Testing the brake warning light switch

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1971 Coupe. 302 2V - C4 trans
Simple question. How can I remove and test the brake warning light switch from the proportioning valve and verify it is working properly?

If this switch is removed from the proportioning block, and it remains attached to the harness, it seems to me that a simple alligator switch from the copper tip on the switch to any ground on the vehicle should illuminate the BRAKE idiot light on dash. Is that correct? Assume the key is in the "run" position.

I have a new one, and when I perform this test - it doesn't work. What am I doing wrong?

 
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Simple question. How can I remove and test the brake warning light switch from the proportioning valve and verify it is working properly?

If this switch is removed from the proportioning block, and it remains attached to the harness, it seems to me that a simple alligator switch from the copper tip on the switch to any ground on the vehicle should illuminate the BRAKE idiot light on dash. Is that correct? Assume the key is in the "run" position.

I have a new one, and when I perform this test - it doesn't work. What am I doing wrong?
The proportioning valve directs fluid pressure equally to both the front and rear brakes. If you should lose pressure to either the imbalance forces the piston to the right or left depending on which brake pressure is lost. On the pic with 2 images the right side is when the brake system is operating properly. Notice the V groove and the position of the switch plunger which is fully extended so the switch is open and the light is off. The left picture shows that there is a problem with a pressure imbalance and the piston is forced to the left. The switch plunger is forced up and makes contact and the light lights.

To test remove the switch and with key in run depress the plunger rod (Copper tip) and the light should come on. You don't ground the rod.

th04OW3GCC.jpg

brakevalve.jpg

 
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Well, I tried this too - the light doesn't work when the plunger is depressed either.

I - perhaps mistakenly - convinced myself that it couldn't be a "regular switch" (such as the kind you describe), because the first thing that happens to the two wires inside the plug is they are electrically connected. There is only a single copper rod (held by a spring inside) going down thru the middle of the switch that is connected to both wires all the time.

I interpreted that to mean they are essentially connected all the time, and when the bare wire touches metal ground - when the shuttle piston moves left or right and touches the plunger - the light would illuminate.

When I examine the internals of my old switch, there isn't any way for the plunger to make and brake contact by being pushed. The two wires are in contact all the time.

It would seem if the above assessment is right, then a guy could put a wire across the plug from one socket to the other and the light would light up.

We are already down to the core of my confusion about this switch, but unfortunately, I need some more help here I think. I really appreciate the reply though, it's helping me work through this problem. Great diagram BTW, I hadn't seen that one before.

 
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I dug out the electrical schematics to see how the electrical connections are set up and as you described in your first post grounding the plunger should work. So I checked the shop manual this morning and it shows the same diagram above so there is a discrepancy somewhere between the 2 sources.

So if you check the switch is there continuity between the 2 posts for the connector?

Is there continuity between the plunger and each electrical post?

The electrical diagram shows that when the pressure imbalance occurs the proportional valve provides the ground to light the light. I don't see anything that says how this is done though.

 
Your testing procedure is correct. The brake warning light receives its ground from the switch. It also receives a ground when the ignition switch is in the start position. Both the wire from the ignition switch and the warning light are connected together in the brake light warning light switch. If grounding one of the wires doesn't work, grounding the other one should.

Brake warning light switch schematic.JPG

If the warning light doesn't light when the key is in the start position the bulb is burned out or there is a problem in the connections or circuit board. The gauges and alternator and brake warning light all receive their power from the same source.

 
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Your testing procedure is correct. The brake warning light receives its ground from the switch. It also receives a ground when the ignition switch is in the start position. Both the wire from the ignition switch and the warning light are connected together in the brake light warning light switch.

If the warning light doesn't light when the key is in the start position the bulb is burned out or there is a problem in the connections or circuit board. The gauges and alternator and brake warning light all receive their power from the same source.
Good point on the start position Don, I saw that too but I don't remember my light ever coming on. I know my bulb is good. Does your car brake light come on when you start your car? I will have to run over to storage and check out mine now as I am curious.

 
Your testing procedure is correct. The brake warning light receives its ground from the switch. It also receives a ground when the ignition switch is in the start position. Both the wire from the ignition switch and the warning light are connected together in the brake light warning light switch. If grounding one of the wires doesn't work, grounding the other one should.

If the warning light doesn't light when the key is in the start position the bulb is burned out or there is a problem in the connections or circuit board. The gauges and alternator and brake warning light all receive their power from the same source.
Don C has it half-correct (sorry, Don).

There are two modes for the brake warning light to operate. For the bulb to operate, it needs a voltage difference (~ 12V). Don is correct that during normal operations, if the brake master cylinder internal sleeve moves, it shorts out both lines that go to the MC. One of those lines goes to the lamp, where the lamp has the RUN-only line voltage when the car is running. Hence, there will be a voltage difference (12 V from RUN-only and ground from the MC).

