Speedometer facts
How the speedometer works:
The indicator pointer has no direct connection to the speedo cable. It uses a drum with magnets on it to couple to the pointer. The drum turns and tries to twist the circular steel disk that is mounted on the pointer spindle. The magnetic force is all that connects the drum to the circular disk. There is very little clearance between the disk and drum, only a few thousands of an inch.
Lubrication warning
Use a graphite based lubricant for the speedo cable. It is available at most auto parts stores in a very small tube. Lubricate only the lower half of the cable. The reason for this is that if you use too much lubricant, it works its way up into the speedo head and gets between the rotating magnet and the disk. This causes the speedo to seize up and may wring the indicator needle off the indicator spindle. You may be able to fix things up with non-flammable brake parts cleaner to clean the disk and magnet assembly. Count on replacing the current cable and housing with a new cable and housing. You need to do this to prevent the excess lubricant from causing it to fail again.
A speedometer that bounces around is either a cable problem (top cause) or a speedometer drive gear that is chewed up. When you remove the cable drive end, check the teeth on the driven gear (the one attached to the cable) or the drive gear (the one on the transmission output shaft) Late Model Restoration (
http://www.latemodelrestoration.com/products/79-93-Mustang-Transmission-Speedometer-Correction) has a complete range of replacement speedometer gear sets.
Note: All 89-93 cars have a VSS sensor even if they do not have cruise control. The 87-88 only have a VSS sensor if they have cruise control.
The speedometer is purely a mechanical device on all Mustangs made prior to 1996. The VSS sensor is for cruse control and the speedometer will work perfectly well without it. Mass air computer cars use the VSS signal to reduce the tendency to stall when slowing down to a stop.
Speedo cable housing assembly without cruise control:
The VSS equipped cars have a speedo cable with a different fitting on the transmission end of the cable. It is the fitting on the LH side of the following picture.
Speedo cable housing assembly with cruise control