rjm65

How can you test the stop light switch on a 79 to make sure it functions properly?

I believe there are two switches on the brake master, one to monitor for brake pressure failure and the other monitors for brake pressure to activate the stop lights (brake lights).

Is the switch that is most forward (in relation to the master) the one that turns on the stoplights or is it the rearward one?

Thanks.

dr bob

Either switch will trigger the brake lights.

The switches work together to generate a 'brake' warning when one circuit has pressure while the other doesn't. Note that each switch is a single-pole double throw configuration (called "Form C" in industrial circles). Because they both need to swap state at the same time under system pressure, most consider it good practice to replace them as a pair. The theory is that the switches are the same ones that ATE supplied for VW applications, and there are other rumors that many of the cheap aftermarket VW switches are crap. So shop carefully.

928ntslow

I have had no issues with the VW switches and have used them in 2 cars. Jump the switches and see if your brake lights work...then you will know it is the switches. Some say to just do replace both...the jury is still out on that as far as I am concerned.

rjm65

Ok, here's what I've found out after digging a little deeper on this. After driving the car last night, noticed the "stoplight" warning light did not go out on the first application of the brake. Drove home and parked it.

Bulbs and fuse check out ok.

Well, I figured out that if I step on the brake pedal with a medium amount of force, the brake lights come on. If I lightly apply pressure to the brake pedal, such as when slowing down to turn into a driveway or parking lot, the brake lights never come on.

If either switch will trigger the rear brake lights, then I'm thinking my "good" switch failed yesterday and the other switch is faulty (requiring more than normal pressure to activate the brake lights)?

This sound logical? Or does the brake pressure light come on when one of the two switches go bad?

Do the lines have to be bleed when replacing the switches? Also, are the VW switches generally available at most autoparts stores? Any particular part number or switch for a certain VW I should ask for?

Jim bailey - 928 International

The brake lights only trigger off one switch so you can move the wires from one switch to the other swap them and the lights may work. Both switchs are used for the brake pressure comparison .

rjm65

The brake lights do come on if I apply enough pressure to the brake pedal.

If I'm making a hard stop, the brake lights will function, if I'm making a gentle stop, the brake lights will not come on.

Does this point to both switches having gone bad?

Randy V

I had the same issue on my previous '79.

It is relatively cheap and easy to replace both switches. If done properly, there is no need to bleed the brake system.

928ntslow

The switches are pressure activated, so just go to a VW/Import supply store, buy a switch or two and be done with it.

SharkSkin

The switches could be getting old, it's also possible that they are partially blocked with debris. I suppose it's possible to test them out of the car, but they are relatively cheap to replace. If you pull one out and it's filthy, flush the system with a power bleeder but in this case do NOT bleed the brakes using the pedal to move fluid through the system. If you do, you could move the pedal into an area where it does not normally travel, and if there is corrosion on the inside bore of the MC then you could tear up the piston.

rjm65

Replaced both switches and stop lights work. Old switches looked clean on the inside but did have some corrosion on the outside.

Every car I've ever owned, all it took was the slightest tap down on the brake pedal and the stop lights come on, on this car you actually have to apply a bit of pressure in order for the stop lights to come on.

Is that normal?

928ntslow

Yes...hence "pressure switches".