Beck TD, Part 71: Another Speedometer Sensor

Back in Part 68, I detailed some work that allowed me to use a digital speedometer with a speed sensor, allowing me to experiment with different rear gear ratios and transmissions, and still have an accurate speedometer. It worked great on the M40 4-speed transmission currently in Beck, so I confidently set out to install it on the M41 overdrive trans I had rebuilt previously. That's when the problems started...

That sensor is driven by the stock speedometer gear and pinion arrangement, and it works the same way in both transmissions. I bought the correct gear from Joe Lazenby and confidently installed it, and was surprised to find that it would bind on every rotation. That pinion gear I had just bought is plastic, so that situation would have destroyed it in short order. After several attempts, I finally disassembled the tail of the overdrive and found this:

That is a worm gear that drives the speedometer, and it is very worn. For those interested, you can click this diagram to expand it and see how it works. The worm is #106, and #115 is the plastic driven gear that I bought from Joe.

It's clear from the photo of the worm gear that it is very worn, and that has all my advisors befuddled. How could a plastic gear do that much damage to that steel worm? Anyway, I spent a good bit of time filing on it, and recruited my patient friend, John Zimmerman, to do the same. No joy - it's so worn that the plastic gear seems to be bottoming out in the root of the worm thread.

I searched the web, and that worm gear is not available anywhere. I know there are hundreds in dead transmissions and junk drawers worldwide, but how to find them? Jake Roulstone challenged me to "roll my own" by putting magnets in the yoke and mounting a Hall effect sensor, and I decided I had nothing to lose. I chose this inexpensive sensor from Amazon - search for "Twidec/10mm Hall Effect Proximity Sensor Inductive Switch NPN NO DC5-30V NJK-5002C" It was only 10 bucks, but later I found another vendor that would sell TWO sensors, minus the magnet, for 10 bucks!

The only documentation that came with it was a label on the cable, so small that I had to take a picture and enlarge it to even read it. I later found a diagram online, and am saving it here.

The other part of the equation was magnets. 1/8" diameter by 1/4" long was what would fit in the yoke, and I had to buy 150 of them! A lifetime supply, for sure... Cheap on eBay.

Testing with a bench power supply, I was surprised to discover that the Hall effect sensor is sensitive to magnetic polarity. I carefully marked the right end so I wouldn't make a mess later.

The next step was to decide where to put the magnets, and at first I didn't notice that the four mounting holes are not placed every 90 degrees. Fortunately, I did notice before drilling a bunch of holes, and with some thought, realized that this spacing avoids the mounting holes the best.

I made that shiny adapter to hold the yoke, and mounted it in my spin indexer. With that, it was easy to mark and drill those holes at the right spots.

Those magnets look like they belong there! I installed them slightly proud of the yoke, because I noticed that the driveshaft yoke was slightly larger in diameter.

To test, I installed the yoke in the lathe and started at the slowest speed. That's a spare yoke bolted to it to simulate the mass of the driveshaft yoke - I wanted to be sure that didn't upset the magnetic field. The sensor is in a magnetic base so I could easily align it. It worked! The spec for the sensor was a gap of 0.080" maximum, and that's about what I found. To see a brief video of the test in action, try this link:

https://youtu.be/z3__DCP-Oic

Once that was done, I needed to get this adapted to the trans, and the first step was to plug the hole that was now there, since I couldn't use the speedometer pinion. I used one of the adapters I had made, plugging it with a brass plug. Funny story: I made this and set it aside for the epoxy to dry, and while waiting I refilled the gearbox with oil. I couldn't figure out why it wasn't topping up until I looked at the other end and saw the oil pouring out, since the plug wasn't attached yet!

The last puzzle was to figure out how to mount the sensor on the overdrive unit on the transmission. There were no bolts nearby that I could use to mount anything, so I decided to make a collar to hold the sensor. I started with a piece of aluminum tubing I had gotten from Phil Oles. It's 3.5" in diameter with a 2" hole. 

The hole needed to be 2.5" so I got to work boring that hole - a slow process using my hobby equipment.

I was done when a 2.5" aluminum rod would slip in place.

I carefully marked the middle of the collar using a height gauge, and then used that to align the workpiece in the milling vise. The goal was to bore a pocket for a screw head, and then tap a hole that extended well past the centerline. Here's one side done.

Incidentally, it's hard to align to something round. I needed to be lined up with the exact center of the ring at the marked centerline. I aligned to a parallel and then moved over the correct amount, taking the width of the parallel into account.

Once all that milling was done, I moved to the band saw and carefully cut the collar in two. Then I drilled the top part to allow clearance for the screw to slide through. You can also see that I milled a flat spot and tapped two holes for mounting a bracket.

Here are two views of the collar attached. Since I was careful boring that big hole, it fits tightly and will be easy to adjust.

The last step was to bend a little bracket to hold the sensor. Job done!

Of course, I still have to get the transmission installed in Beck TD, and that's a big job, requiring cutting and redesigning a crossmember and also designing a new transmission mount. I may defer that until next winter since the driving season is upon us. But I'm glad that the  trans is ready to go!

Continue on to Part 72....