Speedometer Insights from a novice - ‘56 Chevrolet panel truck

[Model56s]

Two years ago I bought a ‘56 Chevrolet panel truck and with a few minor items remaining completed it’s restoration last month. The seller included an extra instrument cluster with all gauges and a newer speedo (not brand new, but showing all zeros on the odometer). The original odometer in the truck registered 89,995 miles, I’m assuming actual mileage.

 

When I got the truck on the road in April the newer speedo briefly registered 10-20 mph then quit working, so this past week I swapped it out for the old one in the original cluster, which worked. But it squealed at speeds above 25 and then abruptly stopped working, so I decided to dismantle the newer one to see what I could do. I was surprised and encouraged to find how simple the speedo was and want to pass this on.

I’ll break down what I saw and learned into a couple shorter posts. What I’m going to show won’t be new to many on the forum, but it will hopefully be to other novices like like me.

[Model56s]

This speedo is surprisingly simple and almost certainly differs from others, so please understand the limitations of this example.
 

The speedo cable mates to an internal shaft inside the mechanism, terminating in a magnetized bar. The cable spins the bar clockwise (looking from the face) inside a free-moving copper drum, about 1 1/2” in diameter by 1/2“ deep, inducing the drum to move clockwise on its own shaft. The drum is attached to a spiral hair spring just like those found on a watch balance wheel. As cable spinning slows down and stops with vehicle movement, the spring returns the drum to the full leftward counterclockwise position accordingly until a tab on the drum contacts a stop on its housing, which corresponds  to 0 mph.

 

A tiny shaft attached to the closed face of the drum extends just under the odometer cylinder through the speedo face, onto which the speedo needle is press fit, so however the drum moves, so goes the needle.

 

The first two pictures shows the mechanism with the odometer showing I logged 5.7 miles before it gave out. The copper drum is inside the aluminum cylinder.
 

In the third picture you can see part of the closed face of the copper drum, and the hair spring, inside its housing with the odometer cylinder and its vertical worm drive gear removed (an easy process!). The shaft that the speedo needle attaches to can be seen close to the odometer cylinder in the second picture.

 

In the fourth picture the drum housing with the drum still in place has been removed. One end of the hair spring is attached to the other side of the drum and the other end to the housing.

Edited May 28, 2023 by Model56s (see edit history)

[Model56s]

The first picture below shows the magnetic bar, and a yellow plastic worm gear which is driven on one end by a worm gear on the bar’s shaft.  This worm gear in turn drives a vertical gear shaft (seen being removed in the second picture below) that drives the odometer cylinder. These two gear shafts are needed to translate power 90 degrees since the odometer cylinder lies perpendicular to the input shaft.

 

The next three pictures here show just how simple it is to remove the cylinder and vertical drive gear shaft. The cylinder axle is secured to the housing by a brass clip which fits into a groove in the shaft, secure and very simple. The last picture shows the main components laid out. The drum shaft passes through the hole at the center of bar at the bottom of that picture. I couldn’t figure out how to disassemble the cylinder to see how it works.

 

I reassembled these components in the housing, leaving the lubricant in place. Once I verified that both the drum (needle) and odometer cylinder indeed move when I spin the shaft, I put the speedo back in the cluster (just 4 screws and the cable) and installed it in the dash.
 

Unfortunately, this exercise didn’t reveal why this speedometer stopped working after just a few miles, but I do know from using both speedometers that the squeal of the original speedo did not come from the shaft. Also, I can’t get the needle off the original speedo to disassemble and inspect it, but I’ll work on that later - these are so easy to remove that I’d like to have a working spare.
 

I’ll test drive the truck this afternoon and post the result.

Edited May 28, 2023 by Model56s (see edit history)

[Model56s]

I test drove the truck with the cluster not secured, just sitting in the dash, and no luck - no needle movement or noise.
 

