help tps low voltage

Discussion in 'Fox 5.0 Mustang Tech' started by 89blufox, Jan 7, 2013.


  1. 89blufox

    89blufox New Member

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    My tps is giving a reading of .14 to .25 at full adjustment with the ignition on and only goes up to 1 volt at full throttle.I've tried 3 different sensors and they all read the same. So car surges and dies If I don't keep it running;but it will idle fine until it warms up.it usually starts the surging at 150 degrees.
     
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  2. jrichker

    jrichker StangNet's favorite TOOL SN Certified Technician Founding Member

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    Setting the TPS at .98 or .99 volt is a Bozo Internet Myth. When the computer powers up and initializes the TPS sensor, whatever it reads is the zero starting point for it. In other words your .98 volt becomes 0 volts and the computer watches for increases in voltage from that point upward.

    Setting the TPS: you'll need a good Digital Voltmeter (DVM) to do the job. Set the TPS voltage at .5- 1.1 range. Because of the variables involved with the tolerances of both computer and DVM, I would shoot for somewhere between .6 and 1.0 volts. Unless you have a Fluke or other high grade DVM, the second digit past the decimal point on cheap DVM’s is probably fantasy. Since the computer zeros out the TPS voltage every time it powers up, playing with the settings isn't an effective aid to performance or drivability. The main purpose of checking the TPS is to make sure it isn't way out of range and causing problems.


    Wire colors & functions:
    Orange/white = 5 volt VREF from the computer
    Dark Green/lt green = TPS output to computer
    Black/white = Signal ground from computer

    TPS troubleshooting steps:
    1.) Use the Orange/white & Black white wires to verify the TPS has the correct 5 volts source from the computer.
    2.) Use the Dark Green/lt green & Black/white wires to set the TPS base voltage. Try this... All you need is less than 1.0 volt at idle and more than 4.25 at Wide Open Throttle (WOT). You'll need a voltmeter with a 1 or 3 volt low scale to do the job.

    The Orange/White wire is the VREF 5 volts from the computer. You use the Dark Green/Lt green wire (TPS signal) and the Black/White wire (TPS ground) to set the TPS. Use a pair of safety pins to probe the TPS connector from the rear of the connector. You may find it a little difficult to make a good connection, but keep trying. Put the safety pins in the Dark Green/Lt green wire and Black/White wire. Make sure the ignition switch is in the Run position but the engine isn't running. Set the voltmeter on the 2 volt range if it doesn’t auto range.

    Here’s a TPS tip I got from NoGo50

    When you installed the sensor make sure you place it on the peg right and then tighten it down properly. Loosen the back screw a tiny bit so the sensor can pivot and loosen the front screw enough so you can move it just a little in very small increments. I wouldn’t try to adjust it using marks.

    (copied from MustangMax, Glendale AZ)

    A.) Always adjust the TPS and Idle with the engine at operating temp. Dive it around for a bit if you can and get it nice and warm.

    B.) When you probe the leads of the TPS, do not use an engine ground, put the ground probe into the lead of the TPS. You should be connecting both meter probes to the TPS and not one to the TPS and the other to ground.

    C.) Always reset the computer whenever you adjust the TPS or clean/change any sensors. I just pull the battery lead for 10 minutes.

    D.) The key is to adjust the TPS voltage and reset the computer whenever the idle screw is changed.

    The TPS is a variable resistor, must like the volume control knob on a cheap radio. We have all heard them crackle and pop when the volume is adjusted. The TPS sensor has the same problem: wear on the resistor element makes places that create electrical noise. This electrical noise confuses the computer, because it expects to see a smooth increase or decrease as the throttle is opened or closed.

    TPS testing: most of the time a failed TPS will set code 23 or 63, but not always. Use either an analog meter or a DVM with an analog bar graph and connect the leads as instructed above. Turn the ignition switch to the Run position, but do not start the engine. Note the voltage with the throttle closed. Slowly open the throttle and watch the voltage increase smoothly, slowly close the throttle and watch the voltage decrease smoothly. If the voltage jumps around and isn’t smooth, the TPS has some worn places in the resistor element. When the throttle is closed, make sure that the voltage is the same as what it was when you started. If it varies more than 10%, the TPS is suspect of being worn in the idle range of its travel.

