Mystical loss of power

Several owners of N43/N53 approached me with the following complaints – the engine periodically as if looses power.
Basic things are tested:

  • throttle opening in 90o: correct;
  • Rail pressure – correct and stable;
  • Lambda: correct (not >1.00);
  • DISA valves checked, no damages detected;
  • error messages regarding the performance of engine and gearbox (included CO catalytic converters): none.

The situation became more interesting when the car owners mentioned that the power loss is detected in the first two gears, in gear No.3 the problem disappears.
Of course, in such a case, special attention should be paid to the gearbox. If the gearbox management unit detects the inadequacy of RPM between “input” and “output” turning speeds, the engine torque will be reduced. Cause – obviously, the friction discs are slipping, or the torque converter slips more intensified. Also, the increased temperature of the gearbox oil can limit the max required torque.

But the situation more interesting is done by the obstacle that in several cases the cars were equipped with a manual gearbox!

One of the most straightforward causes, which often is “missed”: ASC/DTC system. The reason for such “inattention” is simple: no warning symbols light up in KOMBI.

A long time ago, ASC was working very merely: if some wheel slipped (its PRM was higher than for other wheels), the ASC module reduced the torque of the engine. At the same time, in IKE/KOM a yellow warning signal started to blink – it indicated reduced power of the engine and the driver should be careful.

ASC/DTC system works much more advanced. It measures the speed and direction of the wheel turning, also receives the information from accelerometers (using DSC sensor), also knows the exact torque of the engine, and can break each wheel separately. Besides, DTC analyzes the position of the steering wheel and all changes, made by the driver (brakes, changes of required torque of the engine, etc.).
ASC/DTC system is, most probably, one of the most underrated security systems. Its performance is so self-explanatory and even invisible…

Shortly about the invisible work of ASC/DTC. Wouldn’t it be great to predict the slipping of the wheels before it has started? It would! If the system tries to avoid the slipping of the wheels after it has already begun, it’s battle with consequences. If the driver is hugely inept, he can behave very aggressively on a slippery road, and the slipping of the wheels can happen very rapidly and “efficient”. How to guard against such drivers? How to guarantee max safety?
One of the invisible works of ASC/DTC system – it continuously measures the “threshold” of wheel torque slipping and does not allow overshooting of this threshold. In such a way, ASC/DTC already in time limits the torque of wheels before the car has slipped. Of course, this torque threshold is adaptive – it is changed depending on real driving conditions. If ASC/DTC sees that the car behaves very stable, it increases this threshold of torque. Instead, if the ASC/DTC sees that when supplying torque, which is maxed close to the allowed, the wheels start to slip, this max torque threshold is lowered till a safe level.
While ASC/DTC successfully handles management of this torque threshold, no warning lights will appear in KOMBI. Only, if ASC/DTC will detect rapid, unexpected, and strong slipping of wheels, the driver will get an additional warning.
So – if your car regularly (even just a little bit) slips, max torque, which is supplied to the wheels, will be limited. Besides, there will be no warning regarding this limitation!

Why is this loss of power observed precisely in the first two (especially the first) gear?
Check this simple table

Gears of gearbox and differential are rounded to make it simple. This time the main is a principle, not the exact numbers. The most important: gearbox and differential work as RPM/torque converters:

  • RPM is reduced for X times;
  • torque is increased for X times.

X – gear
Note: To make it simple, no losses in the gearbox and differential are included in the calculation.

In the table, the same conditions (RPM of the engine and required torque) in all gears are indicated.
You can see from the table: in first gears, the wheels are turning slower, torque supplied to them – the largest. It is also logical – these gears are intended directly for higher torque. With higher torque, the car can accelerate faster. Switching to the next (higher) gear, torque supplied to the wheels drops – it is inevitable. Power, developed by the engine, is limited. Either high torque to the wheel or high speeds – you can not get everything.
Here, if ASC/DTC sees that, for example, by 2500 Nm torque (which is supplied to the wheels), the car starts to lose stability, the torque, provided to the wheels, will be held below this critical threshold. Accordingly – the most significant torque limitation will be in 1-st gear; the smaller imperfection of the torque will be in the 2-nd gear. Starting with gear No.3 – full torque will be allowed.
Of course, the real performance of ASC/DTC is much more complicated! An example mentioned above is very simplified – just enough to understand the principle and the cause of the problem mentioned at the beginning of this entry.

What to do in the situation, when at the intersection you have to show, what is BMW capable of? Press the DTC button and hold it for 5+ seconds, till the appropriate symbol appears in KOMBI:

This symbol means that not only DTC but also ASC is disconnected. Of course, you should be careful when driving with turned-off anti-slipping and stability systems – the car can slip!