TBF MISCELLANIOUS RAMBLINGS

TBF has been a topic for many years. I think some of Greg's entries came from the old Porschelist. I searched rennlist on my name and came up with a few threads. It seems to come up every year. If you want to read some for a lot more background have at it.

Steve Cattaneo is the most knologeable 928 auto transmission on the list. Here is one of his posts that probably says it all.

I’ve been hearing 928 engines, with automatic transmissions; have a high rate of crankshaft thrust bearing failure.

There is a condition known, in the transmission industry, as torque converter ballooning. The torque converter physically increases its size and exerts an outward pressure. Over revving the engine and excessive torque converter pressure are contributing factors.

The torque converter has two main functions. It acts as a fluid coupling connecting the engine power, through transmission fluid, to the transmissions planetary gear set. It also multiplies the torque of the engine and transmits this torque to the rear driving wheels, propelling the car at a torque multiplication of 2:1 in a Porsche.

The transmission oil pump charges (pressurizes) the torque converter with transmission fluid, when the engine is running. As the TC spins, at engine speed, there are forces in the torque converter exerting pressure on the crankshaft thrust bearing through the torque tube drive shaft.

These pressures are:

1. Rotary fluid flow generated by the rotating fluid. 2. Vortex fluid flow also generated by the rotating fluid in the torque converter.

These forces create a centrifugal force in the torque converter, which push on the torque tube drive shaft in turn pushing the crankshaft forward cutting off lubrication to the thrust bearing.

Excessive torque converter pressures can multiply centrifugal forces. The main causes of excessive pressures are:

1. Restriction in the cooling circuit. 2. A plugged transmission cooler in the radiator. 3. A crimped cooler line. (Torque converter fluid is the feed oil that goes to the radiator to be cooled.) 4. Highline pressure (working pressure). 5. A bad modulator. 6. A stuck pressure regulator valve. 7. A disconnected vacuum line to the modulator.

These can all cause excessive torque converter pressure. The engine rebuilding association blames the torque converter for thrust bearing failure in most cars. The transmission industry claims the failure is due to a flaw in the crankshaft design. Both agree that modifying the thrust bearing or the crankshaft can be beneficial in some engines. If you remove your engine or transmission make absolutely sure that you adjust the pinch clamps on the torque tube drive shaft (according to the workshop manuals). Mercedes Benz issued a tech bulletin on torque converter ballooning. They want you to lay down the torque converter on its primary pump flange (hub) and measure from the work surface to the top of the flywheel mounting ears. If it’s more then 121.5mm, the torque converter has ballooned and needs to be replaced. Everyone knows, and if you don’t know, 98% of the internal components of our transmissions are Mercedes Benz parts. Porsche made the cases for both the 3 and 4 speed. If you’re rebuilding your engine, have the crankshaft or thrust bearing modified for an increase in lubrication. If your one of those who disconnected the vacuum modulators vacuum line, you should reconnect it. If you want a firmer shift install a shift kit or adjust the modulator by one turn only. Steve C The Great White

Here is another theory from Borland

Actually, the earlier models, have a circlip that keeps the pinch collar of the front flex plate from migrating. The later models have a larger diameter drive shaft, so you can't interchange the parts unless you want to use a smaller drive shaft, and that's not recommended. The locktite method takes care of the migration of the front flex plate pinch collar, but the cause of migration is worn torque converter bearings. Replacing these bearings as part of the torque tube rebuild eliminates the migration. For these later cars, the torque converter and torque tube bearings should be all replaced every 75K miles as preventative maintenance. __________________ borland 90'S4