this happens and block the backflow of gases. http://auto.howstuffworks.com/positi...ion-system.htm
How a PCV valve works
Most engines employ a PCV valve at the point where fumes are drawn out of the engine. The PCV valve serves several functions. At an idle, engine vacuum is very high, around 16 to 20 inches. This high vacuum would tend to draw oil as well as fumes from the engine. The PCV valve acts as a buffer against oil being drawn out. It also regulates the amount of vacuum applied to the engine, based on engine load and speed.
At an idle, engine speed is low, around 600 RPM. A relatively small amount of fuel and air travel through the intake at idle speed. If the PCV valve did not regulate air flow, the engine would act like it had a vacuum leak. Too much air flowing into the intake causes the engine to lean out [too much air in relation to the fuel] and misfire. At an idle, the PCV valve restricts air flow, to reduce this problem. At high manifold vacuum [idle], a spring loaded valve is drawn up and partially restricts flow to the crankcase. The first drawing above illustrates the PCV valve position at idle.
On acceleration far more fuel and air move through the engine and intake manifold vacuum is much lower. Air introduced by the PCV valve has much less influence on the fuel-air mixture. Low intake manifold vacuum allows the valve to move to a more central position. In this position the system draws more fumes from the crankcase. The additional flow is very beneficial, without affecting engine performance. The center illustration above, shows the PCV valve in acceleration position.
Any pressure in the intake causes flow in the opposite direction. This could occur during an engine backfire or if the engine is turbo-charged. The PCV valve can act as a check-valve in these situations. By closing the PCV valve, any positive pressure or fuel vapor is prevented from entering the crankcase. Even a very small amount of positive pressure can force oil passed gaskets and seals and blow gaskets out of place. Failure of the valve to seal positive pressure may damage the engine. http://www.agcoauto.com/content/news/p2_articleid/197
What does it look like and where is it at?
It is typically located on the passenger side valve cover. Its really hard to miss.
Easy signs it is bad or stopped up? A stopped up PCV Valve doesnt allow excess gas and mositure to exit the system so oil and moisture can build up under the valve covers and create lots of problems.
How to test for a bad PCV Valve/Hoses. (The factory 90 degree elbow tends to leak alot too) remember this is a closed system and needs vacuum, so any pin hole leads can cause issues.
1) First start your truck up and allow it to idle.
2) Then remove the PCV valve from the valve cover and put your hand on the bottom of the valve and check for vacuum.
3) If no vacuum is present, remove the valve from the hose and check the hose for vacuum.
Case 1: If vacuum is felt with the PCV valve connected, the system is working properly.
Case 2: If vacuum is NOT felt with the PCV valve connect, but vacuum is present at the hose, then your PCV Valve is bad.
Case 3: If vacuum is NOT felt at the PCV valve or hoses, replace both. They are super cheap, and definitely a good maintenance idea.
If you noticed in the pictures above, my PCV Valve has two hoses going around it. This is actually a way of heating the Valve itself and is fairly useless in my opition. The hoses simple wrap around the lip on the Valve and cause more harm than good. You can buy electrically heated PCV Valves that work much better, but I dont think it is all that necessary. The two hoses wrapping around it have no effect on the vacuum. You can see in the picture below what the pieces look like and that the coolant hoses are completely separate.
In the picture below you will see a triple tree of lines coming off the throttle body housing. The one in the middle is the only one that is directly connected to the PCV, the other two are simply coolant lines.
The valve itself is this simple. Just unhook the elbow and twist and pull it from the grommet in the valve cover.
You can pry the grommet out and inspect it for cracks or wear.
These parts are all fairly inexpensive and need to be replaced every 50-80k or whenever a problem arises. Mine were all in good working order, just had some build up inside the valve itself holding it closed. So I used an air compressor and soaked all the parts in Kerosene for a while to clean them. DONT USE THROTTLE BODY CLEANER IT IS TERRIBLE ON RUBBER PARTS.
Once all the parts are clean, make sure to clean off the Valve Cover opening for the gromment and replace it.
I used a little bit of sealant around the grommet and PCV Valve to help, mainly bevause these should be replaced whenever you do a change. Along with the hoses connecting them to the throttle body housing.
4) Engine Coolant Temperature (ECT) sensor
How it works.
Your ECT sensor simply works on resistance and sends that to the PCM then to your gauge. The sensor uses a thermistor which is more accurate at smaller intervals than other types of simple resistance based temperature gauges.
