HARSH NEUTRAL TO REVERSE/DRIVE CHECK VACUUM LINE FOR BLOCKAGE

VEHICLE APPLICATIONS: A, C, H, E/K CARS

Vehicles equipped with a THM440-T4 transaxle may exhibit a condition of transaxle slipping on takeoff and/or acceleration, or harsh engagement. This may be due to a restriction in the modular vacuum fitting at the engine throttle body (TBI) possibly caused by carbon or debris. Another possibility is a plugged vacuum tube from the engine to the transaxle modulator. Restricted vacuum can cause slipping even after transaxle unit replacement with a SRTA assembly. If vacuum is completely restricted, a condition of harsh engagement in drive and reverse ranges could result.

The vacuum modulator controls transaxle line pressure in response to engine vacuum. A restriction in the TBI fitting creates a delayed response of the vacuum signal (vacuum accumulates until it is equal to engine vacuum and is momentarily trapped at the modulator during acceleration). Since the modulator boosts transaxle line pressure as the vacuum sig!lal decreases, a delayed response will cause low transaxle line pressure on takeoff and/or acceleration. Operation at low line pressures during takeoff and/or acceleration will cause slipping and contribute to excessive clutch and band wear.

Verify that a restricted fitting is the cause of this condition by using a vacuum gauge "T"-ed into the vacuum hose at the modulator. Note gauge reaction at engine start-up and during rapid throttle movement. Gauge response should be immediate in both instances if no restriction exists. If gauge response is slow, repair by using a .031" (.76run) drill bit to check and/or clean out the TBI modulator fitting. To prevent recurrence, check the metal vacuum tube connecting the fitting to the modulator for debris by removing the short vacuum hoses at both tube ends and blowing through the tube with air.

Use applicable labour operation and time.

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.