Mustang TPS Adjustment - 0.999999v not necessary

jrichker

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Troubleshooting low or missing 5 volt Vref

All tests done with the ignition switch in the Run position but the engine is not running.

The orange/white wire supplies 5 volts reference (Vref) for the TPS, MAP/Baro and EGR sensors.

Engine Mounted sensors
fuel-injector-wiring-harness-sensors-for-a-5-0-mustang-gif.63347


The TPS and EGR get their 5 volt Vref through the engine mounted fuel injector harness & the 10 pin connectors.

See the graphic for the 10 pin connector circuit layout.
salt-pepper-10-pin-connectors-65-jpg.68512


A bad connection or broken wire in the engine mounted fuel injection harness is not uncommon. Check the EGR sensor orange/white wire for good 5 volt Vref. If it is good there, the TPS has either a broken wire or bad connection.
Good 5 volt Vref at the ERG sensor, then you have a 10 pin salt & pepper connecter problem or a computer problem.

Check for 5 volt Vref on the orange/white wire at the firewall mounted MAP/Baro sensor. Good 5 volt Vref there, then the problem is in the 10 pin salt & pepper connectors or in the engine mounted fuel injection harness.

190627-jpg.537541


No 5 volt Vref at the MAP/Baro sensor, then the problem is either a broken wire in the main fuel injector harness or the computer has died.

Remove the passenger side kick panel to gain access to the computer.
a9x-series-computer-connector-wire-side-view-gif.71316

Locate the orange/white wire (pin 26) on the computer connector and check for 5 volt Vref. Good 5 volt Vref and the computer is OK.

Diagrams courtesy of Tmoss & Stang&2birds

Complete computer, actuator & sensor wiring diagram for 91-93 Mass Air Mustangs
91-93_5.0_EEC_Wiring_Diagram.gif


No 5 volt Vref, then the computer has died. It is time to get out your wallet and go hunting…
Expect to pay $100 or more for a replacement computer. eBay has a repair service that is less expensive.




See the following website for some help from Tmoss (diagram designer) & Stang&2Birds (website host) for help on 88-95 wiring Mustang FAQ - Wiring & Engine Info Everyone should bookmark this site.

94-95 Mustang wiring diagrams
Mustang FAQ - Wiring & Engine Info

Complete computer, actuator & sensor wiring diagram for 91-93 Mass Air Mustangs
http://www.veryuseful.com/mustang/tech/engine/images/91-93_5.0_EEC_Wiring_Diagram.gif

Complete computer, actuator & sensor wiring diagram for 88-91 Mass Air Mustangs
http://www.veryuseful.com/mustang/tech/engine/images/88-91_5.0_EEC_Wiring_Diagram.gif

Ignition switch wiring
http://www.veryuseful.com/mustang/tech/engine/images/IgnitionSwitchWiring.gif

Fuel, alternator, A/C and ignition wiring
http://www.veryuseful.com/mustang/tech/engine/images/fuel-alt-links-ign-ac.gif

O2 sensor wiring harness
http://www.veryuseful.com/mustang/tech/engine/images/mustangO2Harness.gif

Vacuum diagram 89-93 Mustangs
http://www.veryuseful.com/mustang/tech/engine/images/mustangFoxFordVacuumDiagram.jpg

HVAC vacuum diagram
http://www.veryuseful.com/mustang/tech/engine/images/Mustang_AC_heat_vacuum_controls.gif

TFI module differences & pin out
http://www.veryuseful.com/mustang/tech/engine/images/TFI_5.0_comparison.gif

Fuse box layout
http://www.veryuseful.com/mustang/tech/engine/images/MustangFuseBox.gif

Mustang 5.0 Lights and Radio schematic, by TMoss:
http://www.veryuseful.com/mustang/tech/engine/images/mustangFoxLights-Radio_diag.gif

87-92 power window wiring
http://www.veryuseful.com/mustang/tech/engine/images/mustang87-92 PowerWindowWiring.gif

93 power window wiring
http://www.veryuseful.com/mustang/tech/engine/images/mustang93PowerWindows.gif

T5 Cutaway showing T5 internal parts
http://www.veryuseful.com/mustang/tech/engine/images/5_Speed_Cutaway_Illustrated.jpg

Visual comparison of the Ford Fuel Injectors, picture by TMoss:
http://www.veryuseful.com/mustang/tech/engine/images/Ford_Injector_Guide.jpg

