In my experience the following information should get you out of trouble 99% of the time,
Most hall effect senders require a pull up resistor to make the signal visible wich you will be able to buy from any electronics parts store about 5 cents worth, connect the resistor between the ignition supply and the output of the sender unit (Red and white wires). most hall effect senders have approximately 8 pulses per one revolution of the sender, that is, the output will change state from high (12 volts) to low (approx. 0.8 volts) 8 times for one turn of the sender. place the volt meter across the resistor to measure this. with the ignition turned on and earth connected, turn the sender slowly by hand to watch the change on the volt meter,hope this helps.