Venture V6-3.4L VIN E (1997)
Vacuum Modulator: Testing and Inspection
General Diagnosis
For proper operation, the Hydra-Matic 4T60-E transaxle requires 44-57 kPa (13-17 in. Hg) of engine vacuum at hot engine idle checked at the
vacuum modulator with the transaxle in "Drive". An incorrect vacuum supply to the vacuum modulator or a malfunctioning vacuum modulator may
cause some or all of the listed conditions:
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Harsh upshifts and downshifts.
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Harsh "Park" to "Reverse" engagement.
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Harsh "Neutral" to "Drive" engagement.
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Soft upshifts and downshifts.
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Harsh or firm light throttle upshifts.
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Second gear starts.
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Slips in "Low", "Drive" and "Reverse".
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Harsh 3-2 coastdown shifts.
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Rough 4-3 and 3-2 manual downshifts.
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Slipping in "Drive" or "Reverse".
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Pressure regulator valve hydraulic buzz.
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Engine burning transaxle fluid.
An incorrect engine vacuum signal at the vacuum modulator can be caused by a pinched, cut, plugged or disconnected vacuum line. Also, engine
mechanical and operating conditions related to the fuel, ignition, exhaust or emission systems may result in incorrect engine vacuum or poor
transmission performance.
To check for proper vacuum, disconnect the vacuum supply line at the vacuum modulator and install a vacuum gauge to the line. If there is less than
44-57 kPa (13-17 in. Hg) of vacuum at hot engine idle with the transaxle in "Drive", locate the cause and correct as required. The gauge reading must
respond quickly (only 1/1Oth of a second delay) to throttle movement because vacuum is supplied through a 0.031 inch orifice in order to stabilize
line pressure when shifting the transaxle between forward and reverse ranges.
If there is enough engine vacuum available to the vacuum modulator, remove the vacuum modulator and vacuum modulator valve. Inspect the valve
for nicks or scoring. Connect a hand-operated vacuum pump to the vacuum modulator. Pump the device until 51-68 kPa (15-20 in. Hg) of vacuum is
reached. At the same time, observe the vacuum modulator plunger. The vacuum modulator plunger should be drawn in as the vacuum pump is
operated. After reaching 51-68 kPa (15-20 in. Hg), the vacuum should not bleed down for at least 30 seconds. If vacuum modulator checks ok,
perform the next test.
If the following conditions are met:
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Vacuum signal within specifications.
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Vacuum modulator functioning properly.
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Vacuum modulator valve undamaged.
Then the shift problem is not vacuum-related. At this time, an oil pressure check should be performed as outlined in TRANSMISSION/TRANSAXLE
DIAGNOSIS. Note the pressures and, if out of specifications, refer to the appropriate diagnosis.
