Vacuum Modulator Diagnosis

The Hydra-Matic 4T60-E transaxle requires 44-57 kPa (13-17 inch Hg) of engine vacuum at hot engine idle checked at the modulator with the transaxle in Drive for proper operation. An incorrect vacuum supply to the modulator or a malfunctioning modulator may cause some or all of the listed conditions:
- May cause harsh upshifts.
- May cause harsh downshifts.
- May cause harsh Park to Reverse engagement.
- May cause harsh Neutral to Drive engagement.
- May cause soft upshifts.
- May cause soft downshifts.
- May cause harsh light throttle upshifts.
- May cause firm light throttle upshifts.
- May cause second gear shifts.
- May cause slips in the Low position.
- May cause slips in the Drive position.
- May cause slips in the Reverse position.
- May cause harsh 3-2 coastdown shifts.
- May cause rough 4-3 manual downshifts.
- May cause tough 3-2 manual downshifts.
- May cause slipping in the Drive position.
- May cause slipping in the Reverse position.
- May cause pressure regulator valve hydraulic buzz.
- May cause engine burning transaxle fluid.

The following may cause an incorrect engine vacuum signal at the modulator:
- A pinched vacuum line.
- A cut vacuum line.
- A plugged vacuum line.
- A disconnected vacuum line.

Also, the engine mechanical and the operating conditions related to the following may result in incorrect engine vacuum or poor transaxle performance:
- The fuel.
- The ignition.
- The exhaust.
- The emission systems.

Disconnect the vacuum supply line at the modulator and install a vacuum gage to the line to check for the proper vacuum. Locate the cause and correct as required if there is less than 44-57 kPa (13-17 inch Hg) of vacuum at hot engine idle with the transaxle in Drive. The gage reading must respond quickly (only 1/10th of a second delay) to the throttle movement because the vacuum is supplied through a 0.031 inch orifice in order to stabilize the line pressure when shifting the transaxle between forward and reverse ranges.

Remove the modulator and remove the modulator valve if there is enough engine vacuum available to the modulator. Inspect the valve for nicks. Inspect the valve for scoring. Connect a hand-operated vacuum pump to the modulator. Pump the device until 51-68 kPa (15-20 inch Hg) of vacuum is reached while observing the modulator plunger. The modulator plunger should be drawn in as the vacuum pump is operated. The vacuum should not bleed down for at least 30 seconds after reaching 51-68 kPa (15-20 inch Hg). Perform the next test if the modulator checks okay.

If the following conditions are met then the shift problem is not vacuum related:
- The vacuum signal is within the specifications.
- The modulator is functioning properly.
- The modulator valve is undamaged.

Perform the oil pressure check if the above conditions are met. Refer to the appropriate diagnosis if the pressures are out of specifications.