An orifice spark advance control (OSAC) valve is used on all models to delay distributor vacuum advance for about 15-27 seconds during acceleration.
The amount of time-delay varies slightly from one engine size to another.
Some models are equipped with "Maximum Cooling" systems and/or air conditioning have a Thermal Ignition Control (TIC) valve to reduce the possibility of engine overheating under heavy load. When coolant temperature at idle reaches 225°F, the TIC valve automatically opens and applies vacuum directly to the distributor, bypassing the OSAC system. Engine idle speed rises and coolant temperature drops. When normal operating temperature is reached, the TIC bypass shuts off. The TIC valve is usually used on Police Package V8s.
The OSAC valve is located on the air cleaner. Warm the engine to normal operating temperature for this test.
- Check the vacuum hoses and connections for leaks or plugging.
- Detach the vacuum line which runs from the distributor to the OSAC valve at the distributor end. Connect a vacuum gauge to this line.
- Connect a tachometer to the engine. Rapidly open the throttle and then stabilize the engine speed at 2,000 rpm in neutral. When the throttle is rapidly opened the vacuum gauge reading should drop to zero. With the engine speed at a steady 2,000 rpm, it should take about 20 seconds for the vacuum level to rise and then stabilize.
The length of time may vary slightly with different engines; 20 seconds is an approximate figure.
- If the vacuum level rises immediately, the valve is defective and must be replaced.