Seeing as we’re pretty much house-bound at the moment and there’s nothing on the telly, I decided to let my mind wander a bit regarding why MMC might have deleted the pull-up resistor from the M3W speed sensor.
The speed sensor used (Honeywell 1GT101) is primarily designed as a gear tooth sensor.
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In this application the triggering frequency will be much higher than in the M3W. Indeed, the sensor is specified to go up to 10kHz, i.e. 10,000 triggerings per second!
A back of the envelope calculation indicates that, even at 110 miles per hour, the speed sensor will only be working at 111Hz, i.e. 111 times per second. (It is linear, so at 80mph there are roughly 81 sensor pulses per second.)
The purpose of a pull-up resistor is to ‘reset’ the sensor output between pulses. This is probably desirable at 10kHz but not really necessary at 111Hz.
In addition, omitting the resistor will have simplified the production of the sensor assembly by MMC’s supplier which may have saved a few pennies.
All just my ramblings of course but I have verified that the speedo works fine on my car with or without the pull-up resistor. Therefore, I plan to omit the resistor from future ‘Bunny’ speed sensor assemblies I make.
Right, everyone can wake up now!