Edgy01

I'm working on a 1980 with few miles on it and while most things seem straight forward I have gotten a brake warning light, which it also backed up with the general alert/warning light. Could it be an issue with the master cylinder? I plan to simply service it--add brake fluid, and flush fluid from each caliper.

Hold On

Check the brake light switches on bottom of master cylinder. They operate in tandem with the brake light/ warning circuit. If one goes bad it sets the indicator on the dash. The tend to leak after time. They are relatively cheap(about $30 each)

Erik N

If you change out those switches do not overtighten them!

pcar928fan

The general warning light (I presume you are talking about the big "!") comes on with ANY of the "faults". Even low washer fluid and the "!" lights up... you can cancel that one though. I think brake stuff (including worn out pad warning) are not cancelable, ie. you can't push the "!" button just below the HVAC controls to get the "!" light in the pod to go out... that said, it sounds like everything in the warning system is performing as it should which is nice. You just need to figure out what the problem is now...probably a bad switch as noted by the other posters.

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.

Edgy01

The 928 Forum--absolutely the place to go for technical help! Many thanks. I would suspect you're on the right track as the car has been rarely driven. (Total miles are about 45,000 right now.)

Edgy01

I replaced both brake pressure switches on the bottom of the master cylinder but still find that I have the master warning light flashing, along with the "Brake Press" warning in the cluster.

Suspect a wire continuity problem or bad controller. Is the warning controller in the area of the fuse box?

Many thanks.

Edgy01

Bump

GlenL

If you had the flashing "!" alarm then need to disconnect the battery for it to reset. A minute will do fine.

soontobered84

I thought that the warning "!" came on in the older cars everytime you turned the key, but would go away as soon as you pressed the brake the first time. Am I right or did I just make that up?

GlenL

The latter.

Landseer

This might help. You can see what GlenL is talking about with the battery disconnect, depending on which brake warning we are talking about. Looks to me like the text reference to "item 3" corresponds with the brake circuit, counting down the left edge of the list. Its one of the "flashing" signals. Once set, it needs to be disabled like GlenL indicated. Looks like its the only warning that behaves in that manner.

You might need to hold shift and roll your mouse wheel to zoom to read the thing.
Source is CD set available from Rog100, from the supplements, 928S Electrical Troubleshooting & Repair.

Phil Silano

I had the same problem just the other day.

Used Wally's functional description to rule out the pressure switches by using jumpers between the plug terminals to mimic all the different conditions.

Finally tracked it down to the warning light computer under the drivers footrest. Disconnected and reconnected the terminal blocks and problem solved. Probably some corrosion.

Edgy01

I began by disconnecting and later reconnecting the 12V battery. No change.

I went on to step two--locating and disconnecting the warning light computer under the footrest (via three 10mm bolts). Disconnected the two long connectors (rotate off) and lubed things in there with an electrical conductive spray. Put them back on. Tested the system. Played with the connectors a few times (exercising them) and eventually got it to work as per spec!

Many thanks to the Rennlist gang for their smarts and guidance. As we all know on these forums, it's the enthusiasts that know these cars far better than the factory certifiable mechanics many times!

Appreciate the help.

Korwen

If you do this, do you have to jump all of the 3 connections on both bottom switches to get the positive read to make sure the switches are bad or not, or do you just have to jump the power to the brake light switch on one of them for your lights to come on?