khalloudy

All,

I have an issue with my S4 automatic transmission not being fully smooth. Sometimes at partial load or no load/low speed roll, it upshift with apparently no vacuum modulation and it is like a jolt. If i keep the load through the accelerator, then when the upshift happens, it is smooth. This is confusing. Does anyone know what could be wrong? Also sometimes it feels like it is shifting harshly, sometimes it is smooth as silk... I think it is schetzophrenic, but hope not. Any ideas would help.

Sincerely,
Khaled

jeff jackson

I assume you've already checked and verified the vacuum line that goes to the vacuum modulator is intact and connected on both ends ??? If it is...Low fluid level will also cause shifting anomalies, so check that next. Beyond that...PM Steve Cattaneo, he is a Mercedes auto trans specialist. Good Luck.

WallyP

Another possibility is that the vacuum line is connected to the wrong point. It should be on pure manifold vacuum - not venturi vacuum, no check valves, solenoids, etc.

Check the vacuum at the modulator with the engine idling - there should be somewhere around 16"Hg. When the throttle is punched abruptly, the vacuum should instantly drop, then climb.

borland

I think what your describing is a clutch that slips at first then grabs hard. This can happen in 2/3 or 3/4 upshifts.

Possible causes:

modulation pressure out of adjustment
bad clutch
sticky shift valve in the valve body

borland
90' S4, Slate Metallic

khalloudy

Guys,

thanks for the pointers, i will have the car in the air in the weekend. i will check the lines and the pressure.

Sincerely,
Khaled