‘Limp Home Mode’ BMW V12

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In some jobs you get to see sheer terror – this is one of them.

When a potential customer rings and comes in with their car running very badly, with strange messages appearing on their car computer, you know that they will be twitchy, concerned and in need of a blast of anti-perspirant.

On this occasion, the onboard computer on a BMW 850 states “Trans Axle Fail” (causing all sorts of concerns for the owner about possible broken axels and gear boxes, etc).

There was no doubt that something was badly wrong – a labouring, heavy engine sound and general complete lack of power. The key thing that seemed to relate to the ‘Transmission’ was the fact that the car was stuck in top gear.

Experience has taught me to be a bit sceptical of onboard computer display messages. This is especially the case with BMW who seem to take delight in either wanting to be over dramatic (or very poor translators into English) or wanting to implement safety precaution over rides. Someone there in Munich has been watching too many re-runs of ‘2001 A Space Odyssey’ and fallen in love with Hal.

With this in mind, I was not about to believe that disaster had struck and instead decided that, as ever, it was best to work in a methodical way – tea, milk and two sugars. Perhaps toast in reserve if this one becomes a real challenge.

Working on the theory that it was me against Hal the cunning and misleading computer, I decide it was time to try work out what is really going on.

A labouring engine, with no real power, running on just one bank of cylinders (which was the case with this car) is effectively the minimum amount of performance that the engine management system will allow, thinking that something major is going on – almost as if Hal does not trust the driver with more power. This is otherwise known as ‘Limp Home Mode’.

Limp Home can be triggered by many things so time to start at sharp end of the car by establishing which cylinder bank is not working. The first move was to disconnect the MAS air flow meters (one will basically stop the engine from running – the good bank will stay running) then reconnect and note which is the troubled bank.

Next, switch off the engine and swap over the meters. Reconnect the air flow meters and start her up….. has the problem moved to the other bank?

In this case it had, so the diagnosis is nice and easy on this occasion.

We replaced the MAS air flow with a tested second hand unit (ensuring that we used the correct part number as there are I think three different types, all Bosch units) and started her up again. All was well – power up on all twelve cylinders and the heavy sounding agricultural tone now gone.

The’ tele gods’ must be with me as I struck lucky on the first thing I tried – replacing the MAS Air flow meter and the job is done!

Time to go home and watch a Dallas re-run. Well it is a Sunday after all and the Toast is safely still in its packet waiting for Mondays adventures and challenges.

Chris

P.S.

It may be worth looking at the following points if your German lady is feeling the limp mode.

Rather than assuming that your gear box seizing, it might be:

  1. Mas air flow meters
  2. Distributor caps and rotas – ensure that the carbon rod through the centre is protruding out by about 4 mm’ish, as this must make contact with the rota centre
  3. Spark plugs can bring on limp mode – this is unusual, but not impossible as the tolerance range is quite large for plugs. Engine misfire yes!
  4. Intake gaskets
  5. Damaged vacuum connections
  6. Intake air sensors – sometimes!
  7. Fuel pressure
  8. Fuel pressure regulators
  9. Wiring at the back of the alternator this cable and the heavy battery lead
  10. Throttle bodies, also known as EML
  11. DME relays in Dme box
  12. Coils
  13. Speed sensors
  14. Crank sensor – These can make her run very badly indeed – if at all!
  15. Lambda sensors
  16. Engine temperature sensor and the back of the heads – again this may not force LHM on all engines but will cause high fuel consumption and general feebleness of running sweetly
  17. Gear box sensors on manual and auto
  18. Fuel pump relays
  19. Fuel pumps
  20. EML light on and staying on – could be instrument/cluster wiring from the solder point on the circuit board to the bulb carrier

Can’t think of any more causes at the moment – Need more tea and a bourbon!!

Latest Blog July 12th 2010

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