Beck TD, Part 52: Saving a Switch

Here's the second post in a row about extraordinary efforts to try to save a part that is basically junk! The photo below shows two switches removed from the broken shifter in the M41 transmission I'm trying to resurrect (see Part 51 for more details):

These switches thread into holes in the shifter, and one makes sure that the overdrive isn't engaged except while in fourth gear, and the other turns on the reverse light when the trans is in reverse. Both of them were stuck in the "on" position, so I started looking for replacements. This whole saga basically stems from the fact that I didn't know what to call them, so I didn't know how to look them up. All I could find was this eBay ad:

The price for two, including shipping from the Netherlands, was around $128! I resolved to try to fix them. However, after actually getting both switches working, I learned their real name: "overdrive inhibitor switch." Armed with that knowledge, I now know I can buy a very similar switch from Moss Motors for about 10 bucks. But... it sure was satisfying to fix these!

You can see in the first pic that there is a large hex built into the switch. That makes it very easy to mount in a vise - you just clamp on the flats of the hex:

Then, it's easy to use a thin screwdriver with a small hammer to pry the the tabs back. Here's a tip: only work on the tab that is closest to the movable jaw of the vise, so that the taps are in the direction of the clamping force. Otherwise, the switch will want to tilt in the vise. Rotate the switch so that you can release each tab in turn.

Once all three tabs are free, you can extract the parts of the switch:

A closeup of the movable center part shows what was going on:

The O-ring was hard and swollen, causing the switch to bind. I removed it, and found that the 3/16" X 5/16" X 1/16" from my Harbor Freight collection was a pretty good match. I also cleaned up all the switch surfaces, including the contact points, the ball end and the switch body with 200 grit wet-or-dry sandpaper lubricated with oil.

Once done, I reassembled everything. It's important to note that there's a tab in the top plate that matches a notch in the body, and you also need to pay careful attention to the orientation of the contacts on the movable part. Tap the tabs back into place with a small punch.

Testing, I found that the switch now has "infinite" resistance when it's not being pressed, and almost no resistance when pressed, just as it should:

All good... except that in one of the switches, the spring is weak enough that I would be loath to trust it. I'll just buy two of the $10 switches from Moss to actually go in the transmission.

This is a great example of 1952 technology. It was designed to be initially assembled by human hands, and thus could also be disassembled and repaired by human hands. Try that with a switch from a 2019 BMW!

One more fact I want to preserve here: on some Volvo shifters, the hole for the switch is not there, but there is a cast-in boss that you can drill and tap for the switch. After some research, I now believe that hole should be tapped M16-2.0. Obscure, but the tap is pretty cheap from Amazon.

Continue on to Part 53...