marinesniper0318

Anyone know why my back break light warning is always on, usually after I press the break once when starting the car it goes away. I checked the fuse and it is ok, the lights actually work fine... strange...

Landseer

Sniper, find a post made by AO and check out the link for the owneres manual under his sig.

You will find this is normal. The light always illuminates on the dash when you start the car, until you step once on the brake pedal.

Note also, the power for the rear taillights and for the brake lights doesn't travel directly from CE panel to lights.

It runs through a "bulb comparator" or bulb check unit on its way to the rear light fixtures.

This is for autobahn driving. Rear lights are critical there. Dash will give warnings if a bulb is out.

However, with time, due to corrosion... and with mismatched bulbs during replacement, the sensitive comparitor might trigger a false warning. Just be aware of it.

DonT

Here's the link for AO (Andrew Olson) post with the links for the owners manuals.

Link ~

https://rennlist.com/forums/928-foru...al-online.html

marinesniper0318

i have the owners manual, I even noted that normally it goes away after pressing the break peddle. The light is always on ever since I started playing with the cigarette lighter power (which I did fix), but it seems as soon as I had power issues with the cigarette receptacle the light came on conveniently.

thank I will try removing and cleaning the the sensitivity comparator and re-install the break lights...

Landseer

The bulb comparator is mounted in the passenger side parcel tray, against the right wall of the car. Sometimes water leaks from the windshield on S4 and corrodes it.


If he light persists, then you may have corrosion or different bulb on one side of car vs. the other.

WallyP

Sniper,

It always helps to have info on your car in any request for help or info - lots of difference in a '79 and a '94...

I am guessing that you might have an earlier 928.

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 sets 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)

Mrmerlin

nice post Wally!

IrishLegend

Mine comes on only when my headlights are on. I've put new bulbs in and it still comes on?

marinesniper0318

you are right my apologies, I have a 1988 928 S4....I have attached some pics from when I bought it...I will update with new photos this week, 19" inch rims, 2 wht racing stripe...I do have some interior pics I will attach as well....I gutted the exhaust and went with a 2 into 1 3" pipe to a flow-master muffler...it sounds mean...oh also installing 2 10? polk subwoofers tomorrow...I will post when she is complete... : )

Maint wise: I have done a full tune-up, cleaned grounding areas (alot of them), changed brakes. Timing belt was done 2k miles ago so I will wait on this a little longer, replaced the A/C compressor. Changed out all possible bad or corroded fuses for new ones. A lot of TLC...

porschedonald

DD

WallyP

The model year totally changes your question about the brake light warning - ignore my last post completely!

What is the EXACT warning that you get?

When do you get it?

What does it take to turn it off?

tlsmith1999

Brian,
The stop/break light warning comes on, or the tail Light warning? If the latter, then you probably have a marker light that is either out or corroded (but still lighting up) or a replacement on one side that is slightly different than the other and the system thinks one is out.