Ford Full-Size Cars 1968-1988 Repair Guide

Dual Diaphragm Distributors


Dual diaphragm distributors are installed in most 1968 and later models. They appear in many different engine/transmission/equipment combinations.

The dual diaphragm is a two-chambered housing which is mounted on the side of the distributor. The outer side of the housing is a distributor vacuum advance mechanism, connected to the carburetor by a vacuum hose. The purpose of the vacuum advance is to advance ignition timing according to the conditions under which the engine is operating. This device has been used on automobiles for many years not and its chief advantage is economical engine operation. The second side of the dual diaphragm is the side that has been added to help control engine exhaust emissions at idle and during deceleration.

The inner side of the dual diaphragm is connected by a vacuum hose to the intake manifold. When the engine is idling or decelerating, intake manifold vacuum is high and carburetor vacuum is low. Under these conditions, intake manifold vacuum, applied to the inner side of the dual diaphragm, retards ignition timing to promote more complete combustion of the air fuel mixture in the engine combustion chambers.


  1. Connect a timing light to the engine. Check the ignition timing.

Before proceeding with the tests, disconnect any spark control devices, distributor vacuum valves, etc. If these are left connected, inaccurate result may be obtained.

  1. Remove the retard hose from the distributor and plug it. Increase the engine speed. The timing should advance. If it fails to do so, then the vacuum unit is faulty and must be replaced.
  3. Check the timing with the engine at normal idle speed. Unplug the retard hose and connect it to the vacuum unit. The timing should instantly be retarded. If this does not occur, the retard diaphragm has a leak and the vacuum unit must be replaced.


  1. Remove the distributor cap and rotor.
  3. Disconnect the vacuum lines.
  5. Remove the clip that secure the diaphragm arm to the distributor advance plate.
  7. Remove the screws that attach the diaphragm to the distributor (outside of the distributor).
  9. Carefully remove the unit by tilting it downward to disengage the diaphragm arm from the plate.
  11. Installation is the reverse of removal. Consult the instructions that come with the new diaphragm as to the calibration of the unit.