Groud at TPS

[shockertech]

Ground at TPS

Have a 1990 coupe, V8 conversion. Cannot get TPS voltage to adjust, reads 0v. Checked orange wire at TPS, reads 5v when checked with ground at negative post. Where does the TPS ground?Is it internal in the EEC . I have a new EEC installed. Appreciate any info.

 

[jrichker]

The TPS uses a signal ground that connects inside the computer. The EGR, MAP/Baro, ECT and ACT use the same ground.

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.
Use the Dark Green/lt green & Black/white wires to set the TPS base voltage. 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.
2.) Use an analog voltmeter to check the voltage on the Dark Green/lt green & Black/white wires to make sure that it increases smoothly and doesn’t drop out or increase suddenly.
The TPS sensor is a variable resistor like the volume control on most cheap radios. We have all heard them make scratchy noises as you turn them up or down. The carbon element can wear and cause a drop out spot in the output voltage. This confuses the computer because it expects to see a steady TPS voltage increase as the Throttle opens up.

Some basic checks you can make to be sure that the sensor is getting power & ground:
Note that all resistance tests must be done with power off. Measuring resistance with a circuit powered on will give false readings and possibly damage the meter.
Check the resistance between the black/white wire on the MAP/BARO sensor and then the black/white wire on the EGR and the same wire on the TPS. It should be less than 1.5 ohm. Next check the resistance between the black/white wire and the negative battery cable. It should be less than 1.5 ohm.

The following power on check requires you to turn the ignition switch to the Run position.
Use a DVM to check for 5 volts on the orange/white wire. If it is missing, look for +5 volts at the orange/white wire on the TPS or EGR sensors. Use the black/white wire for the ground for the DVM.



Diagram courtesy of Tmoss & Stang&2birds

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


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

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

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 for94-95 Mass Air Mustangs
http://www.veryuseful.com/mustang/tech/engine/images/94-95_5.0_EEC_Wiring_Diagram.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 & pinout
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

 

[shockertech]

Thanks for the information.

 

[shockertech]

Checked all the wiring. Found ground circuit in EEC burnt. Soldered the circuit board and voltage checks good. Just wondering if anybody has seen this and is it common. Any common causes such as EGR, MAP or TPS sensor causing this. Wiring checks good resistance .05 ohms between all connections. Tried wiggling the harness to see any change but found none. Thanks for any info.

 

[jt88gt]

I've got a 88 gt with maf conv. done to it, and i had the same problem. the ground was good, but found that it had burnt the internal ground in the computer. i probably would have never found the problem if jrichker wouldn't have sent me alot of info. on it.

 

[jrichker]

Here's the most common reason the signal ground inside the computer burns up. The power wire for the under hood light has the same kind of connector as the computer self test connector. It always has power to it so that the optional under hood light works anytime you raise the hood. Looks just like it, except that it is dark brown with a black wire with an orange stripe. Someone wants to dump the codes and plugs the scanner or jumpers the diagnostic test ground to the power wire for the under hood light by mistake. POOF!!! in an instant the trace inside the computer vaporizes and you can no longer dump codes. All the engine mounted sensors loose their signal ground and the car runs often runs like a lead sled.

The correct wire for the self test connector has a gray connector shell and a white/red wire.

 

[shockertech]

Appreciate the info once again. I will definately check those wire colors. Just like jt88gt, I appreciate the help and time you take to help people. You always have great info. Thanks!