So I pulled into a parking lot to pull the cluster out enough to disconnect the cable from the speedometer, much easier than it might sound. My idea was to re-insert the cable with the truck moving slowly, thinking it hadn’t mated properly to the square female speedo receptacle. To my surprise I found the end of the cable was almost too hot to handle, so clearly there’s a problem at the end (mating?). I observed the cable end rotating with the truck crawling along at maybe 3 mph, but after slowly reinserting it at that speed still had no luck when I drove off🤔

 

Since it responded so easily on the bench, I’m thinking the issue is not with the speedometer mechanism but somehow with the cable interface. I’ll remove the cable tomorrow and see what I can see. I’ll also get the needle off the original speedo and go through it.

[JamesR]

Thanks for the photo presentation. Very interesting! Best wishes in getting the remaining issue worked out...I'm sure it will turn out great.

 

For folks who don't want to take on the job of fixing the speedometer in their old car, I used Bob's Speedometer Shop in Howell, MI. You can google them easily and make contact. It was close to 20 years ago that they repaired my unit, and it's still working good.

[Model56s]

5 hours ago, Jack Bennett said:

And “So”, said the wicked witch to the little boy, “Come into my kitchen and I’ll fix something to eat”……….Or, as the wise man said “If it seems too easy, you’re doing something wrong”

You are much braver than me so far as tearing into a mechanical speedometer. And, I only say that because they usually work right, until they don’t work at all. 
My immediate suspicion is that you have a gummed up, or totally broke cable.

I eliminate the transmission drive gear from suspect by ignoring it until I am ready to really get down and dirty, mechanically speaking.

And, with the speedometer itself, there is actually very little except a good air bath to blow out debris, a tiny bit (I use a tooth pick) of carefully applied sewing machine oil and a good cleaning of the case and glass.

The cable is another issue. It can usually be easily removed from its case, and the cable itself inspected for wear or damage. The case can be cleaned in acetone or mineral spirits, or whatever other solvent you choose. 
After reinserting the cable back into its case, give’er a few good spins to ensure it is free in its case.

At this point the Speedometer is reassembled in its newly cleaned case. The speedometer can now be tested by reinstalling the cable to the case and giving it a few spins. 
If the needle bounces it’s go time. Rather than reconnecting the drive (square) end of the cable into the chuck of a drill, and, with the driven end connected to the speedometer, slowly increase the speed of the drill.

If the speedometer begins to register speed, good deal, it’s time for a break and back pats.

But if it don’t……..well, remember what I said about the transmission drive gear?

Good luck! 
Jack

As the speedo responded in my bench test, I think it’s OK. The transmission gear is ruled out since the cable inside the sheath turned in the parking lot so I think it may be the wrong cable. I’ll know tomorrow. Thanks for the thoughtful response.

[Model56s]

5 hours ago, JamesR said:

Thanks for the photo presentation. Very interesting! Best wishes in getting the remaining issue worked out...I'm sure it will turn out great.

 

For folks who don't want to take on the job of fixing the speedometer in their old car, I used Bob's Speedometer Shop in Howell, MI. You can google them easily and make contact. It was close to 20 years ago that they repaired my unit, and it's still working good.

Thanks James, the referral is appreciated and along with Mr. Bennett’s fits exactly with the intent of the post to provide some insights. For many years I’ve turned to the AACA forum for education and hope I can return the the favor.

[Model56s]

Success! The truck is now equipped with a functioning and possibly accurate speedometer, the squealing original that was in the dash. This was bothering me so I didn’t put it off to tomorrow.

 

The squeal came from the input shaft which was difficult to turn on the bench. The needle couldn’t be removed without damaging it, so I worked around it and the disconnected speedo face. I removed and cleaned the odometer cylinder, so the numbers are bright again. I used brake cleaner to clean out the input shaft & tunnel and It spins freely, with the needle moving accordingly.  I used a little WD40 on the blade of a small plain end screwdriver to introduce the oil into the shaft tunnel. I’m sure there is a proper speedo lube available.