A thermistor is a type of resistor whose resistance varies significantly with temperature, more so than in standard resistors. The word is a portmanteau of thermal and resistor. Thermistors are widely used as inrush current limiters, temperature sensors, self-resetting overcurrent protectors, and self-regulating heating elements.
Thermistors differ from resistance temperature detectors (RTD) in that the material used in a thermistor is generally a ceramic or polymer, while RTDs use pure metals. The temperature response is also different; RTDs are useful over larger temperature ranges, while thermistors typically achieve a higher precision within a limited temperature range, typically −90 °C to 130 °C.
On my 2003 5.4L it is located on the front of the engine on the drivers side. See picture below for reference.
How can I check it?
First you need to check two things:
1) That your Sensor is working properly, we will use a resistance test for this
2) Check that the plug to the ECT Sensor has voltage.
Checking for a good ECT sensor and plug.
1) To see that the sensor is working proper unplug the connector, it should only have two terminals on it. (This is with the enginge off). Now with the connector off, prob the terminals of the sensor. Place the positive lead on the right hand side and negative lead on the left as show below.
Your resistance reading in OHMs should be 60,000 - 40,000 ohms at around 50 degrees Fahrenheit. If it is colder outside your resistance will be higher, so dont worry, that is the way this system works.
Now with everything out of the way let the truck idle until it gets to operating temperate. One it gets to operating temperate re-do step (1) and prob the terminals to check resistance again. Your resistance readings should be between 4,000 and 1,700 ohms from 180-220 degrees F. So it is obvious that resistance increases the colder the engine is, but resistance decreases with an increase in temperature.
It is around 55 degrees here now so I was expecting a resistance reading of around 30,000-40,000 ohms.
So I can see that my sensor is working properly.
Below is a general physics depiction of the process a guide that can be use to approximate. (This can depend on elevation and of course your model) This is just a general idea, so as long as the resistance is changing adequately, you should be fine.
2) Now all we need to do is prob the connector to make sure that it is getting ~5.0 volts. Simply use your DMM to prob the connector and see what you get. The dark green wire should read +5.0 volts.
Common Symptoms of a BAD Ford MAF Sensor
This is not the most definitive list on the subject, but does cover the majority of symptoms I've seen on these types of Ford Mass Air Flow Sensors:
1) MAF Codes that light up the CHECK ENGINE LIGHT (CEL) on your Instrument Cluster.
P0102 MAF Signal Low Input to PCM.
P0103 MAF Signal High Input to PCM.
P1100 MAF Circuit Intermittent Voltage Input.
P1101 MAF Sensor Circuit Output Voltage low During KOEO Self Test.
MAF Sensor malfunction that DOES NOT light up the CHECK ENGINE LIGHT (CEL).
2) Lean and/or Rich code(s).
3) Fuel Trim code(s).
4) A tremendous lack of power upon acceleration.
5) Black smoke coming from the tail-pipe.
6) BAD gas mileage.
7) Vehicle may idle rough and stall.
What we are dealing with:
Do NOT pull the MAF sensor out to test it. Simply loosen up your clamps and turn your air box up to see the wires.
It is critical that you first check that there are no air leaks between the throttle body and the MAFS. Air entering after the Mass Air Flow Sensor into the engine will negatively impact fuel injection. And will skew the results of your tests which could result in the replacement of a good MAF
Red= 12V +
Tan/Light Blue= Ground provided by ECM
Blue/Red= MAF Signal
How the process works
Use a Digital Multimeter for all tests where a multimeter is called for.
The MAF Sensor produces an analog Voltage signal. This MAF Signal's DC Voltage is directly related to amount of air the engine is breathing. Therefore, if the engine is breathing in more air at 2500 RPM's that at an idle of 900RPM's the Voltage output will be greater at 2500 RPM's than at idle.
Now, when testing this Voltage signal, the important thing to know is not an actual volts number at a specific RPM, but to look for crazy and extreme fluctuations in the voltage signal that do not correspond to the actual air intake (RPM's) of the engine or no Signal at all. For example: If at Idle the Voltage reading starts to spike up and down without you accelerating the engine or if there's no Signal at all.
Later on, I'll show how you'll use a base Voltage reading at idle from the MAF Sensor that will help you to confirm that the MAF Sensor is bad or not.
Test 1: We need +12V
For the next few steps I suggest using a prob or some sort of piercing metal to test the wires so you dont hurt any terminals or insulation. Below is a picture of how i did this.