    TPS will not go below 1 volt
    Engine mounted sensor circuit: Check the resistance between the black/white wire on the TPS and battery ground. It should be less than 1.5 ohms. Higher resistance than 1.5 ohms indicates a problem with the 10 pin connector, computer or the splice inside the main harness where the wire from the 10 pin connectors joins the rest of the black/white wire.

    [​IMG]

    See the graphic for the location of the 10 pin connectors:
    Diagram courtesy of Tmoss & Stang&2birds

    [​IMG]

    See the graphic for the 10 pin connector circuit layout.
    [​IMG]

    Unplug the white 10 pin connector to do some resistance testing. It is good time to clean the connector pins and examine the connector for corrosion, broken wire or other damage. See http://oldfuelinjection.com/?p=85 for help in this department.

    If the resistance on the TPS Black/White wire and pin 1 of the white engine fuel injector harness 10 pin connector, is more than 1.0 ohm, you have bad connection or broken wiring. Repeat the test using the pin 1 of the white body side 10 pin connector and battery ground. You should have less that 1.5 ohm. More than that is a damaged signal ground inside the computer or bad connections or wiring.
     
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  3. 7991LXnSHO

    7991LXnSHO Well-Known Member

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    Did you get this set? I have had to oval the holes a little to add adjustment range with my aftermarket TBody.
     
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  4. jrichker

    jrichker StangNet's favorite TOOL SN Certified Technician Founding Member

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    Your best advice is to read my previous post...

    Setting the TPS at .98 or .99 volt is a Bozo Internet Myth. When the computer powers up and initializes the TPS sensor, whatever it reads is the zero starting point for it. In other words your .98 volt becomes 0 volts and the computer watches for increases in voltage from that point upward.
     
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  5. Mustang5L5

    Mustang5L5 Car used in adult film "Highway Gangbang-InDaButt" SN Certified Technician Founding Member

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    Sounds like he's not able to even get it into the range necessary to "zero" the TPS out on startup.

    If he's really getting .15-.25v on startup, then that would create a code stored in the computer.
     
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  6. 7991LXnSHO

    7991LXnSHO Well-Known Member

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    :Jrichker, Do not worry about my sanity. This has nothing to do with .9999999 volts. It has to do with adjusting lower than 1.35 volts on a modified motor.

    I added the oval holes suggestion after I followed your steps, and I followed BBK's instructions too. In both cases, I had to have the TB idle screw open enough to idle that the TPS would not adjust down into the range where codes were not thrown. I checked grounds (see the 10 pin part of his guide) then used my little round diamond file from my jewelers set from H F Tools. As this lack of adjustment range has been reported in many magazine articles, it seems relevant.
     
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  7. Mustang5L5

    Mustang5L5 Car used in adult film "Highway Gangbang-InDaButt" SN Certified Technician Founding Member

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    Do you have 5V on the power wire?

    I do not recall what wire is which, but the TPS has 3 wires. One is the ground, one is 5V and the other is the return wire with the variable voltage that tells the car the throttle position.

    The TPS is a variable resistor I beleive, so if you are getting lower than expected voltage out of it, i wonder if you have lower than 5V going in??
     
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  8. jrichker

    jrichker StangNet's favorite TOOL SN Certified Technician Founding Member

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    Check for +5 volts on the orange wire,. use the throttle body as a ground point. Recheck the 5 volts on the orange wire using the black white wire as the ground point. Any difference greater than .15 volt is indication that the black/white wire needs to be checked for resistance between the negative battery cable.

    Disconnect the negative battery cable: this insures that the small amount of voltage that always flows through the negative cable doesn't upset your readings. Measure the resistance between the black/white wire on the TPS and the negative battery cable. You should see less than. 1 ohm. More than 1 ohm is a wiring problem or a bad signal ground inside the computer.
     
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