Helping someone on here the other day and on the schematic for the E-brake, there are two ways for this to light the lamp. One is E-brake pulled up, and other is master cylinder imbalance. One line has the diode ( E-brake), MC does not.
Why the difference? Both ground the line to chassis ground.
Could it be to give a different blinking rate to the relay? The diode drops about 0.7 volts across itself.
Tom, curious as heck!

You don't say what year your looking at but the only reason to put a diode in the circuit would be to prevent voltage from back flowing throught the circuit. There must be a path the voltage could back up through and create a problem.

You'd probably need to check the 914/4 current track diagrams to be sure, but my _guess_ is that one of those circuits (like the E-brake) is powered from a switched source, and the other (brake failure at the MC) is unswitched, probably for historical reasons.

Since you only want the parking brake light on with the ignition, it was cheaper for the factory to put a diode in the line than make a new harness powering the MC line from a switched source.

This is a guess. This is only a guess...

Anyone else have an explanation? Dr. 914? George, what do you know ?
Inquiring minds want to know. Well, mine anyway, what's left of it.
Tom

Well what's left of my mind can't quite figure it out.

Both of those switches are just routes to ground after the bulb. Power for the bulb comes from the same flasher that does turn signals so I don't think there is a switched/unswitched power involvement. Both tracks are switched power.

There is another wire attached to the parking brake switch cct. that goes to the seat belt warning module.

My best guess:
The line to the seat belt warning module signals that the parking brake is on and thus the seat belt warning doesn't need to scream. Right? You can sit in the car with the parking brake on and not get a seat belt buzzer until you take the brake off. Maybe the diode is there to so that the brake failure switch doesn't wind up disabling the seat belt warning the way the parking brake switch does. Lord knows you want your seatbelt on if your brakes have failed!

For those of you who want to play along ...
http://www.pelicanparts.com/914/parts/Elec...lectric_73C.jpg

The parking brake switch is #34 Top center of the diagram
The brake failure switch is #32 - just to the right of 34
The diode is #41 just above #32 and the tie to the seatbelt warning module is just to the left of the diode.
Seatbelt warning module is #67 - right edge of the diagram

So you are saying the E-brake light is on at all times, even when the ignition is off?
I don't think so...

But I never owned a 914 with a seatbelt warning, so I can't comment further...

my bad - both are switched. like i said - not much left of my mind

Steve,
At least your eyes are better than mine. I did not see that other wire to the seat belt circuit. I think you got it! It at least has something to do with the seat belt module and isolating the master cylinder circuit from the seat belt module.
Thanks ,
Tom
EDIT" It is a blocking diode used to seperate the two ways of causing the blinking light to come on. Haynes manual says if the E-brake is up, the light and buzzer should not
work. So ,I guess, you can sit in the car with the engine running and seal belt unbelted and not have to listen to the buzzer.