Need help identifying sensor / signal on motorcycle.

[Phoenix]

Hi guys. I'm having a little trouble identifying how to use a sensor on my motorcycle. It's a speed pickup that is on the transmission. I don't have access to a 'scope (that would make it much easier). the pickup has 3 wires. One is 12V ignition power, second is ground, and 3rd is a 5V pulse. The pulse line leads directly to the instrument cluster. With the ignition on but not running, I can read a distinct 5V pulse while turning the rear wheel by hand, but the pulses are not related to wheel position, ie: if I turn the wheel forward until I get a pulse, then turn back a little, and then forward to the same place, I don't get a pulse. It is even more confusing because when I start the engine, I get pulses even when the wheel is not turning!

At first, I assumed that the sensor would be a simple Hall-effect switch, but the behaviour observed seems to indicate otherwise. Is there any way to check this without actually removing the sensor from the engine?

Also, I assumed that the frequency of the pulses would be proportional to the speed, but would it be possible for the signal to be PWM? So that the frequency is constant, but pulse width varies so as to drive the needle motor? Can a simple sensor put out a PWM signal?

Any help here would be greatly appreciated. I've finished programming my MCU, and it works fine using an astable 555 with variable timing up to 5KHz as input, but the bike's sensor is throwing a fish at me.

Alternately, I could place a few small magnets on the wheel rim, and place a separate sensor to detect their passing, but it would be far easier to simply tap into the existing sensors.

Thanks in advance...

 

[Sebi]

Most of Hall-sensors output is an open-collector type. Apply about 1kohm pull-up resistor between output and +5V.

 

[stevez]

Be careful that your attempts to measure might be causing a problem. I recall attempting to troubleshoot the electronic ignition on one of my cars and things got really weird. Eventually I noticed a pattern and discovered that the leads I was using for testing were acting as an antenna - and a powerful local AM broadcast station was triggering the circuit.

Also tried making sense of the temperature sensor on one car recently, with my DVM using the ohm meter. Eventually I discovered that I needed to use the diode checking function because the ohm function would overload the sensor.

 

[Phoenix]

Okay, I've got an update, but still can't figure out what's wrong though.

I got hold of a scope, and the signal is a plain and simple 5V square pulse with a max frequency of 140Hz. Nice clean trace. So I hook it up to my circuit again, and leave the scope on the sensor output. Now my circuit works absolutely perfectly!!! Then I disconnect the scope, and the readings jump all over the place again!

I'm really quite stumped here, so I opened up the dash clocks, and traced the circuits in there. Only components on the speedo signal line are a small coil and a diode in series before the controller.

Any ideas?