Automatic transmission *experts said that *contaminated throttle assemblies ultimately may lead to transmission failure. Left unchecked, the problem may cause repeated transmission burn out.
The 3800-series, Buick-built V6 engine and four-speed automatic transmission is a popular drivetrain in service-age cars built by General Motors Corp.
This transmission has a vacuum modulator connected to a vacuum port in the throttle housing. Coke and carbon often restrict this vacuum port. (Throttle contaminants are discussed elsewhere in this section.)
Because the vacuum connections on the Buick throttle housing don't look like common connections, technicians tend to overlook them. Three different vacuum lines are merged into a single mounting block which is bolted onto the throttle housing.
Sluggish vacuum response leads to transmission clutch slippage and ultimately to complete transmission failure. The transmission's vacuum modulator senses engine vacuum at a throttle housing vacuum port on the 3800 Buick V6.
When the driver accelerates, engine vacuum drops. When vacuum drops, the modulator should respond by increasing the transmission's clutch application pressure. Increased application pressure is needed to prevent the clutches from slipping under heavy engine loads.
A restricted throttle housing vacuum port essentially delays the vital vacuum signal to the modulator. This means the modulator can't adjust application pressure promptly. By the time the modulator senses the vacuum drop, the vehicle is already accelerating with inadequate clutch application pressure.
Consequently, the clutches begin slipping and eventually burn out. When the restricted vacuum goes undetected, a freshly overhauled transmission will also fail prematurely, experts said.