Case Study - E38
Strange Intermittent Electrical Problems

Date of manufacture	All
Mileage	Any
Model	Any
Engine	Any

Symptoms

This is a problem I have not personally seen but have been involved in the diagnosis and repair. The symptoms are quite varied but all have the same cause. One of the usual complaints is I opened one of the sun-visor mirrors and the radio went off! Now my cruise control won't work and the brake lights are stuck on. Sometimes the opposite happens, the radio has not been working for a while and when the sun-visor mirror is opened everything starts working again. Other symptoms are that electrical units intermittently work, sometimes the full gamut of radio, sat-nav, telephone, seat-adjustment and cruise-control work fine, the next day the telephone gives up. At various times one or more units work and the rest don't. There will be no fuses blown and it can be very difficult to trace the fault using normal techniques.

In some cases (less often) the fault stops the engine running, sometimes the car will start fine, the next time it just refuses to start. On other occasions turning the ignition switch fails to even turn the engine over.

The cause

The cause is the ignition switch. The ignition switch on the E38 does not just contain the usual couple of contacts, it has four separate 4-position switches. The outputs from each separate switch controls different aspects of the electronics in the car.

Just to compound the problem, some outputs from the ignition switch do not draw any current, the voltage is used to signal certain units to go into 'operate' mode rather than in standby. This is the case with the radio, cruise-control, telephone, brake-light switch and sat-nav, the only device that draws any current on this switch output is the sun-visor mirror-lights. This is why opening the mirror (which switches on the mirror lights) often affects the other electrical units on the same output.

As the voltage from the ignition switch is used as signals to the units, a wandering voltage (which is supplied from the faulty contacts) intermittently turns ON the units. At other times it can switch them back OFF again! It is no wonder that this common fault takes so long to diagnose. Carsoft diagnostics comes up with a 'Terminal R' error quite often, this is because the Terminal R relay is on the same switch output as the radio etc. It is quite usual for a garage to swap out the Terminal R relay (which is found in the boot), and that, as you would imagine, makes absolutely no difference at all. The circuit diagram for the usual failure (failure of S2) is shown below:

The fix

Fortunately, the switch part of the ignition switch is separate to the mechanical part so you will not need to get new keys cut etc. The ignition switch-block can be found by removing the lower half of the steering clam-shell. This is held on by a single screw under the shell and screws that hold the two halves of the clam-shell together. Disconnect the negative battery terminal and then disconnect the connector to the ignition switch (it has a latch).

The ignition switch has a shroud which is held in by two Torx screws, once the shroud is removed the switch is held in with two screws that are hidden under red paint. These screws must be turned 90 degrees anti-clockwise and then the switch can be removed. Make sure the new switch is in the same position by comparing the two switches before fitting.