Use your DMM and prob the red wire to test for 12V+, ground the black wire of the DMM to the negative battery terminal. If you have 12V (11.7-12.6) there, we are ready to move on. If you do not, then MAF is probably not an issue.
Test 2: We need a common ground to the truck
Obviously, for this circuit to work we need a ground to complete it, so prob the Black/White wire with your negative cable on the DMM. Connect your red cable from the DMM and prob the + battery cable. You should get ~12V or whatever voltage your battery is reading 11.7-12.6 is acceptable. If not then, time to check wiring.
Note: the first two Test were simply power and ground from the truck, nothing to do with ECM. The next to steps check voltage and ground from the ECM.
Test 3: Checking for Ground from the ECM
You will need to turn the ignition on for this test!
Once again connect the Red cable from your DMM to the + battery terminal. Then prob the Tan/Light Blue wire the black cable from the DMM. You should once again see ~12V. If not, ECM or wiring issue.
Test 4: Checking MAF SIGNAL
Crank up the engine and get her to idle.
Once the idle stabilizes, take a look at the Voltage number or numbers your Multimeter is throwing at you. This voltage may fluctuate a little or a lot, and this is OK. Whatever they're doing... this is your base MAF Signal Voltage. Should be fluctuating between .8- 1.1 V
If the MAF Sensor is good, the Voltage numbers on the Multimeter should rise and fall smoothly and without gaps. If the MAF Senosr is BAD, you see one of three things: 1.) you WON'T see a Voltage registered on the Multimeter at all or 2.) the Voltage will stay stuck in one number or 3.) the Voltage numbers will spike up and down crazily even when you're not accelerating the Engine.
**Most credit for this goes to easyautodiagonstics.com, they had a great article on a similar model.
6) Intake Air Temperature (IAT) sensor
Where is the IAT located?
It is located on the intake just after the MAF sensor on the small corrugated plastic tubing.
A faulty one can have the following symptons
1) P0113 : Intake Air Temperature circuit high input.
or any IAT code
2) A tremendous lack of power upon acceleration.
3) Black smoke coming from the tail-pipe and a miss.
6) BAD gas mileage.
7) Vehicle may idle rough and stall.
If you have acess to a scanner get it to read the actual data being sent to the ECM by the IAT sensor. The sensor could be stuck on any thing from -40 degrees F to 150 degrees. Usually auto zone or a mechanic shop can do this.
If you don't posses a scanner that is able to read the information being sent to the ECM, but you would like to test the IAT sensor here is how to do it:
Checking to see your IAT (Intake Air Temp) sensor is working.
What we will be doing:
Using an ohmmeter(Most DMM have this), measure the resistance across the sensor terminals, the resistance should be high when the air temperature is low, start the engine and let it warm up, turn the engine off and measure the resistance again, the resistance across the terminals should be lower once the air temperature is high, if the sensor doesn't exhibit any changes it means that it is bad and it needs to be replaced with a new one.
This one is easy, your sensor should only have two wires coming from it.
Grey (WITH NO STRIP) = + IAT Sensor reading
Grey/Red = Ground
Test 1: Check Ground
Using a DMM place the + probe on the battery terminal and probe the Grey/red wire. You should see +12 V if the sensor is getting ground.
Make sure the sensor is plugged in and the ignition is off. Test 2: Checking Resistance in OHM's
Set you DMM to the 200K Ohm setting.... Ground the negative prob of the DMM to the battery and prob the Solid grey wire to check for resistance (in ohms). I checked with the 2Mil Ohm setting as well. NOTE: We are looking for a base line reading: This reading should be high since the resistance is greater when the temp is low.
Test 3: Crank the truck up and let it warm up for several minutes.
Then repeat test 2 in the exact same way with the ignition off to check the resistance.
Your first Test/Reading should have a higher resistance in OHMS. If your second reading is higher after running the truck, it is more than likely a fault IAT sensor.
Make sure ECM is working with the AIT properly.
Now check the reference voltage being sent to the sensor by the ECM, ( Ignition switch on ), reference voltage should be approximately 5 volts, if the voltage signal is not correct have the PCM diagnosed by a Ford dealer service dept or a repair shop.
Simply ground your DMM to the battery and prob the Grey wire with the ignition on and read the voltage.
Mine is a little low, so I need my PCM flashed or new one installed.
Information taken from HOWSTUFFINMYCARWORKS... all credit to them
Symptoms of a BAD Ford TPS
P0121: Throttle Position (TP) Circuit Performance Problem.
