VCCA Home Forums TECHNICAL - 1945 AND EARLIER 1912-1928 Speedometer Magnets

I worked on my '26 Touring speedometer today and tried to calibrate it more accurately, since it obviously was reading slow. I understand that at 1000 rmp the speedometer should read 60 mph. Spinning the speedometer with my electric drill which supposedly does 600 rpm according to the label, the speedometer should read 36 mph, if my math is correct. However, there was no way I could get the speedometer to read past 25 mph. I figured that the magnet must not be strong enough. As a temporary measure I added a small rare earth magnet to each end to give the magnet a boost. It seems to work for now and I have lots of room for adjustment. However, I tried to find a way to re-magnetize the magnets. I Googled but the best advise I got was to touch the ends with a strong permanent magnet. This didn't work for me but made it weaker. Does anyone know how it's supposed to be done or have any experience with this? Thanks.

Sometimes I have to magnatize a tool for a special reason I rub the strong magnet along the length of the steel in ONE direction, not back and forth. try that instead of just placing it on the end. My 28 speedo is off a bit so I may try to pull it apart and do it while checking the GPS and see if I can get it right on the money.

i am really surprised that your 26 speedo even works at all . i have never found a speedo repair person that would touch one .my 26 was almost dust over the years i have found parts from others to get mine to work some what . wish you a lot of luck

To calibrate an old speedometer, a speedometer calibrating machine is used in conjunction with an electronic magnet. The electronic magnet consists of two strong magnets, each magnet is opposite each other. The speedometer is mounted on the speedometer calibrating machine. On the drum type of speedometer the magnet is held either horizontal or vertical over the drum and then the speedometer is "zapped" with the electronic magnet to either increase the speed of the drum or to decrease the speed of the drum. This is done until the miles per hour on the speedometer coincides with the miles per hour on the calibrating machine.

Any speedometer shop should be able to calibrate your speedometer.

The problem with 25-26 speedos is that the bridge that holds the MPH dial is made from muck metal and there is no replacement part to fix it. I found an old Buick speedo that was identicl to the 25 Chev baring the face plate. My mate who wanted one found it was in pretty good nick. So keep your eyes out for othe GM speedos that may suit
Chris

Other GM units will have an AC and different colors, but will still work.

I really wasn'r interested in how a speedometer shop would calibrate these but wanted to know how to increase the strength of the speedometer magnet. I was intriqued by the use of electromagets in calibrating them in the shop. This means that they were increasing the strength of the magnet. However I found that the unit has to be inside of the case in order to calibrate it since they work quite differently outside of the case than inside of it. The reading I get is about twice as high - I have no idea why this is.

I did find an easy way to make the magnet stronger. You make up a coil of magnet wire by wrapping the wire around a rectangular flat bar a bit wider and thicker than the magnet. I did one layer about 5 inches long. Tape the outside with electrical tape so the windings will stay put and slide the coil off the flat bar. Insert the speedometer magnet inside of the coil and briefly touch the ends of the winding wire to the car battery terminals a couple of times just as you would to polarize your generator. Don't keep it on for more than a split second because you'll short out the battery, with potentially disastrous results. At this point I don't know which end of the magnet should be the North pole or if this makes a difference.

You calibrate the unit by moving the lever to increase or decrease the tension on the hair spring attached to the drum.

Hi Arend,

I made the same discovery regarding the the effect of the casing on the speedo. I recod 3 of them about 2 years ago, had them running on a test jig like a charm, put one back in the case, bingo, running about 20 miles slower than it should. After a while worked out that the casing was affecting the setting of the magnet. Took a while to work out, thought I was going crazy. I think in the end I worked out the difference and set them high outside the case, and they were spot on once assembled. Was a good proccess of restoration, made 3 recod speedos from 5, and imcluded the refacing kit from the Filling Station. I have also heard from others re the wire coil around a magnet connected to a battery, so it must work.

Below is the entry from my blog detailing the whole process.

http://my28chev.blogspot.com/2011/06/2009-speedo-restoration.html

Cheers

Ray

I did the math the easy way using equal ratios. 60mph /1000 rpm = X mph (36 mph for mine) / speed of your drill (mine was 600 rpm). I think the drill must be slower than the rated 600. I drove the car down the highway today and it must be too fast since it said I was cruising comfortably at 55 mph which I don't believe. I'll try calibrating with another drill tomorrow.

I used a similar formula adjusted to the speed of the pedestal drill, and checked it with a lazer tacho that I picked up on the net.All I had to do was use a figure of 8 on the pedestal drill belt to get it to run the wright way.

I do like the common sense formula on USA speedometers, very easy to work with.

Cheers

Ray

Tested the speed today with my wife driving behind me. Turned out when my speedometer showed 50 mph she was showing 40 mph so I had to adjust by 0.8 and set it to 26 mph turned with the drill which must be a lot slower than the rated 600 rpm. I think I'm pretty close now. I also found that my coil didn't make the magnet strong enough so I used up the rest of my wire and almost trippled the number of turns on the coil. That should do it.