Shift Modulator: Testing and Inspection

GENERAL INFORMATION
The vacuum modulator system controls shift feel (harshness) by sensing changes in engine load, which is indicated by engine vacuum. The modulator does this by controlling main line pressure boost. The vacuum modulator system may be suspect for harsh or slipping shift conditions.

VACUUM MODULATOR DIAGNOSIS
For proper operation, the 4T60-E transaxle requires 44 to 57 kPa (13 to 17 inches. Hg) of engine vacuum at hot engine idle, checked at the modulator with the transaxle in "DRIVE." Incorrect vacuum supply to the modulator, or a malfunctioning modulator may cause some or all of the following:
^ Harsh upshifts and downshifts.
^ Harsh "PARK" to "REVERSE" engagement.
^ Harsh "NEUTRAL" to "DRIVE" engagement.
^ Soft upshifts and downshifts.
^ Harsh or firm light throttle upshifts.
^ Slips in "LOW," "DRIVE," and "REVERSE."
^ Harsh 3-2 coast down shifts.
^ Rough 4-3 and 3-2 manual downshifts.
^ Slipping in "DRIVE" or "REVERSE."
^ Pressure regulator valve hydraulic buzz.
^ Engine burning transmission fluid.

An incorrect engine vacuum signal at the 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 modulator and install a vacuum gage to the line. If there is less than 44 to 57 kPa (13 to 17 inches. Hg) of vacuum at hot engine idle with the transaxle in "DRIVE," locate the cause and repair as needed. The gage reading may respond slowly to throttle movement because vacuum is supplied through an orifice to stabilize line pressure when shifting the transaxle between forward and reverse ranges.

If engine vacuum to the modulator is okay, remove the modulator assembly and modulator valve. Inspect the valve for nicks or scoring. Connect a hand operated vacuum pump to the modulator. Pump the device until 51 to 68 kPa (15 to 20 inches. Hg) of vacuum is reached. At the same time, watch the modulator plunger: it should be drawn in as the pump is operated. After reaching 51 to 68 kPa (15 to 20 inches. Hg), the vacuum should not bleed down for at least 30 seconds. If the modulator checks ok, perform the next test.

If the vacuum signal is within specifications, and the modulator is functioning properly and the valve is not damaged, the shift problem is not vacuum related. At this time, an oil pressure check should be performed as outlined in the TRANSMISSION UNIT REPAIR. Note the pressures and, if out of specification, refer to the appropriate diagnosis.

VACUUM DIAPHRAGM CHECK
Inspect
^ Turn modulator so that the vacuum connector faces down. If any liquid (transmission fluid, water condensate, or gasoline) drains out, replace the modulator.

ATMOSPHERIC LEAK CHECK
Inspect
^ Apply soap solution to vacuum connector and crimped seam.
^ Attach a short hose to the vacuum connector.
^ Blow into hose and check for leaks.

NOTICE: Do not use compressed shop air. Pressures in excess of 0.9 kPa (6 psi) may damage the modulator.

LOAD CHECK






^ Tool Required:
- J 36619 Aneroid Modulator Checking Tool

1. Install a known good modulator of the same type on one side of J 36619.
2. Install the modulator to be tested on the other side of J 36619.
3. Holding the assembly level, slowly push the two modulators together.
^ If the gage line remains black, the modulator is not acceptable.
^ If the gage line is white, the modulator is acceptable.

SLEEVE ALIGNMENT CHECK
Inspect
1. Roll modulator on a flat surface. Observe sleeve for concentricity with the modulator can.
2. Plunger for freedom of movement.

CAUSES OF IMPROPER VACUUM AT MODULATOR
1. Engine.
^ Tune up.
^ Loose vacuum fittings or improperly routed hoses/lines.
^ Vacuum operated accessory leak (hoses, vacuum valve, etc.).
^ Engine exhaust system restricted.
2. Vacuum line to modulator.
^ Leaks.
^ Loose fitting.
^ Restricted or incorrect size orifice.
^ Carbon buildup at modulator vacuum fitting.
^ Pinched line.
^ Grease in pipe.