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How to: Ford Explorer - Ranger TPS Test Procedure

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Thanks Pontisteve for the excellent ideas. Before chasing vacuum leaks, which sounds like the likely culprit, could a faulty IAC with a weak internal spring, allowing it to stay open without a signal from the PCM to do so, cause my symptoms? Presently when the engine is warm and finally idling at 750, the rpm's will drop to 550 when I disconnect the IAC. This suggests that the IAC opens on demand. Should it also be open and allowing additional air flow between shifts or when coasting in neutral? I would guess that it should be closed at these times, but if it remained open, the rpm's would over-rev between shifts (as it does) and hang higher than 750 rpm when coasting downhill in neutral (as it does). Could I test this idea by disconnecting the IAC, adjusting the throttle to idle at 750 on its own, and driving the truck without an IAC? If a vacuum leak was still the culprit, the over-revving symptoms should continue. Are there other reasons not to operate the truck without an IAC other than the challenge of cold weather starting and warmup?
 



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Dont do that or youll lose the factory throttle body stop screw setting! Iac is used while driving too. More later on.
 






So if I get 0 voltage from the TPS with key on, the sensor is bad? But I get 5.0 volts from the TPS wire harness with is disconnected from the sensor with the key on the the TPS sensor is bad right? I am just trying to figure out my no start issue and I am at the test the TPS sensor part of the No-start procedure and those were the readings I got.
 






So if I get 0 voltage from the TPS with key on, the sensor is bad? But I get 5.0 volts from the TPS wire harness with is disconnected from the sensor with the key on the the TPS sensor is bad right? I am just trying to figure out my no start issue and I am at the test the TPS sensor part of the No-start procedure and those were the readings I got.

The TPS signal wire should have some voltage at idle. Usually about .98 volts, give or take.

If you have zero volts, first check your meter to make sure when you touch a power source that you are getting 12 volts. If not, the meter ground is bad. Next make sure you are measuring the TPS signal wire, not TPS ground wire.

If still 0 volts, then check the vref wire to make sure the sensor is getting 5v from the computer. If it is, the TPS sensor is bad.
 






These days, an OBD2 scan tool is just about mandatory. There's just so much you can't do without one. Plus, they've gotten cheaper. Most low cost scan tools use generic obd2 data. Better tools will have "enhanced" data, meaning they bought a license from the manufacturer to use their proprietary data. This data is much higher quality, and covers more stuff. Generic obd2 only covers emissions related stuff. Enhanced covers things that don't necessarily affect emissions.

Better yet, get a scan tool that has all 9 modes. Most $100 - 200 tools only have the basic engine mode. Some have transmission too. Better tools will have all 9 modes, including ABS, airbag, body control modules, etc. While it's not the fastest refresh rate, I bought an OTC Nemisys because it came with all 9 modes, plus Ford, GM, Dodge enhanced OBD2 data. I later added Japanese enhanced data package. I paid like $500 for it, which was far cheaper than anywhere else.

For a more basic tool, I would look at an engine/trans scan tool made by Electronic Specialties maybe. Their top of the line is like $200. You could also look at obd interfaces for laptops, which have great datalogging and nice software, but also cost more to upgrade to enhanced data. Plus, you have to have the laptop in the car. That can be a pro or a con. The handheld is much faster and easier to use, but slower refresh rate. When I want seriously powerful data, I hook up my laptop and SCT Xcal3, which does all enhanced engine/trans PIDs and DMRs (data).

You'll pretty much need a scan tool with live datastream to read the coolant and IAT sensors, but you can read the TPS with a voltmeter. To do that correctly, you need some real thin backprobe pins, to poke into the back side of the harness connector. Shoving in regular voltmeter tips is a good way to damage the connector. A low buck way of doing this would be to use straightened paperclips as back probe pins, and then use the alligator clamp tips for your voltmeter probes.

TPS should be around .98v at idle. If it's under .9 or over 1.05, that's very suspect. Be sure to use both the power and ground from the TPS connector. If you ground the voltmeter to the engine or body, you don't have an actual PCM ground loop, and the accuracy could be off depending on ground offsets from the PCM to the battery. Just use the black TPS wire as a ground, and that issue goes away.
I have back probed wires , while connected and obtained good readings. When I disconnected and put the probes on the other side of the connector the signal reading doesn't budge when I move the throttle. Can the connector possibly have gone bad ?
 












If you don't have a wiring diagram, it's easy to tell what's what with some simple voltage checks. Take a multimeter and connect the black lead to battery negative. Then backprobe each of the three wires. A sewing T-pin makes a very good backprobe, and you can buy a box of them at Walmart in the sewing section for $2. Just only use one T-pin at a time, so you don't accidentally touch two wires together.

The key should be on and the engine should be off for this test.

One of the three wires should have right at 5.0 volts. That's what we call VREF, or reference voltage. No less than 4.9 volts, no more than 5.1v.

One of the wires should have a small amount of voltage on it, like 20 to 50 millivolts. This is your sensor ground wire. It's also called Signal Return or Ground Ref.

One should have about 1 volt on it. That's your signal wire. This is the one we're really interested in.

