Re: diode on brake fail switch... EF
Hi there. The function of the diode is to provide means to check that the brake fail circuit is operational every time you start the car. It's set up in such a way that it tests the lamp (or LED) and that the connector on the brake M/C is plugged in. The test is performed when the key is turned to "start".
The trick to it is this. 12v pos from the cluster runs through the lamp, the switch, the diode and then to the line that is common to the BCM and the igntion switch. The secret is that this line goes low (ie earthy) when the key is in "start", thus providing a current path to illuminate the lamp - but only while you are in "start".
Now, if you do get a brake fault while you are driving, the switch will operate and provide the earth to operate the lamp. But you will see that under this condiiton the diode is blocking and the earth cannot get onto the BCM line and cause havoc.
If the lamp is faulty or if the connector falls out of the M/C, the lamp will not light. Probably lost on most people, given the usual powers of observation!