Electrical Tps Troubles. Low Voltage At The Vref Or Org/wht Wire.

Bob91vert

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so my car is running great above idle. But is surging at idle. Running thru the checklist Im getting about 1 volt to green. But only .4 to org/wht vref wire. What would cause this low voltage?

Also, can I check continuity on the black white wire by going from the wire to any good ground?
 
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Bob91vert

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Some things to consider...
When I unplugged the computer 60 pin connector I checked ohms at pin 46 to both the signal return on the tps and the diagnostic connectors. The tps was was .3 and the other was .4.
When I plug the computer back in and check both to ground on the battery the tps is at 1130 and the diagnostic plug is 3.78. I'm thinking I have a short in the computer.
I also checked the grounds at pins 20, 40 and 60 and all were .3 or less.
 

jrichker

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Any time the battery is connected to the battery cables there is current flow that affects the accuracy of resistance readings. Disconnect the battery from the ground cable prior to making any resistance measurements.

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


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.


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.



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.

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


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/tech/engine/images/mustang88VertTopMotorCkt.gifhttp://www.veryuseful.com/mustang/tech/engine/images/mustang88VertTopMotorCkt
 

Bob91vert

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For all signal return checks the battery was disconnected.
But how would you get 5 volts on the vref if the battery isn't connected?
 

jrichker

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I am talking about making resistance measurements when I tell you to disconnect the battery.

Start at the top of the test path I posted and work your way down. Do not skip any steps, do not pass go and do not collect $200....
 
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Bob91vert

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Lol, got Ya. I haven't done any resistance checks on the vref yet. But like I said, I did have the battery disconnected when checking the signal return in 2 different places. When I did connect the battery again and check them with the computer plugged in I did get high readings. I do believe I have a fried ground strap from the ground to the sig return on the pc board in the computer but will verify everything in the morning.
 

Bob91vert

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Jrichker,
When checking pin 26 at the computer, do I check that at the back of the connector with the harness still pluged into the computer and the ignition on correct?

I'm not getting 5 volts at any location. Couple other things to note.
Check engine light is on and the car will not go into diagnostic mode.
 

jrichker

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Make ALL checks for voltage with the battery, computer wiring and ignition switch set as if the engine is running. There is no 5 volt Vref if the computer isn't plugged into the wiring harness and the ignition switch in the Run position.

Check for 12 volts on the fuel injector red wires. No 12 volts and the computer isn't getting power. If the computer isn't getting power, there will not be any 5 volt Vref anywhere.
 

Bob91vert

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Well I'm a doomass. I had the fluke on vac instead of vdc. I have 12 volts at the computer and 5 vref volts at all sensors. I have bad resistance thru the computer on the sig return.
I'm going with the computer has a bad ground inside because like I said, when the computer is unplugged I have no resistance thru the harness.
 

jrichker

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Mar 10, 2000
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Well I'm a doomass. I had the fluke on vac instead of vdc. I have 12 volts at the computer and 5 vref volts at all sensors. I have bad resistance thru the computer on the sig return.
I'm going with the computer has a bad ground inside because like I said, when the computer is unplugged I have no resistance thru the harness.
Easy mistake to make, especially if you don't use the meter very often. I have to look closely to see the little markings on the selector dial. I wish they would realize that it doesn't cost more to make them bigger....
 

Bob91vert

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Is there a way to check the signal return in just the computer? I thought since it was a groud, I could check resistance at pin 46 and 20. But I'm getting nothing. The computer is on my bench. Out of the car.
 

jrichker

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Pin 46 and either pin 40 or pin 60. Check the wiring diagram I posted of the computer wiring for 91-93 5.0 Mustang

Pin 20 is the shield ground for the TFI module. It does not connect to the internal power ground on pins 40 & 60.

See Computer issue? | Mustang Forums at StangNet for Joel5.0’s fix for the computer internal signal ground.
 

Bob91vert

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Computer is fixed car is running good idols great going to take it for a spin so I can cut the PPF warmed up but no more surging idle and no check engine light.
 
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Bob91vert

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Fixed. I cant thank guys like Jrichker enough. What an awesome site and people.
Now that the car runs sweet, its off to other tasks on the car. Heater core, new seats, carpet and a new top is in line now.
With the new engine mods done it sure runs strong. I'm very happy with the performance and not even sure I want to re-gear it now. Thanks again Jrichker.
 

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