Erratic Speedometer behavior

[patman0911]

For a couple of months now my speedo has been hanging up occasionally
at 50 MPH and then jumping up to the correct speed once you get up to
60 or 65 and then it tracks correctly unless you dip below 50 and it
gets hung up again. It was a mild annoyance but it wan't annoying
enough to do anything about it yet, I figured the needle was rubbing
on the face or the coil was developing dead spot or something.

Now it's doing something new. The last couple of days it occasionally reads high between 1-40 MPH, like it jumps up to about 20 as soon as you start rolling more than a couple MPH and it reads 40 when I'm really doing about 30. From 40~50 it eases up and then it stays between 50~55 no matter how much faster I go. Sometimes it starts doing this right away, sometimes it doesn't start doing it until I've been driving for a little while and sometimes it goes back to normal after some more driving.

Tachometer reads correctly as do the other dash gauges.

PCM reports the correct vehicle speed regardless of what the speedo is saying which I think rules out a bad VSS gear or cable, right?

So, speedo going bad then?

 

[nyuk98gt]

patman0911:

Remove the VSS from the transmission and inspect the gear. it is possible that the teeth are worn off the driven gear, or, the VSS itself might be failing.

Clean the connector contacts on the VSS. That is under the car and subject to a lot of water and road crud. A good spritze with contact cleaner (keep it away from your paint) should help.

If you have a SpeedCal installed then check the connections.

If you have had overlay gauge covers then the speedo needle may be pushed onto the spindle too far. You can use a salad fork to lift it a bit and that might clear up the problem.

HTH,

Chris

 

[patman0911]

Does the PCM get its vehicle speed info from somewhere other than the VSS? If not, I don't think it could be the gear or cable since the speed is still correct from the PCM.

No SpeedCal installed.

No overlays but I did have the panel out once to replace the odometer gears. That was my first thought when it stated hanging at 50, that it was rubbing but I don't think that's it now since friction wouldn't make it read proportionally higher.

 

[nyuk98gt]

patman0911:

The VSS signal goes to the speedo and to the PCM. There is an output shaft sensor (OSS) in our cars (pre-99) that is also used by the transmission (via the PCM). Anyway, it looks like the VSS is generating a good signal. I am not sure how to test the speedo driver motor on the instrument cluster but you may want to begin looking for a replacement cluster now. When you fixed the odo gear, it is possible that the speedo needle hub got pushed in a little bit and the motor was damaged. You can try the salad fork method to move the needle up on the spindle just a bit (don't lift it all the way off).

HTH,

Chris

 

[patman0911]

Yeah, that's what I was thinking.

I may pull the instrument panel and see if there's any flaking looking components on the driver board for the speedo.

There's a junkyard down the road with a wrecked GT vert sitting in it... may be paying them a visit soon.