Electrical Speedometer Issues

[mlpeace]

I just bought my first Mustang a 2002 gt convertible. It has a couple small issues, the one I'm currently working on is the speedometer. It goes from dead to bouncing around at idle and while driving or sometimes working perfectly. I changed the vss and it did not fix anything. Any help would be appreciated on how to figure this out.

Thank you.

 

[wmburns]

For the 1999+ model years the output of the OSS sensor goes only to the PCM. The PCM then "tells" the cluster what speed to display. One way to trouble shoot this issue is to use an ODB2 reader to "ask" the PCM what the car's speed is. Compare the speedometer to the PCM's values.

IF the PCM is correct, this proves the OSS sensor and PCM. It also means that the problem is in the cluster.

IF the PCM speed is incorrect, this proves the problem is towards the OSS sensor.

For a possible cluster issue, a common problem is with the Printed circuit foil traces becoming de-soldered at the connectors. In some cases it's possible to fix by re-soldering.

EDIT: Corrected reference to OSS sensor.

1996-1998 model years use an VSS sensor. The VSS sensor is a worm gear driven sensor.
1999+ model years use an OSS sensor. The OSS sensor is a non contact reluctor wheel sensor.

 

[mlpeace]

Thank you for the tips. I just did the obd2 idea and it did not read correctly. So I guess that means vss is not working correctly. The vss was changed a couple days ago. Someone said there could be a bad ground, is that possible? I was just told the clips on the connector for the vss are broken off but it plugged in firm. Could it be wiggling around and cause these issues? Im going to put a ziptie on the connector to hold it more firmly today or tomorrow to see if it fixes it before I change the connector. Thank you again for the response.

 

[wmburns]

What happens from time to time is sometimes people don't realize the difference between a VSS and OSS sensor. They think that the Mustang uses the same transmission setup from 1996 through 2004 and incorrectly think they will interchange.

This is important should someone take a transmission from a 1996-1998 model year (VSS) and install into a 1999-2004 model year (OSS).

The output of the OSS vs VSS sensors are not compatible to each other. In addition the sensors are wired and powered differently. This means that the sensor used needs to match the wiring harness and what the PCM is expecting.

One of the symptoms when someone electrically "forces" a VSS sensor into an OSS application is a wildly swinging speedometer.

 

[mlpeace]

A sn7140 from bwd was put in, an oss. No gear on the end. It looked just like the old one that was removed. I haven't had time to put a zip tie on yet, but I will make sure to post the fix if we do get this solved.

Thank you