Joe Twill

This only applies to early 928’s. I’m about to change my pressure switches at the master cylinder because the STOP light in the Pod doesn’t go out unless I really push hard on the brake pedal. It takes a 'panic stop level' push on the pedal to extinguish the warning light which then also gets the brake lights on. So I don’t get brake lights for normal stops.
Before I get started I would feel better if what I was doing made sense, because it doesn’t seem to add up.
If there was one bad switch, I would guess that the central warning computer would interpret that as no pressure in one brake circuit and LATCH the warning light on. And one bad switch should not stop the brake lights from coming on with normal pedal force. The only way I can see this being a switch problem is if both switches shifted their trip point by the same amount. Doesn’t seem likely, does it ?.
I don’t think it’s a real brake circuit problem because the pedal feel is excellent for stops. Not mushy, no extra travel, and a reasonably light pedal pressure slows the car down nicely.

Can anybody make sense of this ?

Seems that the switches are the same used in the old VW Beetle (113 945 515 G) and can be found for under $5. That just seems too cheap and I don’t want to use inferior parts, not only because I don’t want to have to change them out again anytime soon, but also because it is a critical application. I’m thinking the safe thing to do is go to the VW dealer and pay dealer price, but if anyone knows a way to get a GOOD switch for a fair price, let me know how and what to avoid.

Any pointers would be appreciated. I’m going to try and do this without removing the master cylinder. Are there Orings that go on these switches ? Do they come with the switch ? What surprises are waiting in this job ?

Thanks
JoeT
79 5sp

nilknarf

Hi Joe- I'm about to do the same procedure for my '79- I got the switches from Pelican (only about $13 each as I recall)... I'll let you know what I learn through the process.

Curt
79 5sp too

nilknarf

uh, nevermind... I just noticed that your post was from may of 2007! I don't think "better late than never" really applies here.

But now that I have your attention, how did that project go?

Curt

pcarsrule

Curt,

Were you able to replace your switch from Pelican? and did it resolve the warning light problem?

Thanks in advance,

Don

Mrmerlin

FWIW you need to replace both of the switches on the MC to restore correct operation

jheis

Be gentle when you thread those new switches into the M/C. If you hear a little "tink" and it feels like you've stripped the threads, you've just bought a new master cylinder.

The master cylinder is made of rather brittle cast iron & it is very easy to snap off the threaded boss. Ask me how I know....

James

Maleficio

I've got the same issue on my 82.

fbarnhill

Well I spent about 3 hours today doing this job. I bought new switches as well as the rubber seals between the resovoir and the MC from 928 Specialist. First I removed all the fluid from the MC by removing the right front tire, attaching a line from the bleeder valve to a bottle, then using a pressure bleeder to push the fluid out of the MC. Then I removed the resovoir, and MC in that order. I cleaned everything spotless. Removed the old pressure switches and replaced them with the new ones. At this point I used some electrical contact cleaner to clean the existing connectors. I reinstalled everything in reverse being carefull not to strip anything. I blead the system. Next, I removed the battery cable for a few minutes to reset the computer. As soon as I started the car and hit the breaks, I knew all that work was worth it. A new master cylender is around 300.00 so this was well worth the effort.

Good luck to all who are attempting it next. It is just tedious but not very hard to do.

Landseer

Frank, what symptoms were you seeing? Did you have any leaking or vac loss?

Mrmerlin

FWIW to clean out the brake reservoir I use brake cleaner from the red can of CRC, rinse a few times.

Then spray in some simple green and agitate rinse with hot water, a few times.

Then add a few oz of isopropil alcohol agitate well then take a hair drier and blow warm air into the reservoir, this will leave the reservoir looking like new.

pcarsrule

I removed my brake pressure switch from the bottom of the MC and reset my battery ( > 10 minutes) and the fault light wasn't cleared.

Are there (2) switches underneath the MasterCylinder?

Thanks in advance,

Don

dr bob

Don--

What car do you have?


