NO/HARSH SHIFT ALL GEARS LOSS OF VACUUM

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 at idle. A loss of engine vacuum 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 and install a vacuum gage to the line. If there is insufficient vacuum, locate 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 drawn 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.

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 gage 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 GAGE* 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.

NOTICE: total running time not to exceed 2 minutes.

CAUTION

brakes must be applied at all times.

General Motors bulletins are intended for use by professional technicians, not a "do-it-yourselfer". They are written to inform those technicians of conditions that may occur on some vehicles, or to provide information that could assist in the proper service of a vehicle. Properly trained technicians have the equipment, tools, safety instructions and know-how to do a job properly and safely. If a condition is described, do not assume that the bulletin applies to your vehicle, or that your vehicle will have that condition. See a General Motors dealer servicing your brand of General Motors vehicle for information on whether your vehicle may benefit from the information.