My daughter has a ’91 Chevrolet S-10 and the speedometer has just quit working. Everything else works. What do we look at to find out what is causing the problem? Someone at the parts place told us it does not have a cable. How does it work without a cable?

– John and Betty Lynch, Huntsville, Ala.

During the 1980s, most automotive speedometers and odometers took a leap forward in reliability by abandoning the traditional cable drive and aluminum cup/magnet speedometer head.

A dry or worn speedometer cable occasionally brought complaints of noise and needle bounce, and this mechanical connection between the transmission and vehicle body transmitted a touch of noise and vibration to the passenger compartment. In addition, the engine/transmission computer, cruise control and other new systems needed a look at vehicle speed.

How do they do it on newer vehicles? A vehicle speed sensor (VSS), which is attached to the transmission, sends electrical pulses to a central location. From there the signal can be shared by whichever modules need it. In your case the destination is a little plastic box called a digital ratio adapter controller (DRAC), which is located below and behind the left side of the instrument panel.

Your speedometer is an air core gauge, similar to a fuel or temperature gauge, and the mechanical odometer is driven by a stepper motor. Stepper motors rotate a partial revolution with each electrical pulse applied.

You mentioned everything else works besides the speedometer. Does this include the odometer? How about the cruise control? And finally, has the service-engine-soon (also known as SES, MIL or check-engine) light come on recently? If the odometer works, this means the VSS, DRAC and wiring to the instrument cluster are all OK. The fault would have to lie in the speedometer gauge or a faulty connection in close proximity.

If the speedometer and odometer are both inoperative, and the cruise control works OK (and there’s no SES light), this steers us to a wiring fault between the DRAC and instrument cluster, or faulty DRAC output to this one circuit.

If the odometer and cruise control are both inoperative, and there is an illuminated SES light, the VSS, DRAC, or wiring between them might be faulty.

Testing this system should be a snap for a pro with the right tools and electrical diagrams. I’d start at the DRAC, checking for proper power and ground, and incoming and outgoing voltage pulses, and work in the direction indicated. A less costly option, assuming the DRAC or VSS are suspect, might be to replace one or both with inexpensive wrecking-yard parts. The DRAC would need to come from an S-10 with identical gearing and tire size, while the VSS would be identical to similar S-10 trucks.

Normally I would never recommend throwing parts at a car as a diagnostic method, but when a part costs less than the labor to test it then it’s worth considering.

Brad Bergholdt is an automotive technology instructor at Evergreen Valley College in San Jose. E-mail him at [email protected] He cannot make personal replies.

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