It sends timing signals to the ECU.  Senses changes in magnetic current.

Hall "effect" sensor is likely a better name than "Hall Sender" 
On Apr 8, 2013, at 6:58 AM, john schaper <tahljohn@GMX.COM> wrote:

> Still trying to diagnose and fix my intermittent start (latest theory is that it is do to dew point). Checked all grounds; saw some cracked wires to the distributor, spliced and repaired, vacuum lines connected, getting fuel, AFM is rebuilt, connections to fuel injection is good, coil is new, distributor is 2 years new as are spark plug wires. Swapped out a known good ECU with no effect. Now it won't start at all--just grinds with the engine turning. Now, after my recent repair fixes, it won't start and its 80 degrees! Reading the list and Bentley, the one item I haven't replaced or diagnosed is the Halls sender, generator or whatever is in the distributor. What does this thing do and could it be the problem?