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BMW E46 Three Series - Mechanics

BMW E46 Compact

Cutaway drawing of a 325ti Compact.


BMW m40 engine


Four cylinder 8 valve

1.6, 1.8 and 1.9

1988 - 2001

BMW m52 engine

M52TU M54

Six cylinder 24 valve

2.0, 2.2, 2.5, 2.8 and 3.0

1989 - 2005

BMW n42 engine


Four cylinder 16 valve, first Valvetronic engine

1.8 and 2.0

2001 - 2004

BMW m57 diesel engine

M47 M57 M67

Four, six and eight cylinder turbo diesels

1999 - 2006

BMW s50 engine


Six cylinder 24 valve

1992 - 2007


The first petrol E46's used the engines from the final E36s, namely the M52TU six and M43TU four cylinder. The 318i and 320i designations remained unchanged but when the N42 Valvetronic engine arrived in 2000 capacities rose from 1.9 to 2.0 and 2.0 to 2.2 respectively. Confusingly the 323i was always a 2.5 and it's M54 replacement was now called the 325i. With a bit more logic the M52TU 328i became the three litre 330i.

In terms of technology all engines went from cable to electronic throttles. The four cylinder update gave BMW it's first Valvetronic engine which had no normal throttle plate (except for initial starting and emergency use) and used variable valve lift to control air intake. The later E90 three series also had this on it's six cylinder N52 engines.

From launch the E46 used the new M47 four and M57 six cylinder diesels. These high performance units set new standards and led to the 320d being one of the biggest selling E46s. In 2001 the M47TU grew in size by 50cc and delivered 12bhp and almost 20% more torque. A year later the M57TU gave the 330d an extra 20bhp and 10% more torque.


BMW E46 suspension

Front lower control arm and bushing, aluminium for the E46.



The E36 followed the same basic pattern as the E36 mechanicals with Macpherson struts at the front and the Z-axle multi-link suspension in the rear. However early cars had overly soft and even vague handling made worse by overly assisted power steering. I had this on a 2001 325Ci and disliked it so much I sold the car. BMW made various changes in the early years but by roughly late 2001 things had settled into a well handling car.

Care has to be taken with parts, for example early cars have 60mm lower control arm bushings and later examples 66mm. On some models such as the 325ti Sport even the geometry is different due to custom control arms.

BMW replaced the flimsy plastic underbody shielding on the E36 (which often fell off) with radically different metal under trays that actually increased the stiffness of the front end.

The rear anti-roll bar on the M3 won't fit other models. It's a different shape to fit next to the larger subframe carrier for the //M differential. Bit of a shame as this was a good modification on the E36.


BMW E46 suspension

E46 suspension and steering are evolved from the E36.


Early cars had a different power steering rack from later models and its hydraulic assistance was hugely overdone. It almost feels like driving a Japanese city car or a Mercedes E class and makes the car seem very disconnected from the road. It was finally fixed in late 2001 with a system that feels every bit as good as the older cars. A fair number of US cars were retrofitted with the good rack under a dealer complaints programme but owners had to request it. If you're buying an early car it's vital to test drive it and see what you think of the steering, the early stuff really is that bad.

The M3 and 330 ClubSport had faster ratio steering racks and the M3 CSL took it one stage further with it's own custom rack.


BMW E46 brakes

M3 rear brake and shock absorber.


BMW went to town on the E46's brakes and fitted some serious anchors to the faster cars. The E36 had been limited by the idea of fitting 15" wheels until surprisingly late in production. Contrast this with the 330i and 330d whose brakes wouldn't fit under anything short of 17" rims. It must be remembered that larger brakes mean more unsprung weight with the resultant effects on ride quality, engineering is always a compromise of competing design requirements.

All cars had ventilated front discs but cheaper E46s made do with solid rears. The 318i used 286x22mm fronts as did all other four cylinder petrol and diesel cars, the 320 and 323i. It had 280x10 non-vented or 276x19 vented on tourings or "increased load" packages (both the same part as the E36).

The 325i and 328i used larger 300x22 vented front discs which need 16" or larger wheels. Rears were 294x19 vented. The 330i had monster 325x25 front discs and highly unusually a cross-drilled disc is available from BMW. Rear fitment was 320x22 on 330's. Because of the huge discs at least 17" wheels must be fitted.

ABS was standard and the handbrake was the usual Porsche style operating on the inside of the rear disc's "top hat". Bear in mind that four wheel drive cars will have different front discs to other models. The 325Xi was sold with both 300 and 325mm discs so take care when ordering parts.

M3 brakes are described on the M3 page.



Cutaway drawing of an SMG gearbox and sensors on an M3.


As you'd expect the E46 drivetrain was little different to the E36 template. A notable exception was the introduction of six speed gearboxes from March 2003 for the 330i, 330d and 325ti Sport. The lower five ratios are almost identical but sixth is now a fuel saving 0.85:1 overdrive. 325ti Sports had an easier final drive ratio than regular 325ti's which is why they got it and other 325's didn't.

The M43 powered four cylinder cars had four speed automatics but all other models including the later N42 318i's and 316i's were five speed. Steptronic was available as an option and added paddle shifters to the steering wheel. This is a bit of a con, it's just an electronic interface to the normal gearbox and shifts at the the same (slow) speed as a normal automatic.

Six speed SMG-II was available on the M3 and SSG on the other cars. SSG (Sports Sequential Gearbox) used a totally different gearbox and didn't receive such a good press, I've heard it's less reliable than SMG but have no evidence to that effect. SSG was five speed and was made by ZF with an Italian Alfaspeed controller (SMG was Siemens / ZF). SSG cost 1000 extra when new.

