A/T - THM440-T4 Slips On Take Off or Acceleration

Bulletin No. T 87-33A
File in Group 7
Number 146
Corp. Ref. No. 707101R
Date July '87
SUBJECT: THM440-T4 TRANSAXLE SLIPS ON TAKE OFF OR ACCELERATION, OR HARSH ENGAGEMENT

MODELS AFFECTED: 1985-1987 DE VILLE AND FLEETWOOD 1986-1987 ELDORADO AND SEVILLE

This bulletin supercedes Serviceman Bulletin T-87-33, which should be destroyed.

Some Cadillacs equipped with a THM440-T4 transaxle may exhibit a condition of transaxle slipping on take off and/or acceleration, or harsh engagement. This may be due to a restriction in the modulator 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 take off and/or acceleration. Operation at low line pressures during take off 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.

For warranty purposes, use Labor Operation T6225 at .2 hours.