pjbgravely
pjbgravely HalfDork
2/1/21 12:39 a.m.

    Last week my 2000 Mustang, V6, 5 speed started acting up. When I am stopped the PCM thinks the car is going from 80 to 113MPH. When it swings to 113 the engine stalls. It is fine when dead cold, and when acting up it doesn't stall until it reaches open loop. When I get faster than 20MPH everything is normal. It is getting there that is the problem. Also the higher the engine revs the lower the speedometer is .

    I tried to change the sensor ( which is just a magnetic pickup) but I got the wrong one. I did confirm that unplugging it made the problem go away. This confuses me, I figured some sort of cross talk was making the fake MPH and that when the sensor started outputting it was overriding the cross talk. Could the sensor be causing the high signal?
    
    I do have the Ford shop computer manuals, but when I tried them on a Win7 virtual machine the wiring diagrams didn't work. The ones I got online show no devices between the VSS and the PCM. Is this correct?

 

Update: Of course changing the sensor did nothing. I  unplugged it again and the false speed stopped. I then pulled on the wire and it came back for a second. I assumed that the problem was a wire from an O2 sensor shorting to the speed sensor wire. I then removed the harness up to the upstream O2 sensors ( I could not get those connectors off with out a lift) and inspected and tested the wires and nothing wrong was found. I put it all back together. I was having trouble running it because the cam sensor decided to cause trouble. I unplugged the sensor and ran the engine while wiggling all the wires on the harness with the speed sensor hooked up. Nothing I did would make the false signal stop.

The next day I worked on the cam sensor, I found a crack in the magnet when I removed it. I started the engine and ran it to full temperature with out the speed sensor problem coming back. The next day I replaced the cam sensor. The engine and speed have been normal since. I have no idea what the cause was. It has been normal for weeks.

I still haven’t been able to access the wiring diagram on the program, I even tried on win2000 and winXP.

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