View Single Post
  #4  
Old 09-09-2014, 11:10 PM
Frank Reiner Frank Reiner is offline
Registered User
 
Join Date: Feb 2011
Location: Modesto CA
Posts: 4,078
Hugh E.:

Tecqboy's suggestion of the crankshaft ignition pickup seems a good starting point. In more sophisticated engine management systems, such as those with timed fuel injection and ignition all under computer control, sensors are used to determine the angular position of the crankshaft. In this case the pickup, which is called a variable reluctor (VR), provides a pulse three times per revolution which signals the ignition system to provide a spark. The VR consists of a permanent magnet surrounded by many, many turns of fine enameled copper wire. The flywheel has three short segments which pass close to the magnet as the flywheel turns, causing a variation in the magnetic field which surrounds the magnet. As the field changes a low voltage (~1-3V) current is induced in the windings. That current is the signal to create a spark. As the pickup ages, some of the insulating properties of the enamel deteriorate, and small fractures can also occur in the wire. When the pickup is cold, and the resistance of the wire is low, the voltage will be above the threshold to be recognized as a valid signal, and the ignition system will produce sparks. With increasing temperature the output can fall below the threshold due to increasing internal resistance. Why will the engine continue to run if the RPMs are up? Because the output of the pickup rises with the speed of the segments passing the VR. Once the pickup is warm, and engine speed drops to idle, the output is too low to be valid.

With all that said, the first steps will be to clean all the electrical connections that are associated with the VR, and check the cold resistance of the windings (700-1200 ohms).
Reply With Quote