MorganRock

So... my tail/brake lights seem to be out, but the bulbs look fine, all the connectors look fine, and I checked the fuse and it checked out fine. Is there a relay that I don't know about for the brake/tail lights? I checked the 928GT fuse/relay chart and there doesn't appear to be a relay for them... maybe I just overlooked it... Any thoughts?

Also... headlight motor's not running either... anyone know what relay that is?

I have the manuals, but i'd like to hear if u guys have any experience with it before i go dragging them out and sitting poring over wiring diagrams for the rest of the morning...

AO

What year?

MorganRock

86

Alan

Headlight motor has its own fuse, relay is combined into the main headlight relay (one of the double-wides).

Did these both go out at the same time? Did something happen?

Brake Light circuit is very simple - no relay. ignition switch to switch above pedal via bulb monitoring curcuit - to rear brake light bulbs to ground. check the ground connections at the rear check all the plugs on Central Electric Panel are well seated... check fuse, check brake switch.

Alan

SharkSkin

I thought there were two brake light switches, attached to the master cylinder?

MorganRock

well, i think the headlamp motors have been narrowed down to a bad relay... i've gotten them to go down, so they seat themselves if already up, but they wont come up when required to.

with the brake lights... it's not just the brake lights, but the tail lights as well, none of the rearward red lights work. i was wondering if there was a relay because it's weird that they all dont work and i dont get my achtung light for stop lamps. ideas?

MorganRock

tail lights status - connectors at CEP show 12V from switch and into Lamp Testing Unit. Voltages at rear lamps are all 0. Anyone know where the central lamp testing unit is so i can check voltages there?

MarkRobinson

Did you check every fuse on your board? have you tried bridging a/the relay to see if a bypass works?

Alan

Dave

On my GTS its a single switch unit abiove the pedal - was it different earlier - I assumed this would have stayed the same ...probably a bad assumption?

Looked at an 86 - looks the same but it also looks like it has 2 switches on a '79 why did they change that ?

Alan

SharkSkin

You've checked for power to the bulbs -- have you checked them for a good ground? Or try checking for power with the ground on your tester connected directly to the battery? Sounds like it's a ground issue. Do your turn signals & reverse lights work?

Mrmerlin

Hi I had a similar problem with my 88 S4 it wasnt untill I plugged in the central bulb checking computer then all of the tail and brake lights worked, this box is over or very near to the right hand side of the fuse panel/board, and it may be mounted to the engine computer board or just above it, also there is another set of boxes for the lights and this one is under the dead pedal on the drivers side make shure you have all of the boxes plugged in and also the center console warning reset switch.
Dave the switches on the master cylinder are for a failed brake circuit, the tail light brake circuit are turned on with the switch above the brake pedal. Stan

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, $28.23 each plus shipping) Replacement of the switches requires bleeding the brake system, so this is a good time to flush the brake and clutch systems.

Alan

Wally,
Interesting - thanks.

I like the idea of the dual switches for redundancy AND failure monitoring for either. The master cylinder mount was not the most convenient though. Just having 2 redundant switches with no monitoring would be fairly pointless in the long run.

However it seems the newer single switch is a backwards step functionally (both for switch failure and actual brake pressure monitoring) - yet it does not seem to be a major faiulre point... better switches? better protected in the nice ~warm interior?

Its unlike Poirsche to go backwards - must have been some issues...

Alan

AO

I asked what year because I knew the OB's had the pressure switches. Didn't know the 85-86's use a similar setup.

WallyP

The change from the two hydraulic switches to the single pedal switch happened between '83 and '84.