A/T - THM 440-T4 Slips On Takeoff or Acceleration
Number: 87-190Section: 7A
Date: JULY, 1987
Subject: THM 440-T4 TRANSAXLE SLIPS ON TAKEOFF
OR ACCELERATION, OR HARSH ENGAGEMENT
Model and Year: 1985-87 CELEBRITY
TO: ALL CHEVROLET DEALERS
Some Celebrity models 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. A restricted vacuum fitting 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 signal 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 gage "T"-ed into the vacuum hose at the modulator. Note gage reaction at engine start-up and during rapid throttle movement. Gage response should be immediate in both instances if no restriction exists. If gage response is slow, repair by using a .031" (.76 mm) 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.
Labor Operation Number: T6225
Labor Time: .2 Hours