Brake Light Bulb Controversy

Some of us have heard for years about brake light bulbs and their effects on the cruise control system. I suppose many beliefs are firmly grounded somewhere in folk lore because of some caveman's personal experience. Don't forget... the world was originally flat.

Here are the facts:

I'll begin with some moderately technical information and then boil it down to very simple lay conclusions.

In all VDO Cruise Control Systems the filaments of both brake light bulbs are connected to chassis ground at one end and are switched to approximately battery voltage at the other. When the brake lights are off the switched end sees a wire (the filament) to ground; when on, that ground disappears and that end of the filament goes to somewhere in the vicinity of 13.8 Vdc, current flows and the filament glows. The key is that at the "hot" (in electrical terms) end of the bulb the ground is lost when the bulb lights.

The Cruise Control System uses the above condition in the following manner. Amongst the couple hundred electrical connections inside the Cruise Control Amplifier, there is one, that's ONE! connection to the switched end of the brake light bulb. In the systems from 1981 to 1993 this connection provides a ground path the to emitter of an npn switching transistor which controls the digital flip-flop circuit which holds the cruise system on. In earlier systems the configuration is slightly different. In either case it is a totally passive connection. In other words, it is just a wire which provides a current path to chassis ground for that circuit. When the ground is lost the brake lights turn on, the cruise system switches off. It's a simple as that.

Thus, while there may be several ohms difference between brake light bulbs of different brands, there is no way they can affect operation of or damage the Cruise Control Amplifier. Use the brand of brake light bulb of your choice!

Since the bulb provides a critical system ground, it is true that at least one, preferably both, brake light bulbs must be working and seated properly for the Cruise Control System to work. If your brake light bulbs are burned out the cruise control system will not work.

The notion that it matters what brand of brake light bulb you use is a myth which has no merit based in technical reality.