AMC Coupes/Sedans/Wagons 1975-1988 Repair Information

Vacuum Throttle Modulating System (VTM)



See Figure 1

Click image to see an enlarged view

Fig. Fig. 1: Common vacuum throttle modulating system components and their locations

This system is designed to reduce the level of hydrocarbon emission during rapid throttle closure at high speed. It is used on some 49 states models, and all models with a V8 engine.

The system consists of a deceleration valve located at the right front of the intake manifold, and a throttle modulating diaphragm located at the carburetor base. The valve and the diaphragm are connected by a vacuum hose and the valve is connected to direct manifold vacuum. During deceleration, manifold vacuum acts to delay, slightly, the closing of the throttle plate.


Timing, idle speed and air fuel mixture should be correct before beginning this test.

  1. Connect a vacuum gauge to the distributor vacuum advance line.
  3. If the carburetor has a dashpot, tape its plunger down so that it cannot touch the throttle lever at idle.
  5. Speed the engine up to about 2000 rpm and retain this speed for about ten seconds.
  7. Release the throttle, allowing the engine to return to normal idle.
  9. The vacuum reading should rise to about 20 in.Hg and stay there for one second. It should take about three seconds for the reading to return to normal, the valve should be adjusted.

To check for a leaking valve diaphragm:
  1. Remove the vacuum gauge and connect it to the manifold vacuum line with a T-connector.
  3. Clamp shut the valve-to-distributor vacuum line and, with the engine at normal idle speed, check the vacuum reading.
  5. Clamp the line shut between the deceleration valve and the T-connection. Check the vacuum gauge reading again.
  7. If the second reading is higher than the first, the valve diaphragm is leaking and the valve should be replaced.

ADJUSTMENT - Deceleration Valve

If the deceleration valve test indicted a need for adjustment, proceed as follows:

  1. Remove the cover to gain access to the adjusting screw.
  3. If an increase in valve opening time is desired, turn the adjusting screw counterclockwise.
  5. If a decrease in time is desired, turn the adjusting screw clockwise.

Each complete turn of the adjusting screw equals1/2in. Hg.

After finishing the adjustments, retest the valve. If the valve cannot be adjusted, it is defective and must be replaced.

Throttle Modulating Diaphragm

  1. Run the engine to normal operating temperature and set the idle speed to specification. Shut off the engine.
  3. Position the throttle lever against the curb idle adjusting screw.
  5. Measure the clearance between the throttle modulating diaphragm plunger and the throttle lever. A clearance of 1 / 16 in. (1.5875mm) should exist.
  7. Adjust the clearance, if necessary, by loosening the jam nut and turning the diaphragm assembly.