 

I swapped out the non-functioning speedo for the now-cleaned and lubed original, attached the cable and took it for a test drive. It responded immediately and smoothly registered credible speeds, with the odometer rolling like it should, and making no sound.

 

The only conclusion I could come to from this exercise is that the newer speedo is incompatible with the cable. The only part number on the device was L565 on the cast housing (1st picture on the third post). It’ll take some work to match a new cable to this one if I ever need it.
 

I’ll buy a GPS speedometer app for my iPhone and see how it performs, but this wraps up the effort. Thanks all for following this, and for your comments.

 

[Model56s]

I stumbled on this video which explains why the designer used a copper drum with a magnetic bar in the mechanism, so I’ll conclude with this.

Enjoy.

 

 

[Dan Cluley]

Something to keep in mind when you are checking the accuracy.

 

The number of teeth on the transmission gear can vary and it needs to be the correct one to match up with the rear end gears and the size of your tires.

 

As you saw, the odometer is a direct gear drive from the cable, so if your odometer measures correctly then you have the right transmission gear.  If the milage on the odometer is over or under the actual distance traveled then the transmission gear needs to be changed.

 

Once the odometer is correct, then you can see if the speedometer itself is calibrated right.

 

 

I was just working on the one from my '65 Chrysler.  You can see that the details are different, but it is the same basic sort of mechanism.

[Sonomatic]

Great pictures, great info, great post.  Thanks for sharing this!

[Model56s]

8 hours ago, Dan Cluley said:

Something to keep in mind when you are checking the accuracy.

 

The number of teeth on the transmission gear can vary and it needs to be the correct one to match up with the rear end gears and the size of your tires.

 

As you saw, the odometer is a direct gear drive from the cable, so if your odometer measures correctly then you have the right transmission gear.  If the milage on the odometer is over or under the actual distance traveled then the transmission gear needs to be changed.

 

Once the odometer is correct, then you can see if the speedometer itself is calibrated right.

 

 

I was just working on the one from my '65 Chrysler.  You can see that the details are different, but it is the same basic sort of mechanism.

Hi Dan,

I appreciate the info! It looks like the odometer wheels in your Chrysler might use a different internal mechanism than the Chevy’s. Again, I couldn’t figure out how to disassemble the wheels to see how their ratcheting mechanism works. It’s neat to see the similarity of the Chrysler mechanism using a copper inductive drum influenced by a rotating magnet. Thanks again for posting. 
 

As a novice, the only variable in accuracy that came to mind was overall tire diameter, but what you point out is logical. The likelihood is that the Hydramatic transmission speedo drive gear in my truck is correct since it came out of another 55-57 Chevrolet light truck, and my speedo was very likely the original.

[Model56s]

1 hour ago, Sonomatic said:

Great pictures, great info, great post.  Thanks for sharing this!

Thanks! Also, looks like you have a 50 Buick. My avatar shows my 49 Super Sedanette which I bought in February 2012.

[Randiego]

Goeff,

 

Reading through your threads and from what I sent you in the email, the FIRST thing that you check is the cable and housing.  From sitting, (sometimes many years) the lubricant in the housing congeales and puts an immense amount of resistance on the speedo cable inside of the housing producing heat.  The heat can soften and melt the old grease but it is just a matter of time before the cable catches, binds and snaps.  If you ever examine a speedo cable, you will see that it is a bundle of very small wires wrapped into a spiral.  the ends are usually "tinned" with a hard solder and then machined into a square to fit into the drive head of the transmission gear and the speedo/odo unit.

 

After rescuing a lot of old cars, the first order of agenda are the obvious;  Oil and filter change, air filter, tranny oil (and filter if automatic), belts, hoses, coolant and any other maintenance item that needs attention.  The overlooked items;  the speedo cables, e-brake cables, choke cables, hood release cables, etc.,etc.  Any difference in the lubricants there than in the engine?  Invariably, they get overlooked.  Until there is a problem.  THEN it gets attention.  Or not. 

