Speedometer Cable Replacement
Dialing in correct needle performance
If your speedometer has been acting weird—the needle jerking wildly or not moving at all—chances are that its cable is bad. Like most other automobile parts, speedo cables eventually wear out and need to be replaced. Throughout the Fifties, Sixties and Seventies, many vehicles came with completely mechanical cables that run from the speedometer in the dash directly to the transmission. The smaller end of the cable attaches to the back of the speedometer, and the larger fitting screws onto a housing on the transmission or transfer case.
For GM vehicles, most replacement cables are available from a dealership or one of the many restoration suppliers. In some of GM's later-model Sixties cars, the cable snap-fitted into the back of the instrument instead of screwing on. This project doesn't require any special tools and can be accomplished at home. The degree of difficulty changes with each vehicle, but if you have any mechanical experience at all, you can install a new speedo cable. Start by disconnecting the cable at the speedo, and then feed the end through the rubber grommet at the firewall and work your way down to the transmission connection.
Here is what the speedo end of the cable looks like. The old cable has already been removed from the vehicle, and installation simply involves reversing the process. Inside the outer rubber insulation is the cable itself. It has square ends that correspond to the fitting in the back of the speedometer. Make sure the cable is seated correctly then screw the retainer onto the instrument. On many non-air-conditioned cars, you can easily crawl under the dash and look at the back of the speedo. If you have A/C, some ductwork might have to be removed to get a good look.
Normally, the cable passes through a large rubber grommet in the firewall. If the rubber is dried out and hard, either replace the grommet or slit it with a razor blade in order to push the cable end through. This cable needed a little persuasion, but it made it through. For the early GM A-body vehicles (GTO, Chevelle, 4-4-2 or Gran Sport), the cable is approximately 56 inches long. Most of the cables for these cars originally came from AC Delco; the part number for this application is 6480250.
The cable is usually held in place by a clamp at the lower part of the firewall. Remove the retainers and loosen the clip, but don't attach the cable permanently until you've routed and attached it to the housing. Avoid any tight bends in the cable—use the excess length to make wide arcs into all the connections, making sure to keep the cable clear of the exhaust pipe. Once the cable is in place, you can adjust the play to approximately the same as the old cable.
The speedo connection is usually on the tail shaft of the transmission or transfer case in 4x4s. In this case, the housing is on a 1965 Super Turbine 300 2-speed automatic. Inside the connection is the transmission speedometer gear, and the end of the cable must be inserted into the square receptacle end of this gear. We are routing the cable exactly as the original, and the connection area can be a little tight. This install can be difficult if you're doing it on the garage floor, but elevating the car with a floor jack and jack stands will provide adequate access. No lubrication is required on the cable; it's ready to install straight out of the package.
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