Dear Tom and Ray:

I have just recently purchased a 1993 Dodge Caravan. I was mystified when I heard that the speedometer doesn't have a cable anymore. Could you explain how the speedometer works without a cable?-- Steve

Ray: Will miracles never cease! Next thing you know, they'll have spray-on hair in a can.

Tom: The speedometer cable used to run from the output shaft of the transmission right up to the back of the speedometer on the dashboard. Then the speedometer translated the spinning of that cable into miles per hour.

Ray: But then the Vehicle Speed Sensor (VSS) came into being. The VSS is an electromagnetic pickup at the output end of the transmission which reads the number of times the driveshaft turns (or the axle if it's a front-drive car) and sends that information to the engine's computer. The computer then interprets that data and figures out how fast the car is going.

Tom: And even before it occurred to anyone to use this data for the speedometer, that VSS information was used to control other devices, like the cruise control and the electronically controlled automatic transmission.

Ray: From there, it was hardly a giant leap for mankind to create a speedometer that used this same VSS information that the computer was already receiving. So now, on many cars, the VSS reads the vehicle speed and sends a signal to the computer, which sends a signal to the speedometer, which sends a visual signal to you, which explains why there are sirens and red lights in your rear-view mirror.

Tom and Ray lead you step by step through the process of finding and buying a reliable used car in their pamphlet How to Buy a Used Car: Things That Detroit and Tokyo Don't Want You to Know. To order, To order, send (check or money order) to Ruin, P.O. Box 536475, Orlando, FL 32853-6475. You can also order online.

© 1996 by Tom and Ray Magliozzi and Doug Berman Distributed by King Features Syndicate, Inc.

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