I'd trouble shoot this problem like this:

Remove the distributor cap.

Crank the engine over and stop with the points closed.

Leave the key on.

Measure voltage from battery + to battery - Call this voltage A

Measure voltage from ballast resistor "input" to ground. It should be within 0.2 volts of voltage A. Call this voltage B.

Measure voltage from ballast resistor "output" to ground. It should be between 7 and 9 volts. Call this voltage D.

Measure voltage from coil + to ground. It should be within 0.2 volts of voltage D.

Measure voltage from coil - to ground. It should be 0 volts. Call this voltage E.

Measure voltage from the coil-to-points terminal on
the outside of the distributor to ground. It should be 0 volts. Call this voltage F.

Measure voltage from the coil-to-points terminal on
the inside of the distributor to ground. It should be 0 volts. Call this voltage G.

Measure voltage from the moving points arm coil to ground. It should be 0 volts. Call this voltage H.

Measure voltage from the fixed points to ground. It should be 0 volts. Call this voltage I.

Measure voltage between the distributor case to ground. It should be 0 volts. Call this voltage J.

Now open the points with an insulated tool and place a insulator of some kind between the points. I use a business card.

Measure voltage from battery + to battery - Call this voltage P.

Measure voltage from ballast resistor "input" to ground. It should be the same as voltage P. Call this voltage Q.

Measure voltage from ballast resistor "output" to ground. It should be the same as voltage P. Call this voltage R.

Measure voltage from coil + to ground. It should be the same as voltage P. Call this voltage S.

Measure voltage from coil - to ground. It should be the same as voltage P. Call this voltage T.

Measure voltage from the coil-to-points terminal on
the outside of the distributor to ground. It should be the same as voltage P. Call this voltage U.

Measure voltage from the coil-to-points terminal on
the inside of the distributor to ground. It should be the same as voltage P. Call this voltage V.

Measure voltage from the moving points arm coil to ground. It should be the same as voltage P. Call this voltage W.

Measure voltage from the fixed points to ground. It should be zero volts. Call this voltage X.

Measure voltage between the distributor case to ground. It should be zero volts. Call this voltage Y.


Report any incorrect findings.

Last edited by Bill Hanlon; 05/25/2026 2:40 AM.
'57 GMC 102, Original 347 V8, HydraMatic, 3.08 rear gear, added A/C, disk front brakes, HEI, AFB carb, '98 Honda Black Currant paint. T-boned and totaled 10/12
'52 GMC 152 Stake Bed, Original 228, SM420, added A/C, HEI, disk front brakes, '67 Chev 3.55 rear gear. Gets used as a real truck.