Electra V6-181 3.0L (1985)
Shift Modulator: Customer Interest
A/T - Vacuum Modulator & Throttle Valve Shift Conditions
Bulletin 85-7440-4
Date Oct. '84
Ref. No. 85-005-7440
SUBJECT:
VACUUM MODULATOR AND THROTTLE VALVE (TV) RELATED SHIFT CONDITIONS - THM 440-T4 TRANSAXLES
MODELS AFFECTED: 1985 ELECTRAS
This bulletin is intended to clarify the difference between vacuum modulator and Throttle Valve (TV) system related shift conditions on the THM
440-T4 transaxle used in 1985 Electras. The THM 440-T4 transaxle has unique shifting controls, it uses a vacuum modulator for shift feel (harshness)
and a TV system to determine shift timing (speed of shift).
Vacuum Modulator
The vacuum moulator system on the THM 440-T4 transaxle 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 on this transaxle.
For proper operation, the THM 440-T4 transaxle requires 44-57 kPa (13-17 in. Hg) of engine vacuum at hot engine idle checked at the modulator with
the transaxle in DRIVE. An incorrect vacuum supply to the modulator may cause some or all of the listed conditions:
^
Harsh park to reverse engagement
^
Harsh neutral to drive engagement
^
Harsh or firm light throttle upshifts
^
2nd speed starts
^
Harsh 3-2 coastdown shifts
^
Rough 4-3 and 3-2 manual downshifts
^
Slipping in drive or reverse
^
Pressure regulator valve hydraulic buzz
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 and poor transmission performance.
To check for proper vacuum, disconnect the vacuum supply line at the 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 may respond
slowly to throttle movement because vacuum is supplied through an orifice in order to stabilize line pressure when shifting the transaxle between forward
and reverse ranges.
If there is sufficient engine vacuum available to the modulator, 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 51-68 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 51-68 kPa (15-20 in. Hg), the vacuum should
not bleed down for at least 30 seconds. Additional modulator checking procedures are outlined in the 1985 Buick Chassis Service Manual, Section 7A1.
Replace any modulator that fails one or more tests.
If the vacuum signal is within specification and the modulator is found to be 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 440-T4 section. Note the pressures and, if out of specification,
refer to the appropriate diagnosis information in the Chassis Service Manual.
Throttle Valve (TV) System
The TV system used on the THM 440-T4 is unique in its operation. The TV cable system in the THM 440-T4 is used only for controlling shift points
(shift speeds) and detent downshifts. Other TV cable equipped transmissions currently in use by Buick use the TV system to control shift points and
detent downshifts as well as shift feel (harshness).
The TV system on the THM 440-T4 can be suspect for conditions of shift timing (shifts occurring at incorrect vehicle speeds). To determine if the TV
cable is properly adjusted, refer to T.V. Cable Adjustment in Section 7A of the 1985 Buick Chassis Service Manual.
