A vacuum chamber (or advance) senses engine load and changes the timing to compensate.
The vacuum advance unit is bolted to the side of the distributor housing. On some models, the vacuum hose for the vacuum advance is connected directly to the intake manifold, whereas on other models this hose is connected to a fitting right above the throttle plates.
Typical vacuum advance unit operation. Courtesy of DaimlerChrysler Corporation.
To check the operation of this unit:
- Loosen the distributor holddown bolt or clamp.
- Use a hand-held vacuum pump to draw on the diaphragm until the vacuum specification is reached.
- Raise engine speed to 2,000 rpm or the speed specified in the service manual.
- Then shine the timing light on the vibration damper timing marks and adjust the advance timing light until the correct base timing is registering.
- Observe the reading on the timing meter of the timing light. It tells how many degrees the flash of light has been delayed.
- This meter reading should match the specified advance at that rpm level.
- The amount of flash delay matches the amount of vacuum advance present.
- If the reading does not match up, the vacuum advance mechanism is faulty.