I'm having a speedometer issue:

Ever since I purchased my 1973 coupe about 18 months ago, my speedometer does not hold a steady speed. I even installed a new speedometer cable a few months ago, which seemed to help a little, but not enough. The speedometer needle "bounces around" to the point I had to install a GPS based external speedometer to provide me an accurate speed reading.

Well, yesterday I picked the car up from my mechanic, who had just installed a new starter & flex plate, which of course necessitated removing and reinstalling the transmission. I noticed that on the way home, for the first time since I bought the car, the speedometer needle held steady. I mean rock solid steady. I was thrilled to say the least. I was so happy, that this morning I took the car out for a 10-minute drive to Lowes. It was cold outside (just above freezing), and I noticed during the short drive, that the speedometer was bouncing around again (damn!). Well, I finished my shopping at Lowes and drove home. As soon as I started rolling across the parking lot, I looked at the speedometer, and wouldn't you know it? "Rock steady" yet again. The needle did not wiggle once during the entire drive home. It was good as new.

I know this must have something to do with the speedometer connection at the transmission. Anyone know for sure?

 

Don't take this as gospel, because it has been nearly 40 years since I have owned a 73, and this is from memory. I believe the gear insert that the cable screws into on the transmission (the one you change when you change rear gears to correct the speedo) is nylon and the square recepticle that the cable tip slips into can become rounded a bit. That allows the cable to "jump" every few rotations, causing the needle to bounce.

 

You can put the car on jack stands and set a drip pan under the transmission. Remove the speedometer cable where it connects to the transmission. clean the female square opening with a Q-Tip and use a bright light to look into the small square hole to see if it is wallowed out/no longer square.
As Ron C! said...the square hole that the speedo-core-cable fits into could be damaged causing needle bounce.
Let us know what you discover.

 

Glad to see my memory hasn't got as bad as I thought! LOL

 

I would disconnect the cable at the gearbox end and connect a drill to the square end, run the drill in reverse and see if the speedo moves smoothly in line with the drill speed. If it does, then the problem is with the gear in the transmission most likely the driven gear (either the gear is worn or the square hole is worn) although it could be the drive gear. The driven gear is easy and cheap to replace, so I'd do that first. If the speedo is erratic when driven by the drill then the problem is in the cable or the speedo head.

 

I agree w/ your hunch... Over the years a worn driven gear can produce eratic results and ultimately stop spinning... Since it doesn't sound like it was serviced when you put in the new speedo cable - I believe your plastic speedometer driven gear may be worn and need replacement (super cheap and pretty easy) - it's at the transmission connection - inspect it and see what it looks like.

Disconnect the cable at the transmission and remove the metal 'bullet' at the transmission that retains the plastic driven gear for the speedometer cable. It's mounted to the transmission with a bolted clip is all. There's also an o-ring oil seal on the bullet retainer that should be replaced as well.

 

No one KNOWS for sure.
You may need a new cable or have it cleaned and Re-lubed.
Sometimes the squared ends of cable (e.g. 0.104" square-drive industry standard) become worn rounded and slip.
BUT it also may be internal to speedo gage. OR BOTH.
Camden, eh? I love the place. and its history. It's also where a favorite meat n three buffet-lunch spot for me is DeBruhl's Cafe.
SC's had some crazy weather lately and fixin' to get even worse !
That COLD can affect lube of all sorts, in a cable Or inside a gage.
Sometimes ignorant folk use wrong lube (or more often TOO much) on speedo/tach cables.
After 50-plus years, Your speedo and/or tach may need service. Typically NOT a DIY job; most can't repair a wristwatch either.
There's a specialty shop at Greenville SC (D&M) that routinely cleans, repairs, restores & calibrates automotive gages.
? Perhaps you've experienced the joy of removing tach or speedo from a C3 ? No ? It's A Chore !
Suggest send speedo to D&M. Tip: if ya go to the trouble to R&R a speedo, makes sense to also R&R the tach in one swell foop.
D&M Restoration (CALL 'EM)
57-B Creekside Park Court
Greenville, SC 29615

864-254-9989

support@dandmrestoration.com

https://dandmrestoration.com/

 

If you go for replacing the gear, you'll want a new 1240382 seal, and an o-ring seal (may be 10054241; sorry, not certain)

 

You only want to pull the driver side dash out ONE TIME. If you need the speedometer serviced, get the mechanical tach RESTORED at the same time. My speedometer stopped working and a repair shop replaced the plastic gear, by the time I arrived home the speedometer had stopped working. They had to remove the transmission and refurbish the part that holds the gear in place. I had them install a new clutch, and they also found a missing tooth on the flywheel, so that needed to be replaced.