GM Cadillac 1967-1989 Repair Guide

Distributor Components Testing

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See Figures 1, 2 and 3

If the trouble has been narrowed down to the units within the distributor, the following tests can help pinpoint the defective component. An ohmmeter with both high and low ranges should be used. These tests are made with the cap assembly removed and the battery wire disconnected. If a tachometer is connected to the TACH terminal, disconnect it before making these tests.

  1. Connect an ohmmeter between the TACH and BAT terminals in the distributor cap. The primary coil resistance should be less than one ohm.
  2.  
  3. To check the coil secondary resistance, connect an ohmmeter between the rotor button and the BAT terminal. Note the reading. Connect the ohmmeter between the rotor button and the TACH terminal. Note the reading. The resistance in both cases should be between 6,000-30,000 ohms. Be sure to test between the rotor button and both the BAT and TACH terminals.
  4.  
  5. Replace the coil only if the readings in Step 1 and Step 2 are infinite.
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These resistance checks will not disclose shorted coil windings. This condition can only be detected with scope analysis or a suitably designed coil tester. If these instruments are unavailable, replace the coil with a known good coil as a final coil test.

  1. To test the pick-up coil, first disconnect the white and green module leads. Set the ohmmeter on the high scale and connect it between a ground and either the white or green lead. Any resistance measurement less than infinity requires replacement of the pick-up coil.
  2.  
  3. Pick-up coil continuity is tested by connecting the ohmmeter (on low range) between the white and green leads. Normal resistance is between 650-850 ohms, or 500-1500 ohms on 1977 and later models. Move the vacuum advance arm while performing this test. This will detect any break in coil continuity. Such a condition can cause intermittent misfiring. Replace the pick-up coil if the reading is outside the specified limits.
  4.  
  5. If no defects have been found at this time, and you still have a problem, then the module will have to be checked. If you do not have access to a module tester, the only possible alternative is a substitution test. If the module fails the substitution test, replace it.
  6.  



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Fig. Fig. 1: Checking coil resistance - Ohmmeter 1 shows primary coil resistance connection - Ohmmeter 2 shows the secondary resistance connection (1980 shown, others similar)



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Fig. Fig. 2: Pickup coil removed and disassembled



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Fig. Fig. 3: Testing the pickup coil

 
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