So, I am fairly new to the forum here having only recently acquired a 78 928 with a 5 speed.

seemingly it appeared to have all sorts of electrical gremlins with flashing warning lights etc.

I took the advice previously offered here and read through the new members frequently asked questions section and helpful tips area.

lo and behold the issues/gremlins are rapidly falling by the wayside.

a new alternator and battery cured the no charge problem and as an added bonus extinguished an additioal warning light too.

read the operators manual to learn the the low coolant light comes on when the overflow tank is low on juice. Topped it up to the cold fill line...voila! Light went out.

I thought for sure the parking brake warning light on the central warning panel was illuminated due to an electrical fault...nope, it was on because the parking brake lever was not fully in the down position.

now the only trouble indicator flashing is the brake pressure light, need to do some searching for answers on that one yet.

so the moral of the story is follow the advice offered and scour the new members section and read your owners manual!

thanks for all the help!

craig

 

so common causes for the flashing brake pressure light is a leaking brake pressure switch located on the bottom of the brake reservoir. There are two and if one is bad/ seeping the variance in pressure between the two will trigger light, Disconnect battery to reset light and inspect the two brake pressure switches.

 

Went through a similar path to yours when I first got my car!
And replace the switches in pairs. They are a VW/Audi part.
jp
83 Euro S AT 56k

 

Be careful replacing the presure switches (VW switches work) on the underside of the M/C. If you hear a little "dink" and it feels like you've stripped the threads, you've bought a new M/C. Don't ask me how I know...

James

 

Amazingly, besides detecting when one of the switches goes bad, that warning light is sometimes set when pressures in the two diagonal systems aren't balanced when you push on the brake pedal.

If/when you change the switches, note carefully the connections to each old switch so you can get them back correctly on the new switch. As others suggest, buy good switches and in pairs. The switches (and the rest of the system) suffer when old brake fluid is left. Flush and bleed the system --before-- you replace the switches, and you'll avoid fiddling with stuff again trying to get the light to reset. Battery disconnected while you have the switch wiring disconnected of course.

 

I just checked the electrical diagram. It seems that the two switches are in parallel so if one failes, the system still works. That means, that is the system is not functioning properly, both switches are bad. Or am I totally wrong at this?

 

Your car has two independent hydraulic brake circuits.

You have two brake light switches, one in each circuit.

Each switch is a double-throw switch - That is, there is one set of contacts that are made with the switch in the OFF position, and open in the ON position; and another set of contacts that are open in the OFF position, and made in the ON position.

When you step on the brakes:
If the hydraulic pressure is equal in the two systems, both switches move at the same time, breaking the NC (Normally Closed) contacts, and making the NO (Normally Open) contacts, disconnecting the warning system and connecting the brake lights to 12 vdc.
If the hydraulic pressure is not equal in the two circuits, one switch will remain open for an instant, while the other will close. The closed switch will connect the brake lights to 12 vdc AND at the same time, the warning system will be connected to 12 vdc thru the NC contacts in the switch that remained open. This sets the warning system.

A couple of possibilities:
- Hydraulic pressure is not quite equal in the two circuits, perhaps due to a sticky caliper, worn seal in the master cylinder, low brake fluid, etc.

- One of the brake light switches is faulty.

If you replace one brake light switch, I strongly suggest that you replace both. One old switch and one new switch usually causes brake warnings.

 

The 928 was so far ahead of its time!

 

Keep up the good work, Craig!

Post some exterior and interior shots of your car for us.

 

The plot thickens...

I flushed/bled the brakes.

I disconnected the battery and replaced both brake pressure switches with brand new units. Of course made note of where the wires were attached and replaced them in the same positions.

hooked the battery back up turned on the key and the central warning light and brake pressure warning light were still urgently flashing. I gathered that the system should reset with the battery disconnected.

since I had not even touched the brake I would have thought I should at least start out with a clean slate no?

any other ideas on what I should chase down at this point?

admittidly I have not tracked down all ground points and cleaned them...got a few of them done.

I suspect that the light has been flashing for some time as some po had put a piece of tape over the light on the instrment panel.

yes, yes, I know I need to take and post photos, I will work on that tomorrow.

thanks for the help!

craig

 

I also replaced the left rear caliper with the rebuilt unit I got off ebay and now all 4 corners are doing what they should.

 

Make that right rear caliper...

 

Here is a photos taken this morning at the bus stop.

 

Sounds like two wires are switched. Each brake light switch has three electrical terminals: 81, 81a and 82a.

A BLACK wire starts at Terminal F6 on the bottom of the Central Electric Panel, fed from Fuse S10. This BLACK wire runs to Terminal 82a of one switch, and from there to Terminal 82a of the other switch. This wire furnishes power to the brake light switches.

A BLUE/YELLOW wire starts at Terminal 9 of the Central Warning Computer, then runs to Terminal 81a on one switch, and from there to Terminal 81a on the other switch. This wire triggers the Brake Pressure Warning Light.

A RED/BLACK wire starts at Terminal 81 of one switch, then runs to Termianl 81 on the other switch, then runs to Terminal F7 at the bottom of the Central Electric Panel. This wire operates the brake lights.

 

Thanks Wally P I will check it out. One set of wires has a rubber boot with the terminals set inside of it and it will only fit one way on the bottom of the switch, the other wires for the other switch have no such boot and although I replaced them in the exact same positions they came off that pre-supposes some prior person put them in the right order in the first place.