TCVAS

Revision as of 17:19, 28 December 2007; view current revision
<-Older revision | Newer revision->

'Transmission Controlled Vacuum Advance System (TCVAS) is an emission control system used starting with 1972 USA models. This system provides the vacuum advance only when the gear is in the top (4th) position, and retarded spark timing (with no vacuum advance) at other positions for complete combustion. For satisfactory performance during engine warm-up, TCVAS is disabled until the temperature warms up.

Thermo Switch

The thermo switch is located inside the passenger compartment:
Location: Under dash, right side of car

The thermostatic switch:

• Conducts between 41F and 55 F as temperature rises
• Conducts above 34F when temperature falls

Tip: To prevent advance cut-out, leave heater off
and roll down the windows.

Schematic

Both vacuum and electricity are used.

In cold weather, vacuum advance is normal
Until car warms up...

56-series 4-speed (LHD)

• Reverse lights switch
• Top switch (emission controlled models)

When current flows to the Vacuum Cutting Solenoid, it cuts the vacuum advance.

If the Solenoid is disconnected, 
vacuum advance works normally.

The Top Switch is normally open (allowing current). Only in top gear (4th) does it become electrically closed.

If you neglect to connect the top switch,
 vacuum advance works normally.

So this is nearly a foolproof system. If any electrical component goes bad or is not connected, normal operation occurs. Only when every electric circuit is working does it cut vacuum advance.