Toyota Celica/Supra 1971-1985 Repair Guide

Engine Modifications System



Toyota uses an assortment of engine modifications to regulate exhaust emissions. Most of these devices fall into the category of engine vacuum controls. There are three principal components used on the engine modifications system, as well as a number of smaller parts. The three major components are: a speed sensor; a computer (speed marker); and a vacuum switching valve.

The vacuum switching valve and computer circuit operates most of the emission control components. Depending upon year and engine usage, the vacuum switching valve and computer may operate the purge control for the evaporative emission control system; the Transmission Controlled Spark (TCS) or Speed Controlled Spark (SCS); the dual-diaphragm distributor, the throttle positioner systems, the EGR system, the catalyst protection system, etc.

The functions of the evaporative emission control system, the throttle positioner, and the dual-diaphragm distributor are described in detail in the preceding sections. However, a word is necessary about the functions of the TCS and SCS systems before discussing operation of the vacuum switching valve/computer circuit.

The major difference between the transmission controlled spark and speed controlled spark systems is the manner in which system operation is determined. Toyota TCS systems use a mechanical switch to determine which gear is selected; SCS systems use a speed sensor built into the speedometer cable.

Below a predetermined speed, or any gear other than Fourth, the vacuum advance unit on the distributor is rendered inoperative or the timing retarded. By changing the distributor advance curve in this manner, it is possible to reduce emissions of oxides of nitrogen (NOx).

Some engines are equipped with a thermo-sensor so the the TCS or SCS system only operates when the coolant temperature is 140-212°deg;F (60-100°deg;).

Aside from determining the preceding conditions, the vacuum switching valve computer circuit operates other devices in the emission control system (EGR, catalytic converter, etc.).

The computer acts as a speed marker; at certain speeds it sends a signal to the vacuum switching valve which acts as a gate, opening and closing the emission control system vacuum circuits.

The vacuum switching valve on some 1971 engines is a simple affair; a single solenoid operates a valve which uncovers certain vacuum ports at the same time others are covered.

The valve used on all 1972-85 and some 1971 engines contains several solenoid and valve assemblies so that different combinations of opened and closed vacuum ports are possible. This allows greater flexibility of operation for the emission control system.


Due to the complexity of the components involved, about the only engine modification system checks which can be made, are the following:

  1. Examine the vacuum lines to ensure that they are not clogged, pinched, or loose.
  3. Check the electrical connections for tightness and corrosion.
  5. Be sure that the vacuum sources for the vacuum switching valve are not plugged.
  7. On models equipped with speed controlled spark, a broken speedometer cable could also render the system inoperative.

Beyond these checks, servicing the engine modifications system is best left to an authorized service facility.

A faulty vacuum switching valve or computer could cause more than one of the emission control systems to fail. Therefore, if several systems are out, these two units (and the speedometer cable) would be the first things to check.