Very few E46s had limited slip differentials, BMW by now was relying on less expensive electronic traction control instead (see below). Sadly E36s LSD's won't bolt into an E46 as the E46 subframe is a good deal larger. The M3's unique differential is described on the M3 page.

BMW E46 four wheel drive

Four wheel drive iX drivetrain. Front drive shaft is above gearbox in picture.

Four wheel drive

BMW made four wheel drive E46s as the 325xi, 330xi with the M54 engine and a diesel 330xd M57. Their four wheel drive system is described on the xDrive page. Sadly there were no right hand drive xi's but rear wheel drive E46s are pretty good with the right snow tyres (I drive in snow a lot).


Technology and Systems

The stability control system is a complex beast previously seen on the 1994 E38 seven series and 1995 E39 five. It's based on the pioneering work done in the E31 850i and has evolved new functions along the way. Some of these have hardware components but many are purely software features.

They make use of external sensors to detect the car's behaviour, driver inputs and individual wheel speeds. The ABS valve pack can be used to control each wheel and the computer interfaces via the CAN bus to the electronic throttle and automatic gearbox (if fitted) to modify the power input.

I remember a Top Gear feature on traction and stability control systems from around 2002. Jeremy Clarkson drove a selection of BMWs, Jaguars, Mercedes and other luxury brands and concluded that the BMW system was the only one that really worked. When you see the depth of thought and engineering below it's hardly surprising. The R&D for all this is one reason you pay more for a BMW. You'll never know it's there until you need it.


The E46 was the first three series to have Dynamic Stability Control. From late 1999 this was the same Teves Mk.20 DSC-III used on the E38 and E39. In addition to the ABS sensors DSC adds sensors for steering wheel angle, yaw (rotation) rate, lateral acceleration (cornering G-force), brake fluid pressure (two, front and rear brake circuits). The sensors communicate on the car's CAN bus. These give the computer a picture of what the car's doing and what the driver wants it to do. The computer adjusts the throttle and individual wheel brakes to minimise the risk of any wheel losing traction and of the car skidding.

Clearly DSC can only operate within the laws of physics but I've found it to be pretty good without being intrusive. You can massively help the system by running good quality summer or winter tyres and regularly checking their pressures.

To give an example, a classic driver mistake on rear wheel drive cars is to brake hard in the middle of a tight corner because you realize you're going too fast. This unloads the rear end of the car due to weight transfer and causes the rear to lose traction and the car to skid. DSC counteracts this be sensing the situation and reducing rear braking effort before grip is lost. ABS alone would not have this affect.

DSC defaults to being fully on. If the button is pressed for under 2.5 seconds the yaw rate sensor is ignored at speeds below 42mph (Mk.60 system only), this may improve traction in winter. If the button is pressed for a longer period the system is totally deactivated but ABS still works.

A year later in 9/2000 the Teves Mk.20 was superseded by the Mk.60. This new version added DBC, DBS and ADB capability. See below for details.

The M3 and four wheel drive cars had slightly different systems operation on the same principles.


Teves Mk.20 DSC-III system architecture.


Automatic Differential Brake attempts to mimic the function of a limited slip differential and enhance rear axle traction when one wheel is slipping. In an open differential if one wheel slips no power can be transmitted to the other. To prevent this ADB brakes the slipping wheel until it has traction.


Dynamic Brake Control senses the case where the brake pedal is depressed suddenly and forcefully as if in an emergency. It instantly responds to this by exerting maximum available braking force by means of the car's hydraulic pump in the ABS valve pack. DBC is a software feature designed to ensure minimum stopping distance in an emergency.


Maximum Brake Control increases the braking force to the rear wheels if the front wheels have poor traction and are under ABS control as a result. MBC control can occur when the driver isn't braking hard enough to engage DBC (see above).


Dynamic Brake System combines DBC with MDC (see above).


If instability is detected Corner Braking Control reduces the braking force on the inside wheels whilst the car is cornering. This give different amounts of braking force on either side of the car and reduces the possibility of a skid.

This differs from DSC as CBC is used when the driver is braking, DSC can be applied by the car whether the driver is braking or not.


Electronic Brake Proportioning (which I'm guessing comes out as EBV in German) adjusts the brake bias between the front and rear wheels under computer control. All cars have a bias that sends more braking force to the front wheels than the rear. The reason for this is that weight transfer under deceleration means the front wheels have more load on them and can thus sustain higher braking forces without skidding. It's often the case that the front wheels do two thirds of the work and rears a third, that's why front wheels get covered in brake dust more quickly.

EBV can adjust the front / rear bias to cater for a lightly or heavily loaded vehicle and thus optimise the overall braking effort and reducing the likelihood of the ABS kicking in. You often find a manual adjustable hydraulic bias valve in rally cars.


ASC+T All Season Control + Traction was introduced as an option on the E36 but became standard on the E46. On the M52 they had to fit a second throttle plate under the control of the computer to implement the system as the main throttle was a cable system. The electronic throttle of the M54 and the N42's Valvetronic system did away with all that leaving a much neater and more pleasing design. ASC also speaks to the AGS control unit for automatic gearboxes.

If one wheel slips on an open differential the other will get no power. To prevent this the computer used the ABS sensors to detect if wheels were slipping and could apply the brake to ensure power went to the wheel which wasn't (using the ADB function). The computer can also reduce engine power if needed and prevent the automatic transmission from changing into an inappropriate gear.