GM Cadillac 1967-1989 Repair Guide

Ignition Coil

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TESTING



1967-74 Vehicles

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 wire, 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 ignition switch on (engine stopped) the voltmeter should read 5.5-7 volts for 12 volt systems.
  6.  
  7. Crank the engine. Voltmeter should read at least 9 volts during cranking period.
  8.  
  9. Now remove the jumper wire from the coil. Start the engine. Voltmeter should read from 9.0-11.5 volts (depending upon generator 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- resistance. 12 volt coils, not requiring external ballast resistors, should read about 4.0- resistance.

SECONDARY CIRCUIT WITH OHMMETER



To check ignition coil resistance, secondary side, switch ohmmeter to 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 and Later

See Figures 1 and 2

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 1-.
  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 6K- and 30K-. 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.
  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.



Click image to see an enlarged view

Fig. Fig. 1: Testing the external HEI ignition coil - Test 1: use high scale, reading should be very high or infinite. Test 2: use low scale, reading should be very low or zero. Test 3: use high scale, reading should not be infinite.



Click image to see an enlarged view

Fig. Fig. 2: Ohmmeter 1 shows the primary coil resistance connection. Ohmmeter 2 shows the secondary resistance connection (1980 shown, most models similar)

REMOVAL & INSTALLATION



1967-74
  1. Remove the ignition switch-to-coil lead from the coil.
  2.  
  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 and Later
  1. Disconnect the feed and module wire terminal connectors from the distributor cap.
  2.  
  3. Remove the ignition wire set retainer.
  4.  
  5. Remove the 4 coil cover-to-distributor cap screws.
  6.  
  7. Using a blunt drift, press the coil wire spade terminals up out of distributor cap.
  8.  
  9. Lift the coil up out of the distributor cap.
  10.  
  11. Remove and clean the coil spring, rubber seal washer and coil cavity of the distributor cap.
  12.  
  13. Reverse the above procedures to install.
  14.  

 
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