Check the connections in the engine compartment.  The junction is right
near the Thermostat housing.  They may look good but I've had some fail
right there from all the flexing and shaking the motor does.  All I did
was touch it and one wire fell off.   The way the system works is the
sensor down between the push rod tubes turns the light off when the
pressure builds and the one near the oil pump sends a signal when the
pressure is above 15 lbs. (test light will light when connected to the
sensor wire and a power source)  check to see that this happends first,
before replaceing the control unit in the speedo.
--
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                             -Jim DiGennaro
          LiMBO, The Late Model Bus Organization, international
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