The speedometer tells you how fast your vehicle is travelling for the purpose of obeying speed laws. The speedometer, in combination with the odometer, has usefulness for computing distances and travel time. Most vehicles use a manual cable hooked to the transmission case to decipher the rpm (revolutions per minute) signal. The speedometer cable, although simple in design, breaks down like any other component. Speedometer cables fail from lack of lubrication, impact, wear and cable breakage. Connecting a speedometer cable requires a few simple steps and basic tools.
Place the vehicle in "Park" or neutral with the emergency brake set. Raise the hood, and disconnect the negative ("-") battery cable. Use a floor jack to lift the vehicle high enough to place two jack stands under the front of the frame near each wheel. Use the floor jack to lift the rear of the vehicle and place two jack stands under the frame near each wheel.
Use a tape measure to measure the length of the old speedometer cable, if you have removed it from the vehicle. It should be the exact length. The fittings on each end should be identical, as far as the connection device and the diameter of the coupling device, which will look like a threaded cap. Slide under the vehicle with a shop light. Look for the cable mounted on the transmission case. Remove any heat shield or mud sheath to gain access to the cable using a socket and wrench.
Wipe the speedometer cable connection with a rag. Use a pair of pliers to pull out the metal horseshoe spring clip that connects the cable to the transmission housing, if so equipped. For the threaded cap design, grip the cap with the pliers, and turn it counterclockwise to unscrew it. Pull it out, and notice the square notch on the inside of the cable. Insert the replacement cable into the transmission housing, and screw it on by hand only a few threads in.
Take the other end of the replacement cable, and twist the small square notch. If it does not move, it means the cable has seated in the transmission. Finish screwing in the cap nut, and tighten it with pliers. Follow the old cable up toward the front of the vehicle. Use a socket and wrench to loosen the bolt on the bracket that holds the old cable in place. Slip the old cable out, and place the new cable into the mount. Tighten the bracket bolt with a socket and wrench.
Push the new cable up through the engine compartment, following the same route as the old cable. Move to the engine compartment, and look for another cable bracket low on the firewall. Use a socket and wrench to loosen the bracket bolt. Slip the old cable out, and place the new cable in its place. Replace the bracket bolt, and tighten it with a socket and wrench. Pull the new cable up, and let lay it over a component so it does not fall back down.
Slide underneath the dashboard on the driver's side with the shop light. Look up under the dash, and locate the cable where it screws into the back of the speedometer cluster. Use a pair of pliers to unscrew the cap screw and pull it free. Use pliers to remove the horseshoe spring clip, if so equipped. Move to the engine compartment.
Locate the old cable where it comes through the firewall, inside a rubber grommet. Pull the rubber grommet out of the firewall hole, then pull the old cable through the firewall hole. Use a razor blade to cut a side of the grommet to remove the cable from the grommet. Place the grommet on the new cable about a foot or so behind the cable cap screw. Run the new cable into the firewall hole, and push the grommet back into the firewall hole.
Slide under the dashboard. Use the shop light to spot the new cable end. Use your fingers to screw the cap screw onto the back of the speedometer housing. Tighten it with a pair of pliers. For the other design connector, push the cable end into the speedometer housing, and slip the clip back down over the locking groove by hand, or push it into place with the pliers.
Use the floor jack to lift the vehicle, and remove all the floor jacks. Reconnect the negative battery cable with a socket and wrench. Test drive the vehicle, and note the operation of the dashboard speedometer.