1982 928 Automatic

The brake lights are no longer functioning. I do have the warning indicator in the gauge cluster on. I have checked the fuse and it is working properly. Power is present on both sides of the fuse. I have also checked the bulbs and they are working as well. I do not have power at the bulb sockets.

Help is greatly appreciated.

David

 

Grounds????????

I think there is a ground point on the passenger side rear hatch just behind the trim panel.

That would be my guess.

 

Brake light switches at the m/cyl? Plugs connected to them? One of them (front?) is needed to get the lights on, the other is a cross check that both circuits have pressure. The switches do fail and bring up "Brk Press" warning, which can only be reset by battery disconnection. Replace as a pair - cheap vw/audi part nos from the big 3. IIRC, 24mm wrench needed, and considerable dexterity to get the plugs back on to the connectors. I did mine without needing to do any serious bleeding - I just let gravity get the bubbles out of the m/cyl overnight.
jp 83 Euro S AT 51k

 

Be careful not to over tighten when installing the pressure switches. The cast iron of the master cylinder is rather brittle. If you hear a "plink" and feel like you've stripped the threads, you've just bought a new master cylinder. Don't ask....

James

 

If no power at sockets, find the bulb control unit ( I think its mounted to the right underside of the passenger tray). Power comes from the stoplight fuse to one terminal within this box, and leaves via two terminals (one for each stoplight). Essentially, the power is split within the little box.

Power enters the box on terminal 1 and leaves on terminals 12 and 9 (labeled inside the connector mating surfaces). I think 9 is right side of car and 12 is left. You can run wire continuity checks from there to the connectors at the back of the car, I think they are black/red wires at the stoplights.

Power should be found with pedal pressed on terminal 1. Power should then also be found on both 12 and 9. If not, problem is in this BCU box. 83/84 electronic version is backward compatible with earlier cars which had a reed switch version by design according to an entry in the Moorehouse CD's.

You can also check the power output on these two terminals, 9 and 12, with depress of pedal.

Hope you don't have to dig here, but if you do, it can be done fairly easily.

 

Check Pin F7 on the Central Electric Panel - it is the fused brake supply after the switches... if it not switched with the pedal - its a problem in the switch circuit.

Alan

 

Thanks for all the advice. I'll try the simple things first and work my way toward the master cylinder this weekend.

I'll report my findings and the solution.

David

 

I'd recheck for power at sockets first.

Alan's test of probing the back of plug F, terminal 7 is the crucial next step.

That will tell you whether to move upstream or downstream.

If there is power there with brake pedal applied, the problem is downstream.

 

Worked on the 82 928 Auto yesterday. I checked the grounds at the sockets and all are good. Not sure where plug F, terminal 7 is, but I found the black w/ red wire at the Central Electrical Panel. I removed the red, plastic 8 wire socket and then removed the black/red wire with its femaile receiver. I applied power to the black/red wire and both stop lamps lit -- that means my wiring and grounds are good from the Central Electrical Panel. I verified that I have power on both sides of the fuse. The Stop Lamp warning light remains on. This would make me believe that the problem is in the switch.

All ideas and troubleshooting procedures are greatly appreciated.

Thanks,

David

 

That's great that you can light the tails by applying power.

Couple things.

The mode of failure that most people here have reported is at the switches.

The color of a wire at one end does not necessarily mean it will be same at CE panel, so where it might have worked this time, be careful in general.

You might need to know the CE panel layout. The plugs should be marked with magic marker with a letter. A is far left, B next to it, C next and so on. (true for the most part, although the Z plug is somewhere inserted in the middle of the alphabet). The CE panel has these designations molded into the plastic on the edge below the plugs. Its easiest to remove the wood upper and lower to see this.

The individual terminals corresponding to positions on each plug are designated 1 through 8 (again, get up close, its molded into the male receptacle for any given plug, mounted in the panel). F plug (6th from left) and has terminals are in format as follows, looking at the back of the plug:

4 8
3 7
2 6
1 5

If somebody presses the brake pedal with key on, you should read power on F7. If not, problem is upstream (switch).

All that said, sounds like you may have isolated the issue. Way to go!

 

Winkle the plugs off the m/cyl one at a time. Check for +12V at one of the pins (ign on ). Short 2 together, should get stop lights. IIRC its the font switch that turns the lights on, the rear is to verify pressure in second circuit. As I have said, considerable dexterity is needed to get the plugs back on to the pins IMHO.
jp 83 Euro S AT 51k BTDT

 

Is it only the brake lights? Check the ignition switch.

 

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, Porsche or OEM available at $28.31 or $16.52 each plus shipping) Replacement of the switches usually requires bleeding the brake system, so this is a good time to flush the brake and clutch systems.

 

Finally got back to the 82 Auto over the Turkey Day weekend. I disconnected the plugs for each of the brake pressure switches. With the ignition on (or any other time for that matter), I did not have any power at any of the three female receivers in the plug. This was the case for both brake pressure switch plugs.

Where do I trace from here?

To retrace my steps -- I have power across the stop light fuse. I have good wiring to the stop lights. No power at the brake pressure switches at the master cylinder.

All help is greatly appreciated.

David

 

Unusual to have no power anywhere in the plugs. FWIW, when my brk press light came on, I found one of the leads wires were VERY badly twisted up inside the rubber boot, and either shorted or inducing some current. My favourite method for tracing wires is to get a tracing tool used for networking cables (eg Fox'nHounds). connect the tone generator to one end of the wire in question (eg source at the electrical panel), and try to follow the wire , checking for tone/loss of tone from the far end. This is how I proved my headlight wires were continuous when they failed to rise - relay of course.
jp 83 Euro S AT 51k