gergstuff

I have a '78 928 5 speed and have not been able to get the brake pressure light and central warning light to go out. I replaced the brake pressure switches but the light would not re-set.

Wally P suggested that the wires to the switches were in the improper order. More basically, the wires I seem to have do not match the colors Wally P says they should be so I am left scratching my head a bit.

I do have a solid black wire and have that to the proper terminal on both switches. I have a black wire with a yellow stripe and a blue wire with a red stripe left. They are both connected to the the same terminals on the respective switches.

I am thinking that perhaps the warning computer may have a fault.

Is there any way to test for this?

Any other ideas?

The flashing is starting to drive me crazy!

Thanks,

Craig

davek9

I thought (per a write-up Wally did) that there is a way to jumper out the switches, wouldn't that eliminate the warning computer?
Also some people give up with that mess and install a switch on the brake peddle like the later cars have.
I have also seen new Bad switches installed.

vanster

Have you re booted? ie: disconnect your battery to re-boot.
When you mention the brake pressure switch you mean the one next to master brake cylinder, the one that is a PIA to get off and replace?

dr bob

There are two switches, as Wally shard previously, that do double duty on the brake warning system. One duty is to illuminate the brake lights when pressure is detected in either brake circuit. The other is to illuminate the warning light when pressure is detected on only one brake circuit. The second function requires tha you actually have pressure in both circuits to keep the warning light out, .or. have pressure in neither circuit. The alert "latches" once triggered, even if circuit pressures are not mismatched.

I'm sure there's a way to build a little test-light switch detector, but the effort to do that is greater than the effort to replace the switches.


The problem may be in the warning'light circuitry itself, independent of what the switches are doing. Try disconnecteing all the connections from the switches. Then disconnect the battery for a while, and reconnect. If the warning is still present, the switches and your connection order are not the cause. Let us know what you find, and we'll continue the diagnostsics.

Troubleshooting is a combination of logical thought process, experience, and luck. In this case, isolating all the switch contacts points you in the direction of the switches (and the connection order) .or. the warning indicator system. So you progressively narrow down your list of suspects. You have a potential wildcard in the car with wire colors that don't match with the WSM diagrams, and some obvious past efforts to fix or literally mask the problem. At least by disconnecting the switches as a first step, you can target them or rule them out.

dr bob

Of course, we must not forget that there is a RESET button for the central warning system, that should be used after the problem condition is cleared. No need to disconnect the battery to clear stored faults.

gergstuff

Ok,

so I pulled the wires off the pressure switches and disconnected the battery.

waited awhile then hooked the switches back up then the battery.

turned on the key, light still flashing...

I even pulled the wires off the switches and the light still flashes.

so I am guessing there is either a problem with the computer or the wiring or perhaps both?

I surmise that replacement computers are either unobtainium or very expensive.

Options?

I guess icould live without the warning light but how might I go about disabling it without jacking something else up?

thanks,

craig

jpitman2

After a battery disconnect, the light should not come back on until you hit the brakes....are you saying it came on with ignition on, but brake pedal not pressed? You can get brake lights working with only one of the switches connected....
jp 83 Euro S AT 56k

gergstuff

Yes, turned the ignition on and did not touch the brakes, brake pressure light and central warning light flashing immediately.

I disconnected all the wires from both brake pressure switches and the lights were still flashing which leads me to believe it is the warning system or wiring or both.

Craig

dr bob

Have you pressed the 'reset' button for central warning system? If yes and the warning lights are staying on, test the wiring between the switches and the central warning box, and consider replacing the central warning box if the wiring is intact.

For those playing along at home, the switches are detailed on sheet IV of the '78 wiring diagrams package, for the connection to the bulb warning system and on to the brake lights.

Sheet VIII has the conections to the Central Warning unit and the dash display. Central Warning system has a connection to the smaller wire between the two switches' 81a terminals. This connects directly to terminal 9 on the central warning system controller, with no intermediate connections. When connected through the switches, connection 9 on the central warning box should have no voltage on it, and you would see the resistance of the brake-light bulbs in parallel if measured to ground. When just one of the pressure switches is activated, presumably by an imbalance in the brake circuits, battery voltage would be applied (via the 15 circuit and fuse 10, +12V when key is in 'run' or 'start' position) to terminal 9 in the Central Warning box.

Bottom line: If there's +12V battery voltage at terminal 9 of the Central Warning box, while the switch wiring is still disconnected, you have a wiring fault that needs to be corrected. If terminal 9 shows no voltage (measured to ground) and you still have the warning, the Central Warning box has failed.

I'm reading wiring diagrams for this. Someone might share the actual location of the Central Warning box in the car.

Sheet IX has the connections to the cruise control (Tempostat) controller, so that using the brake can diesengage the cruise control function. These connections parallel connections to the bulb controller, incidentally.

gergstuff

Hi Dr. Bob,

I have pressed the reset button to no avail.

I found the central warning unit, it is under the dead pedal.

I will dig out my multi function meter and see if I can follow your directions.

Is a new central warning unit even available?

Thanks!

Craig

dr bob

Call 928 International for a good used one. "They never fail", so a good used one with a warranty will do you fine.

If you don't have the workshop manual with wiring diagrams, you can get them from Roger at 928srus on CD, along with a bunch of interesting and valuable tech pubs and such.


Remind me where Ferndale, Wa is on the map....

gergstuff

I have a manual in pdf format, need to print it out as its a pain to look things up on my tablet or computer.

Ferndale is tucked up in the nw corner of WA about 20 miles south of the Canada border off I 5

craig

dr bob

Hmmm-

I use the manuals as PDF almost (-almost-) all the time. Except for wiring diagrams, which get printed, or more likely a copy of the original is made so I can color on them as I trace stuff down. Otherwise there would be a LOT of crayon marks on them. Now the marks are just on the screen.

Ferndale is a bit too far from central Oregon for a casual support mission.

gergstuff

Yes Dr. Bob, a 8 hour one way trip is probably a bit much for a Saturday afternoon...

gergstuff

Well finally got a "new" used central warning unit and installed it and the flashing light has now ceased. Yeah! Problem solved. On to the next one!!