[ookinschma]

Hi
I'm working on a 1989 Pontiac Grand Am, automatic with a 2.5 liter engine. The speedometer and odometer have stopped working, and I'm trying to narrow down the possibilities.
To start with, I thought I'd check out the speed sensor located on the transaxle. There is a 2 wire harness connected to it, a purple one and a yellow one. Should there be any voltage on either wire (input)? If so, which one? And if there is input voltage on one, verified with a VOM, should there be intermittent voltage (pulsing) on the other wire, when spinning the gear, to send the speed signal back to the ecm or to the instrument cluster?
To test the sensor, I'm guessing the key would need to be on, and maybe the shifter placed in drive?
Any help would be appreciated!

[TxGreaseMonkey]

. Welcome back, ookinschma.
. Check for a blown fuse in the under dash fuse box.
. Find out the VSS Input Tests for this vehicle--I don't know what it is.
. Replace VSS--most likely solution, since both speedometer, odometer, and likely trip meter don't work.

[CaptainRich]

There is no voltage applied to the VSS. It is a Pulse Width Modulated Signal Generator. The voltage generated is small.

The Vehicle Speed Sensor (VSS) is made up of a coil, which is mounted on the transmission, and a tooth rotor which is mounted to the output shaft in the transmission. As each rotor tooth nears the coil, the coil produces an Alternating Current (AC) voltage pulse. As the vehicle speed increases, the number of AC voltage pulses increase. The Digital Ratio Adapter Controller (DRAC) processes input from the VSS sensor and output signals from the speedometer, computer, and cruise control module. The DRAC takes the voltage pulses from the VSS sensor and uses them to open and close four solid state output switches to ground at a rate proportional to vehicle speed. The DRAC is matched to the vehicle based on final drive ratio and tire size. It is important to make sure the correct DRAC module is replaced on the vehicle if replacement is necessary.(Info online)

Inspect the harness for any type of damage. The VSS can be removed fairly easily for close visual inspection and further testing. Look for physical damage that would suggest contact with internal transmission components. With a proper DVOM the low voltage signal can be seen. Connect the DVOM, set to 10VAC scale, to the VSS and spin the sensors wheel. The meter should show a small spike while the sensor is being rotated. If not, clean the area where the sensor mounts on the transmission, and replace the sensor with a new one.