95 Sport Speedo needle deflecting every time I brake or use LEFT turn signal, or haz

[sabatema]

I have a 95 X Sport 4x4, Auto. Every time I apply the break or the LEFT turn signal, the speedometer needle will deflect wildly to zero. It will pulse from current speed to zero when the Left turn signal is used but is unaffected by the right turn signal? Mechanic ran many tests but can't figure it out...

 

[Stephen S]

Welcome to the site. You'll find everything you want to know about your truck right here!

Now about your problem...that's weird. I would guess that there is a short somewhere in your guage cluster. Maybe the little wires that power the speedo and the left turn signal indicator bulb in the cluster are close together. When you turn on the left signal, it grounds out and effects the speedo. Do both the bulbs outside the truck and the little indicator bulb in the cluster function properly when the speedo is jumping?? Maybe the circuit board in the back of the cluster needs cleaning to prevent the short??

 

[sabatema]

Transmission shifting erratically

That would make sense except the PCM must be "seeing" the false signal also because my Transmission will shift erratically when comming to a stop or at low speeds. Does the Speedo display a signal from the VSS or a Signal from the PCM. In other words does the VSS send a signal to the PCM (Powertrain COntroll Module) and then an output from the PCM to the speedo. A schematic would be awesome but I can't find any with specific info. anywhere...

 

[Stephen S]

Those are very good questions. Finding out where exactly the signal comes from would aid in trying to pinpoint the problem.

But, here's a suggestion though...try disconnecting the battery for a half hour or so. Let the computer forget everything and then reset itself. If the problem lies with the PCM, this may clear it up. It may be an easy fix. I had some trouble with the PCM on mine a while back. It made my O/D OFF light flash and made my tranny act up. The tranny guy reset the computer and the problem hasn't returned. It was just a fluke. Maybe yours is the same.