1977 Spark Control System
The Transmission Controlled Spark (TCS) system has been combined with the Spark Control System on 1977 models. On Federal 2F engine equipped Pickup, a shift point switch is installed to detect transmission gear position. Switch is off in all positions except high gear. Vacuum is allowed to advance normally under all conditions except when coolant temperature is above 122°F (50°C) and Bimetallic Vacuum Switching Valve (BVSV) is open, when transmission is in 1st, 2nd, or 3rd gear, or when throttle valve is opened and EGR vacuum is 3 in. Hg.
On 20R engine equipped Pickup, only a Vacuum Transmitting Valve (VTV) is used in the advance circuit. When check valve closes in VTV, advancer port vacuum acts on distributor diaphragm through orifice in VTV. Vacuum is slowed down and vacuum advance is delayed. If advancer port vacuum drops, check valve opens and allows quicker vacuum transmission.
When coolant temperature is below 95°F (35°C) on Corolla, the Thermostatic Vacuum Transmitting Valve (TVTV) opens a vacuum passage. Vacuum from advance port on carburetor acts directly on distributor diaphragm and normal advance takes place. When coolant temperature is above 95°F (35°C), the TVTV closes vacuum passage. Now vacuum is delayed as vacuum can only be drawn through small orifice in TVTV. If vacuum at carburetor advancer port drops, check valve opens and allows faster transmission of vacuum.
On Corona and Celica, wax in Thermostatic Vacuum Switching Valve (TVSV) is contracted and allows spring tension to force valve down when coolant temperature is below 122°F (50°C). Advancer port vacuum can now act directly on distributor diaphragm for normal advance. With coolant temperature above 122°F (50°C), wax expands and forces valve up. At low speeds, advancer port vacuum is low and only low vacuum can act on distributor diaphragm through small orifice in Vacuum Transmitting Valve (VTV). Vacuum advance cannot operate because of outside air bleed into system. Under acceleration, both advancer port and SC port vacuum act on distributor diaphragm through orifice in VTV and vacuum advance can operate under a delayed condition. If vacuum drops, check valve in VTV opens and transmission of vacuum is faster.
On Land Cruiser, when coolant temperature is below 122°F (50°C), the Bimetallic Vacuum Switching Valve (BVSV) is closed. Carburetor EGR port vacuum can act directly on distributor diaphragm and advance works normally, by-passing all other valves. When coolant temperature rises above 122°F (50°C), the BVSV is opened. Through action of various vacuum switching valves, transmission gear detecting valves and switches, and amount of vacuum available at surge tank and EGR port, spark delay valve is activated. When engine is hot, spark delay is in effect when starting to drive or just after heavy load driving, or under acceleration. Normal advance takes place whenever transmission is in high gear position.