Century Estate Wagon L4-151 2.5L (1985)
Shift Modulator: All Technical Service Bulletins
A/T - Harsh Shifts or 2nd Gear Starts
Bulletin 85-7440-1
Date May '84
Ref. No. 84-223-7440
SUBJECT:
VACUUM RELATED SHIFT CONDITIONS
MODELS AFFECTED:
ALL 1985 THM 440-T4 TRANSMISSIONS
This bulletin is intended to assist the technician when diagnosing vacuum related shift conditions. For proper operation, the THM 440-T4 transmission
requires 44-57 kPa (13-17 in. Hg) of engine vacuum idle. A loss of engine vacuum can cause any of the following shift conditions:
^
Harsh park to reverse engagement
^
Harsh neutral to drive engagement
^
Harsh or firm light throttle upshifts
^
2nd gear starts
^
Harsh 3-2 coastdown shifts
^
Rough 4-3 & 3-2 manual downshifts
^
Slipping in drive or reverse
Low engine vacuum can be caused by a pinched, cut, plugged or disconnected vacuum line. It can also be the result of a poorly tuned engine or blocked
exhaust. Also an incorrectly installed aspirator tee can give the appearance of low vacuum when the air conditioning is in operation. The aspirator tee has
a "flag" with the words "MOD." and "MAN.". Make certain the modulator line is connected to the MOD. nipple, and the manifold line is connected to
the MAN. nipple as indicated by the arrows on the flag.
To check for proper vacuum, disconnect the vacuum line at the modulator install a vacuum gauge to the line. If there is insufficient vacuum, the cause
and correct as required.
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 device (vacuum pump) to the modulator. Pump the device until 51-68kpa (15-20 in. Hg) of vacuum is
reached; at the same time, observe the modulator plunger, it should be in as the vacuum pump is operated. After reaching 51-68kPa (15-20 in. Hg) the
vacuum should not bleed down for at least 30 seconds.
