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post #1 of 12 (permalink) Old 05-19-2017, 05:09 AM Thread Starter
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Brake Light On

My brake light (on the dashboard) is on. Checked the handbrake switch, seems OK. Looking at the master cylinder, looks like 2 leads have no connector (see photo). It this normal? There is a relay-looking device off to the side as well, where most of these wires seem to go.

So, what are the inputs that make the dashboard "brake" light go on? I assume handbrake and fluid level, but there may be others?

Pay no attention to that random white wire going through the frame, that is a PO fix, line to the oil pressure sensor which seems to work at the moment anyway.

Its a 1975, BTW.

Thanks!
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post #2 of 12 (permalink) Old 05-19-2017, 05:20 AM
I think there should be a similar plug on the other side of the filler cap for that loose set of orange wires.
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post #3 of 12 (permalink) Old 05-19-2017, 05:22 AM Thread Starter
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I'll look, thanks. I recently received a nice wiring diagram, I'm printing that and going outside...
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post #4 of 12 (permalink) Old 05-19-2017, 05:49 AM
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What year is your car? I am guessing that you have a car from the 70's which has had its dual chamber brake fluid reservoir replaced with a later single chamber type. I am not completely familiar with the float mechanism of the later type reservoir, but it should be very similar to the mechanism of the older style explained below.

The light you are seeing on the dash is a Low-Brake-Fluid warning. The way the warning light is activated is by a 'sinking float' mechanism. Inside the fluid chamber is a hollow plastic float which floats on the brake fluid. Attached to the top of the float is a (brass? bronze?) nail with a large flat head. The nail slides up and down in a sleeve as the float rises and falls: when the brake fluid chamber is full the nail rides high. The orange and black wires you see are the power and ground leads to the warning light circuit. Each terminates in a contact where you see them entering at the top of the brake fluid reservoir. Usually, the flat head of the nail floats above these contacts and the circuit is incomplete. However if the float sinks enough (usually because the brake fluid is low) the head of the nail moves downward until it comes to rest on the contacts, completing the circuit and lighting the light on your dash. You are thus warned of low brake fluid and impending loss of braking ability.

Sometimes the floats develop a leak, fill with fluid, and gradually sink, lighting the warning light even though the fluid level is fine. Sometimes, though, you have a leak in the brake system and you are about to lose your brakes, so check the problem carefully.

Now, the unconnected orange and black wire pair: the older fluid reservoirs had two chambers, (one for the front brakes and one for the rear) each with a float. The second set of wires should go to the second float. The later reservoirs are single chambered, with a single float, making the second set of wires unnecessary. It doesn't hurt anything to have them hanging there as your single float still activates (as it has) if the brake fluid level gets low.

Here's a picture of the earlier reservoir, shamelessly borrowed from Classic Alfa. You can see the hookup for the second set of wires.

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post #5 of 12 (permalink) Old 05-19-2017, 07:01 AM
I believe Lokki is right about this - someone must have replaced your reservoir with a newer version. Either your float is bad or the pressure switch is bad. If you unplug the wires going into the reservoir and the light goes out then it's likely your float.

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post #6 of 12 (permalink) Old 05-19-2017, 07:06 AM Thread Starter
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Thanks guys, yes I have only a single reservoir. I'll disconnect and see what happens.

Where is the pressure switch?
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post #7 of 12 (permalink) Old 05-19-2017, 07:30 AM
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There also is a brake light switch in the cockpit. It is adjustable and located in front of the brake pedal arm.
Cheers, Jon

Oops!! Just noticed this was for the dash lamp. Lower switch probably not the cause

Last edited by not2old4toys; 05-19-2017 at 08:12 AM.
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post #8 of 12 (permalink) Old 05-19-2017, 09:26 AM Thread Starter
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I found the pressure switch, and disconnected that, light is still on.

Does anyone know if the brake pressure switch is normally open, or normally closed? Its a one lead sensor so it either completes a ground or disconnects a ground if there is a failure.

The switch on the reservoir is a pass-thru so disconnecting that should turn it off.

Or the handbrake switch wire is completing ground somewhere upstream of the actual switch due to a pinched wire or something...

Appreciate the help as always.
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post #9 of 12 (permalink) Old 05-19-2017, 10:19 AM
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afaik BP/Switch completes ground.

maybe disconnect the handbrake switch to eliminate that as a problem (you said 'it looks good')

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post #10 of 12 (permalink) Old 05-19-2017, 11:01 AM
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I had the same problem with my 82. Handbrake switch was fine but had a wire shorted to ground between the indicator light and handbrake. Since the wire was routed under the carpet I decided to simply unplug the wires from the light until I decided it was important enough to fix. Of course I taped the wires off to prevent a short.
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post #11 of 12 (permalink) Old 05-19-2017, 11:58 AM
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Sigh.

I am a bit slow. Which red brake light on the dash are you talking about?

There are two:

One, actually in the dash, lights when the parking brake is on.

The second, actually in the console, lights when the fluid is low.

You may have thrown a red herring with your picture of your brake fluid container.

The two are entirely separate circuits.

If the light is for the Parking Brake warning light, your problem with the wiring on the booster has nothing to do with the light being on. The parking brake is mechanical. The little plunger at the base of the parking brake lever is probably just out of adjustment.

The other light for the driving brakes, is the hydraulic system.


Some people, in answering you are assuming (based on your picture of the booster) that we're talking about the driving brakes.

Others, based, on the use of the word "dash" are assuming parking brake light.

So: which light are we talking about?

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post #12 of 12 (permalink) Old 05-19-2017, 02:31 PM Thread Starter
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Sorry for any confusion... I mean the light on the console that says "Brake"... sorting through the wiring diagram this one light links to the handbrake lever switch on one circuit that is normally open as well as the pressure switch and level switch via another circuit that is normally open as well. Two circuits, but the same light (at least on my 1975).

So to be clear its the rectangular light right under the radio that says "brake". If there is another light, I'm not aware of it, which is entirely possible.

I tested the handbrake switch with a multimeter, and it functions well. When depressed it interrupts the circuit to ground. So the likely cause is the wire is frayed somewhere between the dash (console) and the handbrake switch.
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