Why do speed sensors fail so often?

Question

The typical vehicle speed sensor (variable reluctance sensor) is a very basic device which essentially amounts to a coil of wire wrapped around a permanent magnet.

Simple, yes, but very reliable? Apparently not so these days...

Years ago, I worked on some ancient airport CAT (cash-audit terminals) which would read a magnetic stripe printed on a length of a thick paper card. When the motorist provided the cashier with the card, the machine would read the stripe, calculate the parking fee and using a rather unique printing mechanism, hammer out the text of the charges onto the card as a receipt. The machines were made by a company in San Diego known as 'Electron' - they went out of business in the late 80's.

I bring up the CAT units because of their interesting printing mechanism - an electric motor would turn on, and a round metal disc (platen) with numbers and letters embossed around the perimeter was attached to the motor using a specially made rubber band. As the wheel spun, an ink wheel rolled over the characters on the outside of the wheel. As the card passed through a channel, a cantilevered solenoid with a little hammer-like head would strike - pressing the passing thick paper receipt against the spinning print wheel as a stepper motor positioned the receipt for the next character to be printed.

The timing, speed and precision were amazing - I used to fix them and marveled at how well engineered and reliable they were.

I bring that up because there was a hall-effect sensor that sensed an index mark on the spinning wheel. This signal was sent back to the circuit to adjust the timing of the stepper and hammer strikes. In the 7 years that I worked on those machines, I only recall a very rare few times when we had to replace the printer sensor - maybe less than 5 times in 7 years across about 50 units.

Just about every car I've owned has had wheel speed sensors go out. Now, based on my limited understanding of the manufacture of these items, and of their basic design, I have to wonder what could possibly go bad with a wire coil and permanent magnet encased in epoxy?

What am I missing?

Answer 1

The variable reluctance pickup coils are wound of fine wire, and located in an engine compartment with temperature that cycles from hot to cold.

Often they are potted which can increase forces on the wires if it is not done very carefully. That was not necessarily well understood in the early days so you'd see a lot of open coils, at least on American cars.

Edit: For more information you can read this patent which points to the encapsulation and subsequent temperature cycling as the major source of reliability issues.