I was driving Kiri's car (an '02 Sonata) not too long ago and, suddenly, the speedometer dropped to zero. A few seconds later the check engine light came on. The car also started idling at very low rpm, but I'm not sure if that was related. I had taken the car to the dealer with the airbag issue and immediately suspected them of accidentally screwing it up or giving themselves a little job security. Either way, I didn't want to take it back. I began searching online and found that there are actually three speed sensors on a Hyundai Sonata.
Conveniently, Sonatas do not have repair manuals. Instead, Hyundai provides a cryptic website for diagnostic and repair info. Anyways, the input and output sensors are located on the transmission and help your vehicle know when to shift. Much information regarding these two sensors can be found in online forums and even on the crappy Hyundai service website. Little to no information, however, can be found for the third sensor: the vehicle speed sensor. Fortunately for me, I had a vehicle speed sensor problem. I'm writing this post to maybe save somebody the headache of trying to figure it out on their own. Most of the information I received from conversations with dealer service department people and one particularly helpful dealer mechanic who walked out to my car with me and showed me where the sensor was and described how to fix it. I have a 2002 Sonata and I imagine this applies to all automatic Sonatas with the same body style.
The vehicle speed sensor is located on top of the transmission as shown in the picture below:
Click on the image for a larger view. The sensor is circled in yellow. I began by disconnecting the air filter hose and removing the top of the air filter housing. This gave me better access to the sensor. A close view of the sensor can be seen below:
To remove the speed sensor, I disconnected the electrical plug from the sensor using a flat head screw driver, taking care to not damage the plug. The plug came off easily. If the terminals on the plug look corroded, it's a good idea to clean them and reseat the plug, as a bad connection may have been the only problem. If that doesn't work, keep reading.
Because the sensor plugs into the top of the transmission, it is a good idea to clean the area around the sensor to prevent dirt from entering the transmission when the sensor is removed. Next, I removed the bolt which holds the sensor in place.
The speed sensor assembly is comprised of the speed sensor and a plastic gear. The plastic gear is turned by the transmission. The sensor measures the speed of the shaft on the plastic gear which is held in place by a retaining ring. I purchased both the sensor and the gear for good measure. The new speed sensor assembly is shown below:
The white helical gear can be seen on the right and, just to the left of it, the retaining ring which holds the small gear shaft on the sensor.
After removing the bolt from the transmission, I removed the sensor assembly by gently prying it upward with a flat head screw driver. When I removed the retaining ring and gear from the sensor, small pieces of broken plastic fell out of the sensor. I guess that was my problem (it's good to know the dealer probably wasn't pulling a fast one). The disassembled speed sensor assembly removed from the car is shown below:
The old speed sensor was covered in transmission fluid. So, for good measure, I applied fresh transmission fluid to the o-ring on the new sensor, gear shaft and helical gear. I then inserted the shaft in the speed sensor, installed the keeper ring and installed the assembly by reversing the removal procedure. When the car was put back together, I started the engine, the check engine light turned off and the speedometer worked perfectly!
For those unfortunate enough to own a Sonata, I hope this tutorial helps.