Distributor Advance/Retard Units
See Figures 1 and 2
A vacuum advance and retard unit is attached to the distributor and is controlled by engine vacuum. The advance can be checked with a strobe light by observing the action of the timing mark during the increase in engine speed. The retard side can be checked at idle by removing the retard vacuum line and noting the increase in engine speed of at least 300 rpm.
Some models built for California and high altitude applications and equipped with manual transmissions, have the vacuum advance in operation only when the high gear is engaged. This controlled by an electrical switch connected to the shifting linkage. Automatic transmission 633CSi for California and high altitude, have the vacuum advance inoperative. Late model 530i and 630CSi vehicles are equipped with a vacuum retard unit only. The 528i has both advance and retard.Electric Control Valve (Black Cap)
This control valve stops the retard distributor control over speeds of 3000 rpm.
Remove the outer hose (to distributor) and start engine. At engine rpm lower than 3000 rpm, vacuum should be present in the distributor retard unit hose and not present when the engine speed is increased above 3000 rpm.
Disconnect the wire terminal end at the control valve and have the engine operating at idle. Connect a test lamp to the terminal and check for the presence of current. If current is present, the speed switch is defective.
Increase the engine speed to 300 rpm or above, and the test lamp should light. If the test lamp does not light, the speed switch is defective.Timing Valve
The timing valve is shut off when the engine is cold or in the warm up phase of operation. Turning on the ignition heats the timing valve continuously. To test the timing valve on a cold engine, detach the hose to the distributor vacuum advance unit and check that no vacuum is felt from the valve connection. When the engine reaches operating temperature, disconnect the hose to the throttle housing and check that the engine speed increases about 200 rpm. Any results other than these, replace the timing valve.