i think my computer is toast.

Use the DVM, turn key on, Test the Black wire at the TPS to a good Ground. Whats the Voltage?

Unplug the TPS from its connector and, then test the TP signal( green wire on TPS)to SIGRTN( Black) wire. Whats the Voltage? This is done on the Main harness connector, not the TPS's connector. If 5v isnt present, you need to test each one seperate to a good ground. If 5v still isnt present on the TP signal wire, you need to ohm out the wire at the TPS and PIN 47 at the ECM, with the TPS and ECM unpluged, battery too.

What exactly did you do when this happened?

How are you trying to pull codes? Scanner? Test Light?

I would also check your 10 PIN connectors.

Alternate way to pull codes is the Diagram in the Bottom Left hand corner. You can watch the CEL or use the test light.

so what do you think it is then? tps?

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If the code 67 is still showing in the KOEO then that is why you can't pull codes in the KOER test. You can try and install a jumper wire or fuse in the White plug on the Clutch switch then try and run the KOER codes.

Also make sure the AC is off, as it will throw the same code 67.

No info about your car's model year, EFI or carb, no mods listed. Makes it hard to troubleshoot when we don't know what you have.

Therefore, no clue as to which wiring diagram you need. The wiring changed several times between 79 & 95.

That's why it is a good idea to use the sig under the user control panel (User CP) option. It allows you to post your car year & mods, which help greatly when troubleshooting things. No, it is not there for us to snoop and see any "Secrets" you have hidden away under the hood. Be a good stangnetter and update you sig for future reference & don't keep us guessing.

dresmnkaos said:

i went by the shop but only had like 10 min. to check some stuff you asked for.
i unplugged the TPS connector and checked the 3 prongs on the main harness connector. i used the DMM's black lead as a common ground. i checked with the DMMs red lead to the TPS connectors black wire (4.86v), green wire (0.01v), orange wire (4.99v). so where do i go from here? ive never had to troubleshoot electrical problems. this is all new to me.

you said in a above post to ohm out pin 47, but im not sure how to do that. on my DMM do i put the selector on the lowest ohm setting? i think the lowest is 200. IIRC, the screen reads 0.00.

i tried the test light way to pul codes but the light stays on. never flashes.

would you be willing to call me and talk to me about this? if so PM me and ill give you my #. be easierthan going back and forth. again thanks for your help

Click to expand...

If the black wire(SIGRTN) for the TPS was tested with the TPS pigtail disconnected. Then thats not good. The SIGRTN should not have any voltage( or minimal). Its a sourced ground from the ECM. And thats a sign of a Burnt circuit on the PCB on the ECM.


PM sent also

Was this car an Auto car at one point?

Computer will not go into diagnostic mode on 86-90 model 5.0 Mustangs

Disconnect the battery positive terminal before making any resistance checks.
The small voltage drop in the battery cables will cause incorrect resistance readings.

Computer diagnostic connector:



How it is supposed to work:
The black/white wire (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. If this ground is bad, none of the sensors mentioned will work
properly. That will severely affect the car's performance. You will have hard starting, low power
and drivability problems. 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 ohms 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 the test connector black/white wire gets jumpered to power.
There is a dark brown connector with a black/orange wire near the diagnostic connector. It is
the 12 volt power to the under the hood light. If this happens, it 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. Only an experienced electronics technician can open the computer up & repair the trace if
it burns up and creates an open circuit.

The STI (Self Test Input ) is jumpered to ground to put the computer into test mode. The STI
has a gray connector shell and a white/red wire Jumpering it to power can produce unknown
results, including damage to the computer. The ohm test simply verifies that there are no breaks
in the wiring between the test connector and the computer input.

How to test the wiring :
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 ohms.



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 on the computer wiring connector: 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.

If the first ground check was good, there are other wires to check. Measure the
resistance between the STI computer self test connector (red/white wire) and pin 48 on
the computer main connector: it should be less than 1.5 ohms. More that 1.5 ohms is a
wiring problem

The following is a view from the computer side of the computer wiring connector.


Diagram courtesy of Tmoss & Stang&2birds

Check out the diagram and notice all the places the black/white wire goes. Almost every
sensor on the engine except the MAF is connected to it.


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



See Computer issue? - Mustang Forums at StangNet for more help on fixing the computer innards.

Ive been told that some 4 banger setups can cause problems in the clutch pedal circuit. I havent found out myself. But the Source of that info is the most reliable I have seen in a long time.

As I stated the fact that this all happend after the Clutch Switch was plugged up is a indication of that. Also the Wrong 02 harness can cause these issues also. There is a mis-match of the arrangement in the harness between an Auto setup and Manual, and from Manual to Manual but different years.

As I said, haveing close to 5v at the SIGRTN wire on the TPS or any other sensor that uses the same SIGRTN circuit, is a indication a burnt trace on the PCB board of the ECM.

After you do JR's test, while you have the ECM out, I would open the ECM up and look for something like this


Another test is just to Unplug the ECM and test the same black wire on the TPS to a ground with the KEY on. If the voltage is gone, its in the ECM, if its still there, its in the wiring.


Keep in mind that if the wrong 02 harness is used or if the 4 banger Clutch circuit is the problem, it only blast the ECM with 12v during cranking. So there a few more test to be done on the harness while the ECM is out, just to confirm that the ECM wont fry( it thats the case) again.