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If you don't own a Multimeter or DVM or don't know how to properly use the one you have, you will need some one on one help from someone who knows how to correctly use your meter.
Troubleshooting loss of signal ground on pin 46 for 86-90 model 5.0 Mustangs.
Revised Sep-22-2019 to add back probing the connections to test the TPS voltages
A fault in the TPS circuit where the voltage goes above. 4.3 volts at idle can shut off the injectors.
TPS adjustment:
You need to do the testing with the TPS sensor plugged in as it would normally be when the engine is running. Use some safety pins to probe the connector from the back side to make contact with the wires,
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
Always use the Dark green/lt green & Black/white wires to set the TPS base voltage; anywhere from.5 to 1.0 volt is OK
Do the test with the ignition switch in the Run position without the engine running.
Use the Orange/white & Black white wires to verify the TPS has the correct 5 volts source from the computer.
Adjusting the TPS fails to resolve the problem:
If the adjustment does not work to get the voltage below 1 volt, you probably have a bad signal ground. The black/white wire is computer pin 46 and is signal ground for many things. If it burns up inside the computer you get multiple faults and cannot pull codes.
Disconnect the positive battery cable to insure correct results when measuring the resistance of grounds. The small voltage drop that is often in a circuit can cause erroneous readings. Since the computer and several other things still draw current even with the ignition switch in the Off position, this is a necessary step.
Check the black/white wire resistance. Connect one ohmmeter lead to the black/white wire on the TPS and one lead to the negative post on the battery. You should see less than 1.5 Ω; more than that indicates a problem. Always take resistance measurements with the circuit powered off.
Check the resistance of the black/white signal ground on the MAP/BARO sensor on the firewall behind the upper intake manifold. The resistance should be less than 1.5 Ω If the resistance value of the black/white signal ground on the MAP/BARO is1.5 Ω and the TPS is higher or doesn't read at all, then there is a problem in the engine mounted fuel injection harness or the 10 pin connector
Clean the 10 pin salt & pepper shaker connectors.
Diagram courtesy of Tmoss & Stang&2birds
See the graphic for the 10 pin connector circuit layout.
The injector power pin is the VPWR pin in the black 10 pin connector. [/b]
How it is supposed to work:
The black/white wire (computer pin 46) is signal ground for the computer. It provides a dedicated ground for the EGR, Baro, ACT, ECT, & TPS sensors as well as the ground to put the computer into self test mode. Since it is a dedicated ground, it passes through the computer on its way to the computer main power ground that terminates at the battery pigtail ground. It should read less than 1.5 Ω when measured from anyplace on the engine harness with the battery pigtail ground as the other reference point for the ohmmeter probe. What sometimes happens is that it gets jumpered to power which either burns up the wiring or burns the trace off the pc board inside the computer. That trace connects pins 46 to pins 40 & 60, which are power ground for the computer.
See
http://www.stangnet.com/mustang-forums/749974-computer-issue.html#post7490537 for Joel5.0’s fix for the computer internal signal ground.
How to test the wiring:
Disconnect the positive battery cable to insure correct results when measuring the resistance of grounds. The small voltage drop that often in a circuit can cause erroneous readings. Since the computer and several other things still draw current even with the ignition switch in the Off position, this is a necessary step.
With the power off, measure the resistance between the computer test ground (black/white wire) on the self test connector and battery ground. You should see less than 1.5 Ω
If that check fails, remove the passenger side kick panel and disconnect the computer connector. There is a 10 MM bolt that holds it in place. Measure the resistance between the black/white wire and pin 46: it should be less than 1.5 ohms. More that 1.5 ohms is a wiring problem. If it reads 1.5 ohms or less, then the computer is suspect. On the computer, measure the resistance between pin 46 and pins 40 & 60: it should be less than 1.5 ohms. More that that and the computer’s internal ground has failed, and the computer needs to be replaced.
Measure the resistance between the black/white wire on each of the following sensors: TPS, ECT, ACT and EGR. If you find one that is greater than 1.5 Ω, measure between that sensor and pin #1 of the white 10 pin connectors. Pin #1 is the center pin and is labeled sig-rtrn on the diagram
See the following website for some help from Tmoss (diagram designer) & Stang&2Birds (website host) for help on 88-95 wiring
http://www.veryuseful.com/mustang/tech/engine/ Everyone should bookmark this site.
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