GM Firebird 1967-1981 Repair Guide

Speed Controlled Spark (SCS) System


See Figure 1

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Fig. Fig. 1: Speed Controlled Spark (SCS) system wiring diagram

Speed Controlled Spark (SCS) systems perform a similar function to the transmission-controlled systems above. Many of the components are the same and used in a similar way in both of the systems. The major difference lies in the switch which is activated by the speed, rather than the gear.

In its simplest form, this switch is nothing more than a centrifugal switch, connected to the speedometer drive, which completes the SCS circuit at or above a predetermined speed. A more complicated type of switch uses a small speedometer gear (or speedometer cable) driven pulse generator to send a signal to an amplifier, which, in turn, completes the SCS system circuit at predetermined speed.

Once the circuit has been completed, vacuum is allowed to flow to the distributor vacuum unit. When the circuit is not energized, vacuum is ported into the atmosphere.

Like the Transmission Controlled Spark systems, SCS systems use temperature switches and various relays.


This test should be performed with the engine temperature between 95ordm; and 230ordm;F.

  1. Raise the rear wheels off the ground and support the car so that it cannot roll forward.
  3. Disconnect the vacuum hose which runs between the distributor vacuum unit and the vacuum advance solenoid at the distributor or valve end. Connect a vacuum gauge to the hose.
  5. Start the engine and shift into Drive. Accelerate until the speedometer registers 38 mph.
  7. Until the specified speed is reached, the vacuum reading should be zero. Once this speed is reached, vacuum should be present.
  9. If no vacuum is registered at or above the specified speed, check the vacuum lines and connections first. Examine the carburetor port to be sure that it is not clogged.

Remember to check the coolant temperature vacuum override valve, if so equipped, as its failure could cause a loss of vacuum. Test procedures for it are given earlier in this section.

  1. If there is nothing wrong with the vacuum supply, disconnect the gauge, reconnect the hose and proceed with the next test.

Transmission Switch

  1. Leave the rear wheels of the car off the ground as in the previous system test.
  3. Disconnect the transmission switch leads. Connect a low-amperage test lamp in series with the switch and the positive side of the battery.
  5. Accelerate to the specified speed (see chart below) and watch the test lamp. It should remain on until the specified speed is reached. If the lamp fails to go out or if it does not light at all, the switch is defective and must be replaced.
  7. If the switch is working properly, reconnect it and go on with the next test.

Vacuum Advance Solenoid

  1. Disconnect the vacuum advance solenoid leads. Connect a vacuum gauge to the solenoid hose as in the system test.
  3. Place the transmission in Neutral and start the engine. Increase engine speed. The gauge should indicate the presence of vacuum.
  5. Connect the hot lead to a 12 V power source. Ground the other lead. Increase the engine speed again. The solenoid should energize, resulting in a vacuum reading of zero.
  7. Replace the vacuum advance solenoid if it is faulty. If it is not, reconnect the wiring and go on with the next appropriate test.

Engine Temperature Switch

If vacuum advance is present when it should not be, i.e., below the speed specified, the temperature switch is defective if the other SCS System components are functioning properly and the engine temperature is 95-230ordm;F. Replace it and repeat the system test.

If, on the other hand, vacuum is not being supplied when it should be, with the engine temperature below 95ordm;F or above 230ordm;F, and the other components are functioning properly, the fault again lies in the temperature switch. Replace it and repeat the system test with the engine below 95ordm;F (cold). Vacuum advance should be present at all speeds.