Crank/Cam sensors

[greenamex2]

A fairly general question but probably of interest to most people on the board -

Assuming a 'blank sheet' what are the best sensors to use for crank and cam position? I have assumed hall effect because of the cleaner signal but they do seem less reliable.

Also, is it OK to mount a cam sensor 'inside' the engine to pick up on the cam (eg a lobe or other sticky out bit)? To me the oil contamination and heat would be a bad thing.

Thanks.

[pat]

Sometimes it pays to employ the KISS principle, the simpler it is the less likely it is to go wrong. VR sensors are an old, reliable, proven design so unless there is a compelling reason not to use them then I see no reason to choose another sensor type. If there is simply insufficient space to get enough teeth onto a wheel and still get reliable detection then it is worth considering optical. Hall effect can offer slightly better noise immunity compared to VR, the output impedance can be quite low especially if it's a totem pole output...

Mounting sensors inside the engine is certainly not a problem if they are designed to be used in that way. Take, for example the Subaru AVCS (variable cam timing) sensors and indeed the main cam sensor on the BMW M57 engine, they both cross the oil boundary... the connectors are external but the sensing head is inside. They tend to have O ring seals and appear to be reliable in operation.

Hope that helps,

Pat.

[greenamex2]

Thanks pat, can you explain the significance of the "the output impedance can be quite low especially if it's a totem pole output" statement?

With my sporadic electronics knowledge the only issue that low impedance creates is a higher current draw, possibly leading to heat related issue in the ECU if not the sensor itself.

Maybe I just need to stop my dad setting the air gap on the existing sensor

[pat]

The output impedance is that impedance which you would "see" if you were to "look" into the sensor's output. If the impedance is high then you can change the voltage at its terminals with relative ease, it won't fight you. If the output impedance is zero then you would require an infinite amount of current to change its output. A low impedance is thus less affected by interference, the interference doesn't have an infinite supply of current to impose its will on the sensor's output, ergo the sensor wins and the interference can't affect it (there is of course very much more going on in reality, but it illustrates a point).

As for current, the input impedance of the ECU is quite high, depending on configuration up to 1 megohm. That doesn't take a lot of current to drive, so there won't be any heat. The point of a low output impedance coupled with high input impedance is that the sensor can impose its will on the ECU, and cabling, and anything else that might be hanging off that signal.

Hope that helps,

Pat.

[greenamex2]

It does, thanks.