Chevrolet Full-size Cars 1968-1978 Repair Guide

Ignition Coil

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TESTING





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Fig. Fig. 1 Engine Component Locator-1977 Caprice With 305 CU. IN. V8

1968-74 Engines; 1975-76 Inline 6-Cylinder Engines With HEI
PRIMARY CIRCUIT WITH VOLTMETER

A quick, tentative check of the 12 volt ignition primary circuit (including ballast resistor) can be made with a simple voltmeter, as follows:

  1. With engine at operating temperature, but stopped, and the distributor side of the ignition coil grounded with a jumper wires, hook up a voltmeter between the ignition coil (switch side) and a good ground.
  2.  
  3. Jiggle the ignition switch (switch on) and watch the meter. An unstable needle will indicate a defective ignition switch.
  4.  
  5. With the ignition switch on (engine stopped) the voltmeter should read 5.5 to 7 volts for 12 volt systems.
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  7. Crank the engine. The voltmeter should read at least 9 volts during the cranking period.
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  9. Now remove the jumper wire from the coil. Start the engine. The voltmeter should read from 9.0 volts to 11.5 volts (depending upon alternator output) while running.
  10.  

PRIMARY CIRCUIT WITH OHMMETER

To check ignition coil resistance, primary side, switch ohmmeter to low scale. Connect the ohmmeter leads across the primary terminals of the coil and read the low ohms scale.

Coils requiring ballast resistors should read about 1.0 ohm resistance. 12 volt coils not requiring external ballast resistors should read about 4.0 ohms resistance.

SECONDARY CIRCUIT WITH OHMMETER

To check ignition coil resistance, secondary side, switch the ohmmeter to the high scale. Connect one test lead to the distributor cap end of the coil secondary cable. Connect the other test lead to the distributor terminal of the coil. A coil in satisfactory condition should show between 4K and 8K on the scale. Some special coils (Mallory, etc.) may show a resistance as high as 13K. If the reading is much lower than 4K, the coil probably has shorted secondary turns. If the reading is extremely high (40K or more) the secondary winding is either open, there is a bad connection at the coil terminal, or resistance is high in the cable.

If both primary and secondary windings of the coil test good, but the ignition system is still unsatisfactory, check the system further.

1975-78 V8 Engines, 1977-78 Inline 6-Cylinder Engines

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 and 30,000 ohms. Be sure to test between the rotor button and both the BAT and TACH terminals.
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  5. Replace the coil only if the readings in Step 1 and Step 2 are infinite.
  6.  

These resistance checks will not disclose shorted coil windings. This condition can only be detected with scope analysis or a suitable designed coil tester. If these instruments are unavailable, replace the coil with a known good coil as a final coil test.

REMOVAL & INSTALLATION



1968-74 Engines; 1975-76 Inline 6-Cylinder Engines With HEI
  1. Disconnect the negative battery cable. Remove the ignition switch-to-coil lead from the coil.
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  3. Unfasten the distributor leads from the coil.
  4.  
  5. Remove the screws which secure the coil to the engine and lift it off.
  6.  
  7. Installation is the reverse of removal.
  8.  

1975-78 V8 Engines, 1977-78 Inline 6-Cylinder Engines
  1. Disconnect the negative battery cable. Disconnect the feed and module wire terminal connectors from the distributor cap.
  2.  
  3. Remove the ignition wire set retainer, if equipped.
  4.  
  5. Remove the 4 coil cover-to-distributor cap screws and the coil cover.
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  7. Remove the 4 coil-to-distributor cap screws.
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  9. Using a blunt drift, press the coil wire spade terminals up out of distributor cap.
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  11. Lift the coil up out of the distributor cap.
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  13. Remove and clean the coil spring, rubber seal washer and coil cavity of the distributor cap.
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  15. Install the coil spring and rubber seal washer. Install the coil in the distributor cavity.
  16.  
  17. Install the coil connections in the cap. Install the coil retaining screws.
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  19. Install the coil assembly cover. Install the ignition wires. Connect the feed and module wire terminal. Connect the negative battery cable.
  20.  

 
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