V8 AUTO in LIMP HOME

[The Quattro King]

I posted this once before:



I have a friend that is working on a V8 auto that has a weird problem that
  is stumping us.

  Symptoms:

  Tranny stays in limp home mode:  Only 3rd and Reverse work.

  Codes:

  Lost power to the TCM.  TCM seems to work though.
  Mulitfunction switch failed.  This was replaced and the computer spit this
  code:  Cruise Control Brake switch sorted, open or going +.  The switch
was
  tested and for resistance and for current and was fine.

  Other symptoms:

  Witht the TCM Disconnected and the key on the Cruise switch on the brake
  pedal had no volts.  With the TCM connected it had 3.5 volts.  Is this
  normal?

  Also, should the brake light switch be sending 12V to the cruise switch
when
  the brake light switch is closed? My thought would be yes only to preserve
  the memory of the cruise so that the resume function works.


  Any ideas how to get this car out of limp home mode????




Got one reply and tried it to no avail:


Check to see if the brake lights are all burned out or if there is a broken
wire to the brakes.  Audi usually uses the brake light circuit as a _ground_
to give one extra fail safe mechanism.  Normally when you push the brake the
CC's ground will go to something like +12v, which will disable it ... but if
there is a problem with the connection to the lights the CC never gets a
ground.

Be advised that the instrument cluster on the V8 is set up so that if it
thinks the tranny is in limp home mode the CC is disabled ...


Also discovered:  

If the battery is disconnected, the limp home mode is reset back to normal,
however, it comes back on as soon as the car is started.




Someone must have more ideas.......



Shayne P.



1972 MB 280 SEL
1984 4ksq
1987 4ks
1991 v8q5
1991 200q Sedan
1992 S4

Parting:  1972 Mercedes Benz 280 SE 4.5