Toyota Pick-ups/Land Cruiser/4Runner 1970-1988

Dual Diaphragm Distributor



See Figure 1

Carbureted Models Only

Some Toyota half-ton Pick-Ups with an 8R-C engine are equipped with a dual diaphragm distributor unit. This distributor has a retard diaphragm as well as an advance diaphragm.

Retarding the timing helps to reduce exhaust emissions as well as compensating for the lack of engine braking caused by the activation of the throttle positioner.


Check all the vacuum hoses for leaks, kinks, or improper connections before making any test, and install replacements where necessary.

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

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

  1. Remove the retard hose from the the distributor and plug it. As the engine speed increases, the timing should advance. If it does not, the vacuum unit is faulty and should be replaced.
  3. Check the ignition timing with the engine running at idle speed. Connect the retard hose to the vacuum unit; the timing should instantly be retarded 4-10°. If this does not occur, the retard diaphragm has a leak and the vacuum unit must be replaced.

Click image to see an enlarged view

Fig. Fig. 1: Dual diaphragm distributor-without the vacuum switching valve (VSV)