A/T - Harsh Shifts or 2nd Gear Starts
Bulletin 85-7440-1Date 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.
OIL PRESSURE CHART:
If the modulator is found to be functioning properly and the valve is not damaged, the shift problem is not vacuum related. At this time attach an oil pressure gauge to the pressure tap as illustrated in figure 1. Note the pressures and refer to diagnosis section in the Service Manual.
As a reminder the T.V. cable does not control line pressure; it is used to control shift points (shift speed) only.
An updated oil pressure chart (figure 1) is attached which replaces the oil pressure chart in the Service Manual.
PRELIMINARY CHECK PROCEDURE
CHECK TRANSMISSION OIL LEVEL CHECK AND ADJUST T.V. CABLE
CHECK OUTSIDE MANUAL LINKAGE AND CORRECT CHECK ENGINE TUNE INSTALL OIL PRESSURE GAUGE CONNECT TACHOMETER TO ENGINE
CHECK OIL PRESSURES IN THE FOLLOWING MANNER:
Minimum Line Pressure Check With the brakes applied, take the line pressure readings in the ranges and at the engine r.p.m.'s indicated in the chart below.
Full Line Pressure Check Full line pressure readings are obtained by removing the vacuum line from modulator, and with the brakes applied, take the line pressure readings in the ranges and at the engine r.p.m.'s indicated in the chart below.
Line pressure is basically controlled by pump output and the pressure regulator valve. In addition, line pressure is boosted in Reverse, and Lo by the reverse boost valve.
Also, in the Neutral, Drive and Reverse positions of the selector lever, the line pressure should increase with throttle opening because of the vacuum modulator.