The following material covers only that service on speedometers which can be performed by the average service man. Repairs on the units themselves are not included as they require special tools and extreme care when making repairs and adjustments and only an experienced speedometer mechanic should attempt such servicing.
The speedometer has two main parts: the indicator head and the speedometer drive cable. When the speedometer fails to indicate speed or mileage, the cable or housing is probably broken.
SPEEDOMETER CABLE
Most cables are broken due to lack of lubrication or a sharp bend or kink in the housing.
A cable might break because the speedometer head mechanism binds. If such is the case, the speedometer head should be repaired or replaced before a new cable or housing is installed.
A jumpy pointer condition, together with a sort of 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.
LUBRICATION
The speedometer cable should be lubricated with special cable lubricant every 10,000 miles.
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 indicating hand.
INSTALLING CABLE
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 when the cable comes out of the transmission and connects to the head so the cable will not be damaged during its installation.
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.
The transmission mainshaft nut must be tight or the speedometer drive gear may slip on the mainshaft and cause slow speed readings.
