Do I have a bad TPS?
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Do I have a bad TPS?
I have a 99 F250 LD 4x4 w/ 5.4
I just back probed my TPS ( Yeah my mind went there too ) and it seems
to be bad. I think. It does just the opposite of what my chilton's say's it
should do . Throttle closed 3.8 volts. Throttle wide open .84 volts. The
reference voltage was 4.85 volts. Seems suspicious that the closed
throttle and open throttle would be opposite of what they're suppose to be.
Did I do it correctly? Grnd middle, signal top, reference bottom. It tells
me the TPS is bad. How about you guys?
I just back probed my TPS ( Yeah my mind went there too ) and it seems
to be bad. I think. It does just the opposite of what my chilton's say's it
should do . Throttle closed 3.8 volts. Throttle wide open .84 volts. The
reference voltage was 4.85 volts. Seems suspicious that the closed
throttle and open throttle would be opposite of what they're suppose to be.
Did I do it correctly? Grnd middle, signal top, reference bottom. It tells
me the TPS is bad. How about you guys?
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Senior User
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Bad TPS ADDITIONAL INFORMATION
I posted a thread earlier about a possibly bad TPS. Here is some additional
information about driveability issues. Slow idle speedwhen cold. Transmission downshifts with slightest touch of the accelerator. I know it could also be my PCM but not sure. Hoping not!
99 F250 LD w/5.4 - 200,000 miles.
information about driveability issues. Slow idle speedwhen cold. Transmission downshifts with slightest touch of the accelerator. I know it could also be my PCM but not sure. Hoping not!
99 F250 LD w/5.4 - 200,000 miles.
Cargo Master
Joined: Apr 2003
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From: Frequently frozen MN
I don't know too much about your specific truck, but it's hard to believe that the TPS would read backwards. It's just a variable resistor.
You might try checking all 3 pins vs a different ground. The reference should stay steady, the ground should be 0 v (duh), and the signal should vary.
I also think that there are other things that could cause your symptoms besides the pcm. I don't know if the computer has a lot of control over cold idle, although it might. How about a bad temp sensor? If it reads warm when the engine is cold, it would screw up the idle (I think). Just a thought.
You might try checking all 3 pins vs a different ground. The reference should stay steady, the ground should be 0 v (duh), and the signal should vary.
I also think that there are other things that could cause your symptoms besides the pcm. I don't know if the computer has a lot of control over cold idle, although it might. How about a bad temp sensor? If it reads warm when the engine is cold, it would screw up the idle (I think). Just a thought.
Elder User
Joined: Jan 2008
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Think about what you're suggesting. If it is actually as you say your engine would rev to WOT on startup and you'd have to push the air pedal(formerly know as the gas pedal) to the floor to get the engine to idle. You'd have to hold your foot to the floor whenever you stop and lift your foot to move forward. Not a very safe condition. Either you're backprobing the wrong pins or your manual is labeled wrong. A TPS doesn't wake up one moring and decide to work backwards. TPS's don't get PMS.
Posting Guru
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"Think about what you're suggesting." The TPS doesn't actually control the throttle, it only senses the throttle's position. Many vehicles that use a TPS will actually run w/o the TPS even hooked up!
Its easy enough to see how back probing a TPS, w/o letting your mind wander, might still end up backwards based on Chilton's. A bad TPS usually throws a code, often the failure is revealed by drop outs/dead spots in the sweep of the TPS when tested w/VOM. The common fault is at idle, where wear is concentrated most & the motor will have inconsistant idle & try to stall and/or surge. Never heard of downshifting at slightest touch of throttle as symptom of TPS.
Its easy enough to see how back probing a TPS, w/o letting your mind wander, might still end up backwards based on Chilton's. A bad TPS usually throws a code, often the failure is revealed by drop outs/dead spots in the sweep of the TPS when tested w/VOM. The common fault is at idle, where wear is concentrated most & the motor will have inconsistant idle & try to stall and/or surge. Never heard of downshifting at slightest touch of throttle as symptom of TPS.
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More Info. on TPS and driveability
Here is more information on driveability. Sometimes it will hesitate before
accelerating. It does go back and forth Idling correctly, and to
slow , when cold. My main concern is the downshifting. I was just thinking that the
tps sends info to the pcm about when to downshift , like for passing. I was just taking a shot in the dark to be honest. I know the idle problem
could also be the icv or dirty maf. sensor. The shifting problem is my
major concern. Tranny shifts smooth . Could the pcm need to be
reflashed ? The old owner did replace the torque converter and the pump
in the tranny recently so I know it's been out of the truck.
I did recheck the tps. I backprobed every combination and it does
the same thing. Actually, I can swap between ground to signal and ground to ground and it gives me the same reading. Strange! The reference wire remains the same at 4.85 volts. Throttle open or closed , so I know that is the correct wire .
accelerating. It does go back and forth Idling correctly, and to
slow , when cold. My main concern is the downshifting. I was just thinking that the
tps sends info to the pcm about when to downshift , like for passing. I was just taking a shot in the dark to be honest. I know the idle problem
could also be the icv or dirty maf. sensor. The shifting problem is my
major concern. Tranny shifts smooth . Could the pcm need to be
reflashed ? The old owner did replace the torque converter and the pump
in the tranny recently so I know it's been out of the truck.
I did recheck the tps. I backprobed every combination and it does
the same thing. Actually, I can swap between ground to signal and ground to ground and it gives me the same reading. Strange! The reference wire remains the same at 4.85 volts. Throttle open or closed , so I know that is the correct wire .
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I tested mine and had the same result, seemed like it read backwards, Swapped the leads and got negative readings, but same higher number at idle and lower at wot. Possible the manual has something wrong. Seems like if it was reading incorrectly it wouldnt run or would run like crap.
Elder User
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Decided to do a little experiment to narrow this problem down.
Disconnected TSP: Engine starts, idles and runs as normal
Disconnected MAF: Same results
Disconnected both TSP and MAF: Starts and runs as normal, but won't idle. Have to hold foot on air pedal to keep engine running. "SES" lite came "ON" immediately.
When TSP is disconnected the PCM must infer throttle postion from MAF and MAP values, but once both the TSP and the MAF are disconnect the PCM says wait a minute you've gone too far. At that ponit you only have engine vacuum(MAP) to base throttle position and air mass on. The PCM knows what vacuum levels relate to specific engine conditions. It still has RPM and MAP so that must be enough to keep the engine running even if it won't idle.
Disconnected TSP: Engine starts, idles and runs as normal
Disconnected MAF: Same results
Disconnected both TSP and MAF: Starts and runs as normal, but won't idle. Have to hold foot on air pedal to keep engine running. "SES" lite came "ON" immediately.
When TSP is disconnected the PCM must infer throttle postion from MAF and MAP values, but once both the TSP and the MAF are disconnect the PCM says wait a minute you've gone too far. At that ponit you only have engine vacuum(MAP) to base throttle position and air mass on. The PCM knows what vacuum levels relate to specific engine conditions. It still has RPM and MAP so that must be enough to keep the engine running even if it won't idle.
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