Speedometer disassembly and calibration

Thought I'd go over basic speedometer disassembly and calibration on a 70-72 speedo.

Here is your basic 70-72 automatic speedo-



First thing you'll want to do is remove the gear selector. Come off with one 1/4" screw



Next remove the lens. There are 4 metal tabs that are bent down to retain the lens. Just bend them back carefully and remove the lens and black bezel. Now would be the time to wash the lens in some diswashing soap and polish it. I use mothers metal polish. Yes, really! Works well



Next remove the printed curcuit board. Comes off with three 1/4" screws. You'll probably find the bulbs are dark and/ or silvered. Replace them with 194 bulbs.



Next remove the two screws holding the speedo head from the cup. You can repaint the inside of the cup with a fresh coat of light blue paint



Next up is removing the needle. The needle comes off very easily. If you notice, the needle rests on a metal stop. If you move the needle, you hear it bang against the stop as it goes back to zero. You can see the metal tang here



All you have to do is grab the needle and twist it counter clockwise against the stop. It comes right off. If you look carefully, the end of the shaft the needle is attached to is tapered. Your essentially breaking the taper like a ball joint.



Next take the two screws off that retain the speedo face.



Once the face is off, you can remove the clip that retains the odometer wheels



Take the odometer wheel assy and slide it towards the left and out. I have another thread that outlines how to reset the mileage.



Now we can take a good look at the clock springand the arms that adjust the calibration.



The long arm coming off the center puts tension on the clock spring wound up around the needle's shaft. Moving that clockwise or counterclockwise adjusts the reading

 

Next we'll go over how to actually calibrate the speedo...

GM speedometers are calibrated to read 60 mph at 1000 rpm. So I needed to figure out how to run a speedo at 1000 rpm.

I made this fixture using a cheap harbor freight drill, a speedo cable, an old extension cord and a dimmer switch to adjust the rpm.



I used a digital photo tachometer to get the rpm to 1000 rpm. My corvette speedo was reading low





Needed a small adjustment. A little more adjustment on the spring and it's now reading at exactly 60 mph



As it's spinning, just use a small screwdriver to move the calibration arm. Either push it down from the top or push it up from the bottom to adjust the mph.

 

One of the issues I had with the Corvette was that the odometer wheels got rusty from decades of sitting. When that happens, the black plastic odometer gears strips out as its the weakest link

So let's get a little deeper into this speedo. Remove the gray plastic bracket that the face was screwed into. It's attached with what looks like brass screws. Be careful at this point because the needle shaft isn't being supported anymore. You csn go ahead and remove the white odometer gear as well.



Now we're going to remove the speed cup. It's attached with those two bright 1/4" screws. Once you remove the screws, grab the speed cup by the shaft and remove it. Be extremely careful with this part. Put it in a safe place.

You can get a closer look at the black odometer gear that runs off the first worm and magnet gear.



To remove that black gear, you have to knock out an aluminum plug.



What you want to do is get a small punch and a hammer. Give the plastic gear a tap or two from the other side and drive out the plug



And here's the blsck odometer gear and plug removed

 

The only thing left to remove is the first worm and magnet assembly out of the casting. It's retained by a very thin snap ring. Unless the first worm and magnet is binding or feel rough, leave it in place.

To reassemble, judt reverse the steps. They do make a repro black odometer gear in metal if your odometer doesn't work.

 

Thanks guys!

I just learned. I had the necessity to fix an odometer that stopped working a few years ago and had to figure out how to take it apart.

In the next posts, I'll recap how to reset the odometer and how to convert an auto speedo to a manual trans speedo. We'll do a half ass resto on this particular one, put it back together and calibrate it

 

You can wash the speedo face in warm water and a little dishwashing soap if it's dirty.



If you want a cheap alternative to an actual 4 speed speedo, you can wipe the gear selector printing off with some windex. Once the printing gets really faded, you can use a little mothers to get the remnants off. Now, it won't be perfect. You'll still see the imprint in the black. But once the speedo is put back together and behind the lens, it will almost undetectable.



If anyone knows a better method, just post it up. But this method is acceptable in my opinion

 

And a little before and after on the lens.

Before-


This lens was really dirty and cloudy. A little warm water and soap, then a quick hand polish with mother's metal polish. The difference is amazing

 

In this next post, I'll go over resetting the mileage. Now, there are alot of non- nefarious reasons for doing this. Maybe it's a nut and bolt, frame off resto and you want to change the mileage back to zero. It's what I did on the beater. I reconfigured the whole car, why not start at zero again? Maybe you need to replace the speedo and you want the mileage to match the original your removing.

Whatever the case, here's how you do it. This is your typical odometer. Right now it's showing 55,482



On the backside is the tamper plate. It's a piece of thin plastic that holds the wheels in place.



If you try messing with the numbers, the tamper plate would break in the center and the two far left wheels would break free. So instead of reading 55,482, the two wheels that read 55 would esentially free wheel. In this image, with the odometer installed without the tamper plate, looking down, you can see the 10k and 1k wheels are free.



So very carefully remove the tamper plate. This things were fragile 53 years ago and they haven't improved any. Don't pry against the wheels! Take a small screwdriver and pry up using your thumb against the little metal tabs. Like how you peel an apple. Keep working it up to the top of the little metal tabs, little by little.



Now that the tamper plate is off, all the wheels are free. Now it's not as simple as just putting whatever mileage on them. You have to work from left to right. Once you start playing with it, you'll get the idea. In this case, we're going to set the odometer back to 00001 miles as if it were a new car!



So you don't get confused, the numbers that will be shown on the speedo will be directly opposite of the metal tabs.



Once you have the miles set, just reinstall the tamper plate back on. Again, very carefully a little at a time. And if you do happen to crack the plastic which is easy to do, don't worry! The parts place repro's it!

So that's it. It's actually fairly simple once you get the gist of how the wheels work.

 

All you need to do in order to put the speedometer back together is just reverse the steps

One important step is to reattach the needle. All you have to do is put the needle on at around the 50 mph mark



Just put a little pressure on it and twist it counter clockwise till you get to zero. You may have to go a little past zero to get it to rest exactly on zero.



All that's left is to put the lens back on and squeeze the tabs snug



There you have it. A budget 4 speed speedo!