Convertible top motor wiring http://www.veryuseful.com/mustang/t...g/tech/engine/images/mustang88VertTopMotorCkt
 
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akaNYr

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Added more tech data. Hopefully any tps questions can be directed here
Added more tech data. Hopefully any tps questions can be directed here
This is a an old post but I'm going to throw this out here anyway. SN95's don't have an adjustable TPS. Drilling the holes does nothing to adjust volts. I tried it and the TPS didn't move. SN95 TPS Volts are adjusted by the screw under the rubber cap on top of the throttle body. I think it's called the idle air bypass screw. It takes an Allen key to adjust It. It took many days of aggravation and research to find this out. There should be separate forums titled 'SN95 tech info' and 'Fox body tech info'.
 

BlakeusMaximus

Still got to try a little lube on my speedo head
Jul 12, 2017
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There are separate ones. Fox 5.0 tech and 94-95 specific tech.
 

jrichker

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This is a an old post but I'm going to throw this out here anyway. SN95's don't have an adjustable TPS. Drilling the holes does nothing to adjust volts. I tried it and the TPS didn't move. SN95 TPS Volts are adjusted by the screw under the rubber cap on top of the throttle body. I think it's called the idle air bypass screw. It takes an Allen key to adjust It. It took many days of aggravation and research to find this out. There should be separate forums titled 'SN95 tech info' and 'Fox body tech info'.
None of the Mustangs had an adjustable TPS.

You missed the point entirely, which is that attempting to adjust the TPS is totally infective for increasing performance.
When the computer powers up, it takes whatever reading the TPS puts out as the zero or starting point to look for increases beyond that value. If you set it at 0.99, the computer thinks that is the zero point and looks for increases beyond that value.
 
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akaNYr

Member
Jun 13, 2017
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None of the Mustangs had an adjustable TPS.

You missed the point entirely, which is that attempting to adjust the TPS is totally infective for increasing performance.
When the computer powers up, it takes whatever reading the TPS puts out as the zero or starting point to look for increases beyond that value. If you set it at 0.99, the computer thinks that is the zero point and looks for increases beyond that value.
I'm not trying to increase performance, I'm trying to stop my car from suddenly surging to 2,500 or 3,000 RPM'S.
 

RaggedGT

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93gtmustang

Member
Oct 21, 2006
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Checked my TPS. Key on was .75 WOT 4.58
That's within the proper range right?
Does it matter if the car is cold or hot?
Is it necessary to check it at idle?
 

Mustang5L5

i'm familiar with penetration
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Feb 18, 2001
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No, not necessary to check with car running. Throttle blade position at idle and with car off is the same. Battery voltage might be a tad bit higher though, so actually value might be higher.

you can double check by running your codes and seeing if any tps codes pop up
 

93gtmustang

Member
Oct 21, 2006
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No, not necessary to check with car running. Throttle blade position at idle and with car off is the same. Battery voltage might be a tad bit higher though, so actually value might be higher.

you can double check by running your codes and seeing if any tps codes pop up
Thanks!
 

Mustang5L5

i'm familiar with penetration
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Feb 18, 2001
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Where did the. 99 volt myth come from?


I believe it was in a Mustang magazine or early website back in the day. Part of the 5-minute tuneup.

It's essentially non-adjustable, but the myth still persists.

It can still be faulty, which causes issues in how the car runs, but setting it to an arbitrary number is meaningless which you take into account what the ECU is doing with this number.
 

Dan02gt

10 Year Member
Mar 2, 2003
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Where did the. 99 volt myth come from?
My guess is that the logic behind the myth is that the ideal voltage for the TPS is 5v at WOT so setting the TPS at .99V with the TB (the max they thought was acceptable) ensures the WOT TPS voltage is at high as possible to ensure the ECM's WOT strategy is used. Now we know there is a range that's acceptable for both and as long as you are in that range it makes no difference in performance.
 

7991LXnSHO

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Sep 1, 2010
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Where did the. 99 volt myth come from?
I remember MM& FF or Hot Rod magazine doing a Ten Minute Tune Up, (maybe it was 5), and a track test. They were trying all the low or no buck tricks they heard about trying to get the cars faster & quicker when they were much newer and parts were still being developed. I which I had saved that article for fun.
 
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