Struggling with a bouncing speedometer needle and poor function? Try these quick tips and look forward to smooth operation.
Although speedometers seem mysterious in terms of function, there's not much to understand about them because they're simple in scope. There's the old mechanical speedometers like we see in old Mustangs. And, there are electronic speedometers like you see in late-model Mustangs today.
A classic Mustang speedometer is little more than a whirling magnet inside a non-magnetic shell (known as a speed cup), which is attached to the needle. As the magnet, which is connected to the speedometer cable from the transmission output shaft, whirls around, it sets up a magnetic field, which makes the speed cup want to follow it around from zero to your intended speed.
The speed cup tends to want to return to zero against the stop because there's a hair thin watch spring (return spring) that returns the cup to zero. As speed increases, the whirling magnet goes faster and faster, drawing the speed cup and needle higher on the speedometer face. As speed decreases, spring pressure returns the cup and needle to zero.
Because speedometers are precision instruments, wear and tear take a toll on accuracy. Speedometers wear out just like any other part of your Mustang. There are instrument repair shops that fix speedometers and electric instruments. We suggest entrusting your speedometer to a reputable instrument repair shop when operation become erratic and you've exhausted all other efforts, such as the cable, drive gear, and head lubrication.
When speedometer needles bounce in rhythm with vehicle speed, you may have a binding cable. Disconnect the speedometer cable and pull it from the casing. The cable should be free of any damage that mind cause it to bind and pop inside the casing. Lay a white grease/penetrating lubricant mixture in the palm of your hand and run the cable through it. You can never have too much lubrication, but be careful. Cable function should be slippery smooth when it's reinstalled in the casing. It should roll around like it was in hot butter.
Inspect the speedometer drive gears down under for damaged teeth. Check o-ring seal at the transmission tail shaft. If damaged, replace it. Lubricate the new o-ring seal with transmission lube.
We will get into electronic speedometer function in another blog.
We'd like to know your speedometer troubleshooting solutions - so lets hear them at www.mustangmonthly.com or Jim.Smart@sorc.com.