Speedometer Cable: Description and Operation
The following material covers only that service on speedometers which is feasible to perform. Repairs on the units themselves are not included as they
require special tools and extreme care when making repairs and adjustments that only an experienced speedometer mechanic should attempt.
The speedometer has two main parts, the speedometer head and the speedometer drive cable. When the speedometer fails to indicate speed or mileage,
the cable or cable housing is probably broken.
Most cables are broken due to lack of lubrication, or a sharp bend or kink in the housing.
A cable might break because of the speedometer head mechanism binds. In such cases, the speedometer head should be repaired or replaced before a
new cable or housing is installed.
A "jumpy" pointer condition, together with a scraping noise, is due, in most instances, to a dry or kinked speedometer cable. The kinked cable rubs on
the housing and winds up, slowing down the pointer. The cable then unwinds and the pointer "jumps."
To check for kinks, remove the cable, lay it on a flat surface and twist one end with the fingers. If it turns over smoothly the cable is not kinked. But if
part of the cable flops over as it is twisted, the cable is kinked and should be replaced.
The speedometer cable should be lubricated with special cable lubricant. Fill the ferrule on the upper end of the housing with the cable lubricant.
Insert the cable in the housing, starting at the upper end. Turn the cable around carefully while feeding it into the housing. Repeat filling the ferrule
except for the last six inches of cable. Too much lubricant at this point may cause the lubricant to work into the speedometer head.
During installation, if the cable sticks when inserted in the housing and will not go through, the housing is damaged inside or kinked. Be sure to check
the housing from one end to the other. Straighten any sharp bends by relocating clamps or elbows. Replace housing if it is badly kinked or broken.
Position the cable and housing so that they lead into the head as straight as possible.
Check the new cable for kinks before installing it. Use wide, sweeping, gradual curves where the cable comes out of the transmission and connects to
the head so the cable will not be damaged during installation.
Arrange the housing so it does not lean against the engine because heat from the engine may dry out the lubricant.
If inspection indicates that the cable and housing are in good condition, yet pointer action is erratic, check the speedometer head for possible binding.
The speedometer drive pinion should also be checked. If the pinion is dry or its teeth are stripped, the speedometer may not register properly.