BMW E31 840i, 850i AND E32 MAGIC RESET
We've all heard of the Magic Reset, and it certainly 'cures'
quite a few problems. The 'cure' is actually a general reset of the 'learned'
values that the ECU uses for settings like timing and injector dwell.
E32 Reset: How do you reset? First....make sure you know your radio code! There is no need to disconnect terminals from the battery, remove the seat or join wires together, that makes absolutely no difference. You simply remove the trim from under the RHS rear seat (unclips), remove the securing bolt which removes the plastic housing and you will find the negative battery lead where it connects to the chassis.
Remove the nut (don't lose the nut down the gap) and then pull the lead away from the stud, pull it out slightly and rest it on the carpeted face of the seat. There is absolutely no voltage retained in the wiring so there is no point joining wires across things or removing the positive terminal, I can assure you that it makes no difference.
E31 840i and 850i Reset: The 840i
and 850i have two batteries and both need to be disconnected to provide a reset.
Please note that this reset does not work on the 840ci or 850ci (post 1996) which has a
later ECU. Disconnect the negative lead from the left-hand battery, lay the
cable away from the battery. Disconnecting the negative lead from the right-hand
battery will provide the reset.
The time required to reset the processor is quite short, 10 seconds or so will reset the values retained in the ECU. If you keep the reset short then you don't lose the Odometer reading. I usually leave it 1 minute just to be on the safe side.
What happens after a reset? For a start the car doesn't usually tick-over properly and may stall. It takes a while for the ECU to get the hang of idling and tick-over usually starts at around 100-150RPM lower than it should do, it takes a few miles of driving before it gets the tick-over correct. Don't be surprised if the power-steering gives a kick when you are reaching a junction, this is because the idle drops below the effective power-steering assist RPM.
Why does a magic reset work? At the very least it 'forgets' the bad habits the ECU has learned while you were driving the car like a Granny. After the reset give the car some welly! It might learn some good habits.
Generally though, we perform a magic reset because the car is not running correctly. In this case the ECU has been given dud input from one or more sensors. The classic fault is due to either the AFM (or MAF on V8's) or one of the lambda (O2 sensors). The lambda is a 'modifier' for the ECU and gradually the ECU leans out the fuel/air mixture to a point where the knock sensors (on the M60 and M62) tell a couple of cylinders to give up (it will rock like a mad thing then) or (on the straight 6) the mixture is so lean that most of the power disappears and things get very hot.
Where a sensor is at fault it takes only a 100 miles or so before things go awry again. Less miles when stop / starting. The 'cure' is to fix the sensor rather than keep resetting. A box of Doom is invaluable.
I must admit I haven't got around to fixing my M60 yet (hole in the exhaust is affecting one lambda) so I am resetting mine every 100 miles.....and I've done 5000 miles so far! By the way....if you lose your battery terminal nut far from home down the gap (yes, I did), the wing-nut that holds the jack in place fits nicely, in fact, I eventually retrieved the nut and stuck it back on the jack.....I've still got the wing-nut on the battery terminal!
Magic Reset can be done in 1 minute......1 hour makes no difference.
Magic Reset is good for forgetting Granny driving
Magic Reset doesn't mend anything, if there's a fault it WILL come back
Magic Reset IS important after replacing any sensor