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

Bob91vert · Dec 23, 2015

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?

Bob91vert · Dec 23, 2015

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 · Dec 23, 2015

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

Bob91vert · Dec 23, 2015

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 · Dec 23, 2015

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....

Bob91vert · Dec 23, 2015

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 · Jan 3, 2016

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 · Jan 3, 2016

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 · Jan 3, 2016

That's what I thought. Thanks! I'll check it after the Seahawks game.
Go Hawks!

Bob91vert · Jan 4, 2016

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.

Bob91vert · Jan 4, 2016

Thanks for the help jrichker. Sorry for any inconvenience. :-/

jrichker · Jan 4, 2016

Bob91vert said:
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 · Jan 5, 2016

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 · Jan 5, 2016

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 · Jan 6, 2016

My trace is fried black. Same one he fixed with the red wire. Time to solder a wire.

Bob91vert · Jan 6, 2016

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.

stang89bidges · Jan 6, 2016

Great troubleshooting guys! Bob, what is the trace? Ground like you thought? Was that internal to the ECU?

stang89bidges · Jan 6, 2016

Oh, is this the trace? The signal return?

Bob91vert · Jan 6, 2016

Bob91vert · Jan 6, 2016

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.

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

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