P0122: Throttle Position (TP) Circuit Low Input.
P0123: Throttle Position (TP) Circuit High Input.
Really BAD gas mileage.
Transmission does not shift out of second gear.
No power as you accelerate the vehicle.
Hesitation when you step on the Accelerator Pedal.
What does it look like and where is it at?
It is located on the right side of your throttle body with screws holding it in place. It has a three prong plug connected into it.
How does it work?
When you crank and start your truck the PCM (Powertrain Control Module=Fuel Injection Computer) feeds the Throttle Position Sensor with 5 Volts and a Ground.
This Ground is known as the Sensor Return. As you step on the Accelerator Pedal, the TP Sensor translates the amount of Throttle Plate opening (caused by the Accelerator Cable) into a Voltage Signal the PCM can use.
The PCM uses this Voltage Signal , that will vary with the amount the Throttle opens as you accelerate or decelerate the vehicle, to inject fuel, control ignition timing, and the rest of the song and dance the Computer has to do to get your vehicle moving or slowing down.
What does the inside of the sensor look like?
I am not sure on all models, but there is a potentiometer “like” dial inside your TPS that connects to your throttle, so as you open up the throttle the potentiometer reads the values and sends that to the PCM. The system is more than likely based off resistance since the ribbon like material can only be turned so far within the sensor. Below is a picture of a simple potentiometer and the parts within it. Again this exactly exactly how yours looks like or works, but gives a good idea of how opening up the throttle turns the ribbon and sends a signal to your PCM. These are used in all sorts of things such as volume control *****, dimming lights, ect.
All rights go to http://easyautodiagnostics.com/ford_...nose_tps_1.php for this section.
Here are the wires off the plug.
It has three pins, one is ground, one is 5.0 volt reference from the PCM, and one should return a variable voltage based upon the throttle position.
Now that we know the Circuitry we can begin our investigation/test.
Note: It is best to do this with a wire insulation piercing probe. Don't damage the plug or wires when trying to test this.
Test 1: PCM Reference Voltage
1)Turn the key to the on position (truck not running)
2)Probe the wire denoted 5.0V (PCM Reference voltage) with the red wire from your DMM.
3)Ground the black wire from the DMM to the battery or a good solid ground. You should have betweenr 4.5-5.0 volts
Outcome 1: If your DMM registered 4.5-5 volts your reference voltage to the PCM is good and you can proceed to the next test.
Outcome 2: If your DMM didnt read ~5.0 volts then double check you have everything in the right position. If you still get a bad reading, you have a bad circuit relaying to and from your PCM, or your PCM is toast. Thus your TPS is probably not the problem.
Test 2: Sensor Return (Ground) Circuit
1) follow the same routine you did in the previous steps, but this time use the black/negative prob from your DMM to check the Sensor Return Ground (bottom wire on the plug). Note: this is a ground wire, see pictures above for reference.
2) Attach the red prob from your DMM to the positive lead on the battery.
3) With the key in the run position (engine off) your DMM should read between 11.6-12.6 Volts.
Outcome 1: If your DMM read 11.6- 12.6 volts then you can proceed to the next test.
Outcome 2: If you DMM didnt read the above voltage you have a bad ground. Either the wire has a short, or the PCM is toast. Thus your TPS is probably not the problem.
Test 3: Testing Throttle Position Signal (may need a friend to help with this one).
1) With the red prob on your DMM pierce and prob the middle wire on the plug (labeled above in the picture).
2) Use the black prob on your DMM and connect it to a good ground, preferably the battery negative lead.
3) Turn the ignition on(truck not running) and check your voltage. You should see .8-1.1 volts.
4) With everything in the same position and the engine off slowly open up the throttle.
5) You should see a steady increase in voltage.
6) Once you get to wide open your voltage should peak around 4.5-5.0 Volts
7) Now slowing let the throttle back to the original position and notice how the voltage readings decrease in a non-spiking manner.
Determining Results from test 3:
Outcome 1: If your readings were the same as posted above your TPS is okay. You should have smooth readings throughout the throttle band with no intermediate gaps or voltage spikes. Opening up the throttle increases voltage, and closing is vice versa.
Outcome 2: If you had no reading what so ever, your TPS is bad. If you are getting large spikes or alternating voltage when increase or decreasing throttle your TPS is pobably bad as well.
PLEASE NOTE: The TPS does not operate your throttle, your throttle actually operates the TPS and TPS just relays the message to the PCM and other necessary components.