Early cars with the 2 pressure switches and diagonal braking circuits have, um, 2 pressure switches.

Later cars with front/rear split systems have no pressure switches. Brake lights work from a pedal-mounted motion switch instead.

The early cars have two electrical circuits, each routed through both switches. One is for brake lights, the other lights the dash indicator if one switch is activated when the other is not. Replacing both switches with new matched parts gives you the best chance for success if the dash light is on -and- it was actually caused by a fault in a switch. Of course, the switches are intended to help identify a hydraulic fault in one circuit; Sometimes they do actually work as designed.

fbarnhill

I started getting the flashing red light with the secondary break failure on the dash. Nothing I could do would turn it off. My breaks were perfect. In fact, I had replaced the master cylender about 1.5 years ago. I had heard that replacing the switches didn't work. However, rather than spend another $350.00 for another new master cylender, I decided to buy both switches and give it a try. I cleaned everything within an inch of its life. I thought I had remembered that removing the MC was a bad/hard job, in retrospect, if you take your time, it was relatively easy. One of the best purchases I have ever made was buying a pressure bleader. I think I bought it from one of our sponsers as it is 928 specific but I dont' remember. I think it was about $50.00. Best money I have ever spent. I have changed my fluid to the new blue racing stuff and blead all the old stuff from each wheel. All by myself. I must admit that I workded all day doing mine. However, I have arthitis so bad I can barely hold a wrench so that probably had something to do with it. I am now back in love with my 79. This car runs so well that I am going to have a hard time selling it. However, If I don't, I will never have the $$$ to get the 88 running as well..... Buy both switches, new resovoir seals and give it a shot. It is worth the savings if your breaks are ok. I do think it is best to replace them both at the same time. I reset the computer after finishing everything else. I was surprised it worked. I am so glad not to have to look at that damn flashing light anymore.

Landseer

Good advice, I'll remember it.

Here's some CW system information ---

WallyP

Up thru 1983, the brake lights are operated by twin pressure switches on the bottom of the brake master cylinder. From 1984 - up, the brake lights are operated by a simple plunger switch mounted on the brake pedal under the dash.

The earlier brake light switches do double-duty, operating both the brake lights and the brake pressure warning signal. These switches can cause some problems.

The switches are identical - one in each hydraulic circuit. They are single-pole, double-throw switches. This means that you have only one circuit (single pole) thru each switch, and that the switch chooses one of two outputs (double throw).

Each switch has three connections: power, central warning computer, and brake light.

Not Activated
There is a power feed into each switch. When the switch is not activated (no pressure in the brake circuit) the power isn't hooked to anything.

When the switch is not activated, the central warning computer and the brake lights are connected inside the switch. Nothing happens, since there is no power applied on either circuit.

Activated
When the switch is activated by brake hydraulic pressure in one of the circuits, the central warning computer connection is broken, and the brake lights are hooked to the power feed. This removes the central warning computer from the circuit, and puts power on the brake lights.

(This might be easier to visualize if you think of three wires: one for 12 vdc, one to the brake lights and one to the warning computer. The lead that goes to the brake lights is connected to the one to the warning computer until brake pressure pushes it off of that wire and over to the one going to power.)

All three leads go to both switches.

When you apply the brakes, both switches should activate. Both switches disconnect the warning computer and connect the power to the brake lights. The brake lights come on, and no signal is sent to the warning computer, so it is happy.

If only one switch activates (either no pressure on one side of the brakes, or a bad switch), then things change.

On the switch that is not activated, the brake light circuit is still hooked to the warning computer.

On the switch that activates, the brake light circuit is hooked to 12 vdc.

The brake lights come on.

In addition, power flows on the brake light circuit back to the non-activated switch. Since in this switch the brake light circuit and warning computer are hooked together, 12 vdc is applied to the warning computer circuit. This set the Brake Pressure Warning light.

If you have a bad brake light switch, I strongly suggest that you replace both switches.
(P/N 113 945 515 G)