Now that you know which wire is the Signal wire, hook your multimeter red lead to that terminal via a backprobe. Then, hook your multimeter's black lead to the Signal Return wire via a backprobe. You don't need to mess with the 5v VREF anymore, once you've confirmed it's there and correct.

When the throttle is closed, you'll have between .9 and 1.1 volts, with .99 to 1.03 being very common.

Dirty Throttle Body test: Open the throttle all the way, and then close it very slowly. We want to land gently here. Once it's closed, write down the TPS voltage. Next, open the throttle all the way again, and slide your foot off the pedal sideways, making the throttle slam shut as fast as possible. You should see the same TPS voltage. If you have less voltage doing this than you do closing it slowly, there is a good chance your throttle body is dirty, and doing a Throttle Body Cleaning service (carb cleaner and a tooth brush to throttle plate and throttle bore) may help you resolve some idle issues.

Next test: The sweep test. Slowly open the throttle by hand (engine off). Watch your multimeter slowly rise in voltage from 1.0 volts to about 4.65 volts. You should see the numbers only rise, never fall. And they should rise just as consistently as your hand opens the throttle. Once fully open, slowly close it and watch the voltage drop. The most likely spot for this test to fail and voltage to "drop out" momentarily is in the first 35% of the throttle range, but it could happen anywhere. If you have a graphing multi-meter or scope, this test is even better.

Repeat the sweep test in the lower 35% of the throttle range multiple times. A failure may not be consistent, so we're trying to catch it in the act. Also, try this test on a cold engine, as well as a hot engine.

Next test: The snap test. Open the throttle rapidly and watch the voltage. It should jump to the top of the range and stay there. Probably about 4.64 volts. Then slam the throttle shut quickly and see if it stays there, around 1.0 volts. You don't want to see it quickly drop below that and recover. Voltage should never get below your idle voltage threshold. A min/max function on a quality multi-meter can help you here, since the meter can actually think faster than it can display.

A final word about TPS testing. While a pretty bad TPS sensor can easily be caught acting up with a voltmeter, a very fast issue may not be so easily caught. Even the best Fluke multi-meter is not really all that fast. And even slower to display the voltage on it's screen. It simply doesn't have a fast enough sample rate to catch any really difficult issues. So all these tests might pass with flying colors, and your TPS could still be bad. The only thing truly fast enough to catch ALL TPS glitches is an oscilloscope or at least a graphing multi-meter.

A good multi-meter probably reads voltage several times a second. My Fluke 88-V updates the digital display 4 times per second. However, it also has an analog bar graph that updates 40 times per second, so you can look at the bar graph during a TPS test and may see a dropout that the numbers don't show.

My Picoscope can measure up to 80 million samples per second. Nothing hides from the Pico. Or any other scope, really. Also, that type of scope records the measurements and stores them, then draws a graph on the screen. So we don't have to catch it with our eyes in the act. We can just go back and look at the picture, and see the dropout.
 






Thanx for your help. I tested as you described and results were good. Got to look for something else I guess !
 






Thanx for your help. I tested as you described and results were good. Got to look for something else I guess !

What is the truck doing wrong? Does it have any codes or a check engine light on? You might want to make a new thread if it's not TPS related.
 






When driving and push the pedal to accelerate there is no power . Have cleaned Maf sensor so felt it had to be TPS, but it seems ok . Idle is fine. There was a check gas cap light on and I cleared it , it has not come back on. Not sure what to check next.
 






When driving and push the pedal to accelerate there is no power . Have cleaned Maf sensor so felt it had to be TPS, but it seems ok . Idle is fine. There was a check gas cap light on and I cleared it , it has not come back on. Not sure what to check next.

When you have no power at WOT, the problem is either going to be a dirty or bad MAF, low fuel pressure, or a plugged exhaust (melted cat typically).

Using a scantool, drive it, floor it, and watch for MAF voltage to jump to at least 4 volts. Watch for the O2 sensors to go to rich (.9v). If you have a fuel pressure gauge, make sure the fuel pressure remains fixed. Thsts probably around 58 to 65 psi.

A clogged cat isn't likely, unless you've had misfired that you let go without fixing for a while. A failing fuel pump is the most likely thing here. Maybe a really clogged fuel filter.
 






When driving and push the pedal to accelerate there is no power . Have cleaned Maf sensor so felt it had to be TPS, but it seems ok . Idle is fine. There was a check gas cap light on and I cleared it , it has not come back on. Not sure what to check next.

When you have no power at WOT, the problem is either going to be a dirty or bad MAF, low fuel pressure, or a plugged exhaust (melted cat typically).

Using a scantool, drive it, floor it, and watch for MAF voltage to jump to at least 4 volts. Watch for the O2 sensors to go to rich (.9v). If you have a fuel pressure gauge, make sure the fuel pressure remains fixed. Thsts probably around 58 to 65 psi.

A clogged cat isn't likely, unless you've had misfires that you let go without fixing for a while. A failing fuel pump is the most likely thing here. Maybe a really clogged fuel filter.
 






Sorry for the necro bump but I've been chasing a 173 code(running rich) on my 92nexplorer for awhile and someone mentioned the Tps...

So I did the test and it was .86 at closed and 4.66 at WOT

So that would mean that the Tps is good right?
 






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