speedometer issue

[rla216]

My 1964 Tbird speedometer only works to about 30 mph then stays there no matter how fast I go. Any idea why? How to repair?

[sseebart]

I would first investigate the speedometer drum, as it may be sticking for some reason and unable to turn beyond the 30mph point.

The speedometer takes some digging to get to. Before you pull it out, it might be worthwhile to check the cable--which is more-or-less on the way--to ensure it's not worn or sticking.

~Steve

[RedBird64]

Seems like it almost got to just be the drum sticking for some reason just as Steve suggested.
The cable spins a magnet that catches the end of the drum. The drum is spring loaded to return back to zero. Everytime the first magnet catches the end of the drum, it tugs it around an amount proportinate to the cable speed. At some speeds you can actually see the tugging. The drum is not physically connected in any way to the cable like normal speedo's.
Since your cable is obviously intact and turning the first magnet, the fault has to be in the drum.
They're pretty simple to take apart. The worst part is getting it out of the dash without scratching something!

Scott

[paulr]

Yep ^^

Per the above post, you’ll either learn to live with it or you’re gonna be removing the speedo pod. Follow the book BUT, about the scratching things...1) remove the steering wheel and protect (easy), 2) drop the column as low as it will go and wrap it overly well with bubble wrap or towels, 3) there are other sharp metal corners around the work area waiting to scratch your painted parts—wrap those too. Then take the speedo pod to the bench to work on. You can operate the drum by using a small square bit in your power drill. Enjoy.

[RAVEN]

Do a "SEARCH" in previous posts under Speedometer and RAVEN (as author). I submitted some points of repair/trouble shooting, many moons ago.
Basically the Speedo is a magnetically driven unit, where a fixed pole magnet is inserted inside a drum. As the magnet is rotated by the Speedo cable, a magnetic flux is created (fluid coupling) within the drum which causes it to rotate in a push/pull action. The speed rotation of the drum is proportional to magnet RPM. (Remember magnets repell of same poles, and attract on opposite poles)
The biggest cause of Speedo problems is the Odometer gears and lubrication, which are directly connected to the Speedo cable as it enters the "POD". The gears either seize or start to jump over the teeth, due to dry grease. This causes the cable to WHIP, as it winds up, releases, then winds up again. This then causes the magnet to pulse as the cable whips, giving rise to a bouncing indicator, or lack of display.
I recommend you first remove the clock pod, and do a test by inserting a drill motor, and drive the Speedo and see it's action. This will indicate whether the cable is faulty or the Speedo. You should be able to hear the gears clicking if the drill is quiet enough. If Speedo is good then check the cable.
Let us know what you find, and we can possibly narrow down the cause.

[paulr]

The biggest cause of Speedo problems is the Odometer gears and lubrication, which are directly connected to the Speedo cable as it enters the "POD". The gears either seize or start to jump over the teeth, due to dry grease.

This was my experience. The 2 nylon ODO gears are pretty soft, and once there is a fracture or abrasion of any kind they will wear down and really affect speedo problem. Since these gears are not reproduced (or are they now?) I had to rummage through several salvage cars before I could find one that did not have bad gears. Why are they soft nylon and not brass? And Wally, what's the right lube that won't aid in breaking down the nylon?

[RAVEN]

Paul, not sure if it was the right stuff, but have had no issues for years, so maybe lucky.
I used a high temp boiler/steam valve silicone lubricant. Figured it would not harm the nylon, was rated for heat, would not break-down or dry-out.
(even went to my shop and found the tube for ya)
Its a Dow-Corning product High Temp Lubricant #111
It looks very similar to the plumbers faucet lube you can buy at your local plumbing jobber. (looks like vasaline, but is a silicone product not a petroleum product) Something like the story of latex and certain body creams do not mix. You get bad results.

[paulr]

Thanks Wally, appreciate the product tip. I used a silicone gel recommended at NAPA but I thought of it as a stopgap for short term. I put a couple pics below of some of the gears I've collected. You can see the one that someone used a petroleum lubricant on; once the grease starts to break down the poly molecule, friction does the rest. I have found a number of gears in this condition in salvage cars. The other semi-clean looking gears, if you enlarge the pic and look closely, you see tiny burrs. This is the beginning of trouble. Eventually, a burr with catch on the other gear and it will tear, then you'll get the famous bouncing speedometer syndrome. You always know it by the clicking sound that comes from the odometer.

[rla216]

Interestingly enough when I ran the speedometer with my drill it went up to 40 mph which is more than when driving. That's all the spped my drill will apply so I wonder if it isn't the cable or trans gear.

[RedBird64]

I would expect the cable to either be broken or working. If it were the gear, it would probably skip or just stop altogether.
I don't see how either of them could smoothly limit the speed on their own but I get surprised everyday.
Maybe a neighbor has a higher speed drill you could try?


Just for fun, here the the gears for the end of the cable (left= new/old) and the drive gear that's attached to the tail shaft of the trans (and shouldn't be cracked like this one...).



If you do get a new gear for the cable and you have P215 75R15 tires, go with a 16 tooth gear so your speedometer is accurate.

S