Speedometer function on modern cars built in the early ’90′s and up is a function of many electrical components sending signals to the computer. By the late ’90′s all cars switched to electronic speedometers. This is one of the primary input signals that controls nearly every function of tranmsission operation. Now a days, if the speedometer quits working, the automatic tranmsission stops shifting. What is most important to know is that a bad VSS can cause all sorts of engine problems too, including serious engine damage or failure.

The “older” cars used small plastic gears and a speedometer cable to relay speed to the speedometer. If the cable or the plastic speedo gears broke, the only thing that quit working was the speedo. With the complexity of a modern day cars computerized electrical system it became necessary to integrate speed with other functions such as:.. when the transmission should shift, how hard it should shift, when to downshift and even effects engine operation. It is all integrated into a complex system.

Generally speaking the “engine light” will come on if the VSS (vehicle speed sensor) stops working, in conjunction with a loss of the automatic shifting features. Realistically speaking it is important to take the vehicle to a competent repair shop  instead of a dentist and have a mechanic hook an automotive electrical scan tool to the adapter. Early hookups were called OBD1 (on board diagnostics1), in 1996 or 1997 all automakers were mandated to use the same universal adapter, which fits all cars, called OBD11 (on board diganostic11). This was a great simplification, because every brand of car had it’s own proprietary adapter causing the need for many adapters.

I think it is a great mistake not to have your car scanned first even if you do the repair yourself. Some of the auto parts stores sell affordable scanners or more precisley ‘code readers’ for the home mechanic. A code is what the scan tool reads on the screen. It may be a number like code 24 or a letter and a number like A347. Once you “pull” the code you can look it up in a code book, usually included in the scan tool kit.

That being said, here is how a VSS works:..There is a trigger wheel on the output shaft of the transmission that spins anytime the vehicle is moving. The VSS is in the transmssion housing and it is aprroximatley thirty thousandths of an inch from the trigger wheel. When the car moves, the VSS counts the revolutions of the trigger wheel and sends it to the cars computer which transforms the ‘pulse counts’ or revolutions into a signal that goes to the speedometer and reads speed and mileage. It also sends all types of signals to the various engine sensors. Not to complicated, right?

I’m writing this in response to a question that one of our readers sent in about:.. no speedometer and a no shift problem. Anytime an engine lite comes on is a good time to go to your favorite repair facility and at least have the car scanned. By not scanning the car, you will be guessing at the problem and may waste money on unnecessary parts, more important, you may cause further damage to your transmission and car. Most people don’t even want to know how much it costs to replalce a transmission with one of the options available.

The whole purpose of this GotEngines.com Blog is to give you real life facts about finding the right repair shop and the most qualified engine and transmission supplier and provide answers to your questions for your needs. We also hope you find it helpful in other ways. Please get your free subscription delivered to youe email box.