Author Topic: Distributor components testing  (Read 7563 times)

Offline Irish_Alley

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Distributor components testing
« on: December 28, 2014, 04:15:24 AM »
Married Ignition Coil  (1975-1986; mounts in the top of the distributor cap):




Divorced Ignition Coil  (1987-1995; mounts separately from the distributor):



Ignition Control Module (ICM):
The ICM can only be tested by specialized equipment that heats the module to a predetermined test temperature.  Some parts stores such as Autozone have ICM testers available, although such tests are not strictly reliable, because the testers cannot duplicate the dynamics of a real working environment.

Pick-Up Coil (PUC):
The PUC can only be tested for hard (non-intermittent) failures.  For the following tests, remove the distributor cap and rotor and carefully disconnect the PUC from the ICM.  Measure the resistance between either PUC lead and the metal frame of the PUC (ground).  The ohmmeter should indicate infinite resistance (open circuit).  Measure the resistance of the PUC with the ohmmeter connected to both PUC leads simultaneously and apply vacuum to the vacuum advance can while watching the meter.  If the distributor does not have a vacuum advance, gently wiggle and lightly tug on the PUC leads while performing the resistance test.  PUC measured resistance should remain constant (unchanging) within the range of 500 - 1,500 ohms.  If the PUC fails any of the resistance measurements it is defective and should be replaced. 

Intermittent failures of a PUC are generally caused by temperature associated expansion and movement of the fine wire that comprises the coil winding, or flexing and internal breakage of the PUC leads that connect to the ICM, which result in temporary opens or shorts as the wire moves.  Typical symptoms are random misfire and/or dying of a warm engine that clears up once the engine cools or is cold.  Tests for intermittent PUC failures with the distributor in the vehicle are generally unreliable.

However, one notable exception may result from turning the distributor shaft at a constant RPM (less than 1,000) with the distributor securely mounted on a bench while monitoring the PUC output using an oscilloscope.  As vacuum is applied to and removed from the vacuum advance canister, repeatedly, an intermittent PUC malfunction often will manifest as an erratic pulse indication or complete loss of PUC output.  Still, because of the difficulty in creating realistic thermal cycling of the distributor, test results may remain inconclusive.
« Last Edit: February 11, 2022, 02:26:38 PM by bd »
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