GM Lumina/Silhouette/Transport APVs 1990-1999 Repair Guide

Vacuum Modulator

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The vacuum modulator system on the Hydra-Matic 4T60-E transaxle controls shift feel (may be soft or firm shifts) by sensing changes in engine load, which is indicated by engine vacuum. The vacuum modulator does this by controlling main line pressure boost. The vacuum modulator system may be suspect for firm or slipping shift conditions.

INSPECTON AND DIAGNOSIS



For proper operation, the transaxle requires 44-57 kPa (13-17 in. Hg) of engine vacuum at hot engine idle checked at the modulator with the transaxle in "D". An incorrect vacuum signal at the modulator can cause shifting problems.

See Figures 1 and 2

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Fig. Fig. 1: The vacuum modulator and O-ring assembly diagram are mounted into the side of the transaxle case



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Fig. Fig. 2: Routing of the vacuum modulator vacuum source

  1. To check for proper vacuum, disconnect the vacuum supply line at the modulator and install a vacuum gauge to the line. Check for the proper vacuum signal, if there is less, locate the cause and correct as required. The gauge reading must respond in less than 1 / 10 of a second delay to throttle movement.
  2.  
  3. Remove the modulator and modulator valve. Connect a hand operated vacuum pump. Pump the device until 51-68 kPa (15-20 in. Hg) of vacuum is reached. At the same time, observe the modulator plunger; it should be drawn in as the vacuum pump is operated. After reaching the 51-68 kPa (15-20 in. Hg) the vacuum should not bleed down for at least 30 seconds.
  4.  

REMOVAL AND INSTALLATION



  1. Slide the conduit back from the vacuum modulator hose.
  2.  
  3. Remove the vacuum line from the vacuum modulator.
  4.  

See Figures 3, 4, 5 and 6

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Fig. Fig. 3: The vacuum modulator determines the shift harshness of the transaxle



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Fig. Fig. 4: Disconnect the vacuum line from the modulator



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Fig. Fig. 5: After unplugging the modulator wiring harness and removing the bolt, pull the vacuum modulator straight out ...



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Fig. Fig. 6: ... and remove the vacuum modulator from the transaxle case. Make sure no debris falls into the opening left by the modulator

  1. Remove the bolt holding the torque converter clutch wiring harness shield and the clamp holding the vacuum modulator.
  2.  
  3. Remove the vacuum modulator and O-ring.
  4.  

To install:
  1. Install the vacuum modulator, using a new O-ring.
  2.  
  3. Install the clamp, torque converter clutch wiring harness and shield. Install the retaining bolt and tighten to 20 ft. lbs. (27 Nm).
  4.  
  5. Connect the vacuum line to the vacuum modulator.
  6.  
  7. Slide the conduit back over the hose and the vacuum modulator.
  8.  

 
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