91 Dynasty
3.3 v6
3 speed + OD transmission (computer controlled)

I was having a problem some time ago where the speedometer suddenly stopped working. Cruise didn't work, locks didn't engage at 15, etc. I took it to a garage and they replaced the sensor and everything seemed fine... except that the cruise is all jerky now. It maintains a speed, but accelerates erratically. Also, I used my GPS the other day and found that the speed is off. At low speeds, the speedo and GPS are very close. However, at 55 on the speedometer the GPS tells me I am going 60. At 70 on the speedo the GPS says 80. In other words, the higher the speed, the higher the difference. I just called up the garage and they are trying to tell me it is a dashboard issue. While they may be correct, I find it odd that both the cruise control and dashboard were working fine before and now that a new sensor was installed are both acting wierd.

I had a similar problem with a chevelle years ago where the speedometer was off more and more at higher speeds after some transmission work. It turned out that the little plastic gear they put in the transmission was the wrong one and was throwing the speedo off. That was a mechanical cable instead of an electrical system but wouldn't the concept be the same? Is it possible they installed the wrong sensor? Any thoughts?

 

That is very posible. With gearing, it's all based on ratios so the faster you go, the further off the actual number will be. If the shop still has the old par, compair the numbers on it verse the one that was installed.

 

The VSS (vehicle speed sensor) is what tells the computer how fast your going. If you have access to a scan tool you can plug it in and read right from the computer how fast your going (this would rule out a faulty speedometer) my thoughts from what your saying is that they installed the wrong VSS.

 

This was my thought too. They used these same transmissions in the caravans and it occurs to me that even something like a different wheel size could be throwing the sensor off. I find it extremely unlikely that the cruise control *and* speedometer would start malfunctioning at the same time that the speed sensor died.