The other day I started driving my 2004 GXP and the shifts were very hard. It would not shift from 1st to 2nd until about 30 mph. From 2nd to 3rd it was the same and so on. I also noticed the speedometer wouldn't move till about 25 mph either. I immediately suspected loss of ATF. I looked under the car and no leak was found. Checked the level on the dipstick and that looked fine as well (although it needed changing bad). I then drove it around again and everything was back to normal. Hmmmm What was wrong? The car only has 50,700 miles.

I thought maybe the fluid was dirty and something was getting clogged somewhere so I did a transmission flush at a local garage. I left there and all seemed fine till later in the day. Went back to the garage and they did a diagnostic check and it said Low Voltage from the VSS (Vehicle Speed Sensor). I went and bought a new speed sensor for about $25 and will have it installed Monday. Has anyone else experienced this phenomenon? Also, after replacing the transmission fluid, I was shocked to see how dirty mine was. Some of you may want to check the condition of your fluid. My level was fine but it was dirty as all get out. Anyone knows what could cause premature dirtying of the ATF?

I look at my tranny fluid quite often because of how I really drive mine hard, At 18,000 it still looks rather translucent. I wonder if the "flush" stirred up things in yours you wouldnt see under normal driving conditions. I only knew about each wheel having a speed sensor, (WSS). The car has a single VSS as well? Where is it located.

Venom I couldn't tell you exactly where the VSS is but we have 2. One is an external and we also have an internal. If you have to replace the internal it costs about $110 but expect very high labor costs.

The Pontiac dealer here said he had never sold one before and didn't have one in stock so he said to go to the local Chevy dealership. They had them in stock and had sold a few. If the VSS gets whacky it affects the shifts since there is no speedo cable in our cars according to the Pontiac parts guy. The wheel speed sensors and the VSS have diferent tasks. Aren't the WSS for the anti-lock or traction control?

As for the fluid change stirring things up, I thought that too but it was doing the radical shifting before the change. In the past 2 days it has shifted smoothly, but I am going to replace this sensor anyway since I bought it and can't return electrical parts.

The GM part number for the external VSS for the GXP is 24204319. It looks sort of like an "L" shaped PCV valve.

There is an external VSS mounted to the tranmission housing extension, near the output shaft of the 4T80E.

Thanks guys, seems I learn a little more about the car each day. I imagine input from the VSS and the WSS'* help StabilTrac and T/C know when things are askew as in a slide.or icy roads. After driving the car in a particularly bad winter, I'm am impressed with both systems.

You'll also find some inertial sensors in the vehicle as well. There should be one under the rear seat if I'm not mistaken and another under the rear deck close to the electronic compass module. There'* actually a fair amount of technology in these cars.