 

At the very least, when I get an old car and am bringing it back to life, I disconnect the cable from the transmission and deal with the speedo and cable at a later date when I have time slated for that maintenance.  It is amazing how many overlook this step.  When I do get time, I pull the cable out of the housing, wash it clean in Acetone and start the process of cleaning out the cable housing.  It is best to start on the end that attaches to the speedo unit. 

 

I ususally pour in ( with a little funnel) acetone into the housing and introduce the cable into the housing slipping it in and out until the gunk has completely exited the bottom end of the tube that is disconnected from the transmission.  I also use my air hose to blow the line clean.  When I am satisfied that the housing is clean, I introduce new lubricant.......liberally.  I run the cable in, spin it with my drill, pull it back out, introduce more lubricant until I can see it exiting the lower end and it is clean. 

 

IT IS IMPERATIVE THAT THE CABLE DOES NOT HAVE ANY BINDING ALONG THE ROUTE !  You can tell if you see your speedo needle vascillate (erratic movement).  If you have your speedo head moving freely and operating corrctly, then the culprit usually is the speedo cable or housing.  Especially if you have a tight bend to navigate.  In my upright tool box, I have a 1' and 2' length of speedo cables (using the one that best will work for the application) that I use to test run my speedo unit while still in the vehicle.  I will insert the square end into the speedo and chuck the other end into my drill, spinning up the speedo to check operation.

 

In the case of the severe curve(s), it is best to remove the cable from that location (if possible) and reroute the cable so that there is not the tight curvature that can lead to issues with the cable.  When you pull your cable out of the housing, inspect the cable from stem to stern. If you see any "shiny" spots on the cable, that is indication that the cable is wearing or binding at that spot and is meeting excessive resistance.  That in turn will lead to failure.  50 to 70% of all speedo issues are usually with the cable, not the head or the transmission gear drive.

 

They have much better synthetic oils today than the  oil that was used back in the day.  The new lubricants do not break down and are resistant to heat and give excellent lubrication to the cable.  One brand that I found that works well I get from the hobby shop. It is: Hob-E-Lube Premium. Ultra-Lite Oil HL661.  It has state of the art lubricant additives, clings well to moving parts and will not break down over time.  There are other lubricants available also.  This is just one.

 

Again, Geoff covers the speedo in detail.  Other than not having a puller to remove the needle from the shaft, the units are relatively simple to dissasemble the working parts. Much like a watchsmith with fine gears and worm drives.  But many restorers are fearful about getting into speedos prefering to send them to the specialists (expensive) vs. repairing or servicing the units themselves.  Of all the jobs involved in restorations, I really enjoy working on the speedo/odo units. 

 

Rules of the game; It is imperative to have the correct tools.   Have a clean workspace. I go one step further.  I use a 16" X 16" ? 1/2" thick melamine faced plywood (white) with a 1 1/2" X 1/4" strip of wood nailed around the perimiter "boxing in" the work surface.  This keeps the errant screw from falling off onto the carpet or ?  Too many hours spent looking for dropped fasteners led to fabricating this accessory.  I have a good "goose neck" lamp that I can zero in on the item that I am working on.  Fine screwdrivers appropriate to fit the small slots on the screw heads, minature hex and open faced wrenches for the small nuts, nut drivers, medical hemostats are an excellent tool to use for handling small parts, tweezers, long q-tips for cleaning hard to get to areas and a jewlers puller for removing needles from the shaft, Small clamps to hold parts together, and I have a Ultrasonic tank for cleaning up the parts. I also have small trays to put the parts in and clean with acetone (not the numbers on the odo. Those I hand clean with mild detergents and a q-tip).  

 

Take your time, shoot pictures of the unit and steps along the way for refrence and re-assembly.  If you run into a problem, there are a lot of sources to help with solving them.  In the end, it is a rewarding portion of your auto restoration.

 

Randiego