BMWs have generally reliable transmissions but do require proper care and feeding. The following should serve as a brief guide.

Automatics

ZF made most of the early transmissions although some mid 70's E3s had Borg Warner units. The three speed ZF 3 HP 22 was replaced by the four speed ZF 4 HP 22 in the mid-80's and in it's EH form had a sport / economy / manual control dial.

The four speed box was a top gear overdrive model, third one-to-one with several first and second gear ratios depending on model and year. The four speed boxes are geared to have no first gear and a low second, this results in poorer 0-60 times but good mid range performance. The last cars to use it were made in '93.

These gearboxes could fail if revved hard in neutral or parked, such as during emissions testing. Pressure would build up and kill the A clutch pack. After mid '87 a pressure relief hole was added in the A clutch to help prevent this. I'd advise an oil change with Dexron II or III ATF every 15-20,000 miles using good quality oil. There is also a filter screen which should be replaced every second oil change, it's under the oil pan. You'll need a new O-ring, oil pan rubber gasket and Torx socket. Clean the two magnets in the oil pan while you're at it. Caution - the oil level needs to be checked when the engine is running!

Later GM five speeds can't be refilled at home, you need special equipment. These units have a learning facility to suit the driver's mood.

All auto boxes should shift and kick down smoothly. At the first sign of slipping change the oil and filter screen. Most older units will creep forwards in Drive, live with it. Auto cars have a relay to prevent the engine starter running except in parked or neutral.

Manuals

Most BMWs gearboxes have been made by Gertrag. Older models, such as those used in the 2002, can have a weak synchromesh on second gear so check for easy engagement. A rumble at idle when warm can be due to too thin an oil, many folk have had luck with Redline synthetic transmission fluid to cure this. Hard shifting when cold can be due to too thick an oil.

The shifter shouldn't be too loose. If it is you may need new shifter parts but this need not be expensive and is a good excuse to upgrade to a short shift gear lever. Gear knobs just pull off if you need to replace them.

Some '80s close ratio sports gearboxes had a dog leg shift pattern as shown below. This allows a quicker change from second to third which is useful in motor sport. They are just as reliable as other BMW gearboxes.

SMG

The euro M3 is available with a six speed sequential manual gearbox (SMG) as an option. This eliminates the clutch pedal and uses a touring car push-pull style shift lever. Having done a few laps in one of these cars I can only say good things about the performance.

They were removed from sale in the UK early on as users experienced problems in traffic, many people tried to drive the cars in Automatic mode and this still needed some work it seems.

Clutch

BMW clutches require more force to operate than a lot of cars, most drivers like this including me. A clutch which is really heavy may have one of the "ears" broken off the part which pushes it and needs a labour intensive repair.

A squishy clutch or one which doesn't return to normal height after use probably has hydraulic problems. Slave cylinders are cheap, master cylinders are dearer. Neither requires removal of the transmission to install. Poor clutch operation may also be due to air in the hydraulics or very old fluid which has picked up moisture.

BMW clutches are pretty reliable and parts aren't too expensive for most. A notable exception is the throw-out bearing on some models which can be pretty dear.

Driveshafts & Transaxles

BMW drive shafts are two part with a splined joint in the middle. This middle section is mounted to the body via a bearing. If this fails it can cause vibrations at certain speeds. If a drive shaft is removed it must be put together the same way to preserve balance, paint an alignment mark before removal!

A clunk when pulling away can be due to worn nylon/rubber guibos at either end of the propshaft. Around '82 BMW changed over to nylon guibos which are much longer lasting and don't perish. Driveshaft universal joints are not readily rebuildable although a few companys will do it.

Transaxles tend to be very reliable. The main problem occurs if the rubber gaiters are torn and allow grit into the universal joints. These gaiters need to be checked to avoid future problems.

Differentials

BMW differentials need a fluid change around every 30,000 miles. If filling a limited slip unit be sure to use the correct fluid with friction modifiers. Undo the filler plug first, this will prevent the embarrassment of having a drained diff and no way to fill it if the plug is stuck!

A limited slip differential is desirable as it provides improved traction, if one wheel slips the other will continue to turn and get the car moving on ice, mud or snow. More modern BMWs have ASC+T electronic traction control and don't need LSDs so much although some cars did have both.

Changing the final drive ratio, from say 3.07 to 3.46, is a good way to improve acceleration at the expense of a seldom experienced top speed. BMW don't use that many different styles of differential so it is possible to swap between models in many cases.

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