Checking Optical Distributors

by Bohdan L. Bodnar

If the engine will not start, go to the 4-way connector. Hook up an analog voltmeter to the gray wire with the black tracer and ground. Crank the engine. The voltmeter should pulse between 5.0 volts and 0.0 volts. If the voltage cycles the problem is not in the distributor. This is what you have just checked:

Battery power to the distributor
The distributor ground circuit
5.0 Volt source from SMEC
correct operation of the photo diodes and LEDs

If the voltage did not cycle, is it 0.0 volts or 5.0 volts all the time? If it is 0.0 volts, the following could be at fault:

the 5.0 volt feed from the SMEC could be open or grounded. Check with an ohmmeter.
the solid state circuit in the distributor could be grounding the wire all the time. Verify by unplugging the distributor connector and if the volts go up to 5.0 volts (key on), you have a bad pickup.

If the voltage stayed at 5.0 volts, the following could be at fault:

no battery voltage at the distributor on the orange wire. Check with a voltmeter (key on)
No ground for the distributor on the Black wire with the light blue tracer. Check with an Ohmmeter. (key off)
The solid state circuit in the distributor stays open all the time.
If you have 9.2-9.4 volts on the orange wire and continuity on the blk/lb wire and the voltmeter did or does not cycle 5.0 -0.0 volts you have a bad pickup.
Note: The sync pickup will never prevent an engine from starting. It can be checked in the same manner as the reference pickup, except test the remaining wire instead of the gray/black wire.

The distributor does have fault codes that the SMEC can store: code 11 for reference pickup and code 54 for the sync pickup. In most cases, either fault code would require replacement of the pickup assembly, however, using the "quick check procedure" discussed earlier will confirm this. Do not replace the pickup solely on the basis of a fault code!