System Testing

Connect voltmeter positive lead to distributor battery terminal and negative lead to ground. Crank engine. If voltage is under 7 volts, repair or replace wiring or components back to battery, including ignition switch and all connections.
If voltmeter reading in step 1) was 7 volts or more, connect positive lead of voltmeter to "TACH" terminal of distributor, while leaving negative lead attached to ground. If voltmeter reading is 10 volts or more, proceed to step 4).
If reading in step 2) was under 1 volt, replace ignition coil. If reading was 1-10 volts, replace electronic HEI module in distributor, and check for spark as instructed in step 9). If spark results, system is OK. If no spark results, replace ignition coil in addition to the module.
If reading in step 2) was 10 volts or more, remove distributor cap, but leave wiring harness attached to cap connector. Connect spark tester (ST-125) or modified spark plug so terminal touches center contact of cap. Ground tester ground wire, and crank engine. See Fig 1.

Fig 1: Checking For Spark At Ignition Coil Output Terminal
Courtesy of GENERAL MOTORS COMPANY

Integral coil model is illustrated.
If spark occurs, inspect cap for water, cracks, or other defects. If cap is OK, replace rotor. If no spark occurred in step 4), remove pick-up coil leads (Green and White wires) from module. Again, check voltage at "TACH" terminal of distributor cap.
Attach voltmeter positive lead to "TACH" terminal and negative lead to ground. See Fig 2. Turn ignition switch on. Attach test light to battery positive terminal.

Fig 2: Checking Distributor Components With Voltmeter And Test Light
Courtesy of GENERAL MOTORS COMPANY

Integral coil model is illustrated.
Observe voltmeter reading as you momentarily (no more than 5 seconds) touch other test light lead to module terminal "P" (small terminal) on Jeep models and Chevrolet and GMC Federal S10 pickups. Touch test light to module "G" terminal on other Chevrolet and GMC models.
If voltage did not drop, check module ground. Also check for open in wires for distributor to cap. If OK, replace HEI module.
If voltage dropped in step 5) when test light was connected, or if 1-10 volts was recorded in step 2), check for spark at coil center contact (using spark tester as before) as test light is removed from module's "G" or "P" terminal.
If spark results, replace pick-up coil assembly. If no spark appears, use module tester to test HEI module. If OK, check ignition coil ground. If ground is OK, replace ignition coil. If module is defective, replace it.
If no module tester is available, check ignition coil ground circuit. If OK, replace ignition coil and repeat step 9). If spark results, system is OK. If no spark results, original coil is OK. Replace module.