The other mode is the proof-out circuit. In this case, power is sent from the ignition switch to the brake MC, and then the return line is compared to the RUN-only line. During CRANK, the RUN-only line is disabled, so if the MC internal sleeve is in its proper position, you would see 12V from the ignition and 0 volts from the RUN-only line. However, if the sleeve is offset, it gets grounded, and the voltage comparison is no volts, so the bulb would not light up.

So...here's how the owner determines if there is a problem.

Case A. Proof-out lamp is lit, normal operations is off: bulb is good, MC is good.

Case B. Proof-out lamp is lit, normal operations is on. This case cannot happen.

Case C. Proof-out lamp is off, normal operations is off: Bulb is bad, MC is indeterminate.

Case D. Proof-out lamp is off, normal operations is on: Bulb is good, MC is out of order.

I rest my case. :thankyouyellow:

 
Midlife,

Using this thread to learn from so bear with me. Assuming MC you reference is Master Cylinder and should be proportional valve instead.

I see 2 ways for the light to come on:

1. When the key is on run there is 12v on one side of the lamp and the switch at the proportional valve provides the ground when brake pressure is lost on either the front or rear brake and the internal shaft is forced over to provide the ground.

2. When the key is in start a ground signal is provided through the ignition switch to the lamp so the light should come on. Looks like a common ground for the cluster and switch illumination as well as the cigarette lighter. A check the light feature.

I see a variety of causes that could cause the light to come on when ignition key is in run.

1. One side of the MC is bad, either front or rear brake.

2. A break in either the front or rear brake line somewhere.

3. The brake fluid reservoir is empty because of a leak on one side. Possibly a seal is bad on the rod and fluid leaking into the power brake booster.

Are my assumptions correct?

 
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Midlife,

Using this thread to learn from so bear with me. Assuming MC you reference is Master Cylinder and should be proportional valve instead. correct.

I see 2 ways for the light to come on:

1. When the key is on run there is 12v on one side of the lamp and the switch at the proportional valve provides the ground when brake pressure is lost on either the front or rear brake and the internal shaft is forced over to provide the ground. correct

2. When the key is in start a ground signal is provided through the ignition switch to the lamp so the light should come on. Looks like a common ground for the cluster and switch illumination as well as the cigarette lighter. A check the light feature.Incorrect. Power is provided by the ignition, but the other side of the lamp is grounded, but only during "start".

I see a variety of causes that could cause the light to come on when ignition key is in run.

1. One side of the MC is bad, either front or rear brake.

2. A break in either the front or rear brake line somewhere.

3. The brake fluid reservoir is empty because of a leak on one side. Possibly a seal is bad on the rod and fluid leaking into the power brake booster.

all correct.

Are my assumptions correct?
There's another situation that can occur. Suppose the RUN-only line is broken and can't supply power to the lamp during running operations. Then whatever the valve position is won't power the lamp. During the Proof-out "start" condition, there is no true ground at the RUN-only line, but the RUN-only line at the lamp itself is also tied to other lamps. During CRANK, the ground will seek a place to go to, and other lamps will likely be lit that shouldn't be lit. Does that make sense?

Just to emphasize things: the ignition does NOT provide a ground path for the brake lamp. The ignition only provides power to the lamp and only when the key is in the CRANK position.

 
Midlife,

Using this thread to learn from so bear with me. Assuming MC you reference is Master Cylinder and should be proportional valve instead. correct.

I see 2 ways for the light to come on:

1. When the key is on run there is 12v on one side of the lamp and the switch at the proportional valve provides the ground when brake pressure is lost on either the front or rear brake and the internal shaft is forced over to provide the ground. correct

2. When the key is in start a ground signal is provided through the ignition switch to the lamp so the light should come on. Looks like a common ground for the cluster and switch illumination as well as the cigarette lighter. A check the light feature.Incorrect. Power is provided by the ignition, but the other side of the lamp is grounded, but only during "start".

I see a variety of causes that could cause the light to come on when ignition key is in run.

1. One side of the MC is bad, either front or rear brake.

2. A break in either the front or rear brake line somewhere.

3. The brake fluid reservoir is empty because of a leak on one side. Possibly a seal is bad on the rod and fluid leaking into the power brake booster.

all correct.

Are my assumptions correct?
There's another situation that can occur. Suppose the RUN-only line is broken and can't supply power to the lamp during running operations. Then whatever the valve position is won't power the lamp. During the Proof-out "start" condition, there is no true ground at the RUN-only line, but the RUN-only line at the lamp itself is also tied to other lamps. During CRANK, the ground will seek a place to go to, and other lamps will likely be lit that shouldn't be lit. Does that make sense?

Just to emphasize things: the ignition does NOT provide a ground path for the brake lamp. The ignition only provides power to the lamp and only when the key is in the CRANK position.
Perfect, thx!!!

 
Wow. I'm amazed how complicated the function of this little switch is. It is literally one of the simplest systems on the car, but I had my mind blown looking into the switch and realizing both wires were hooked together inside the switch.

So, there is another case I am wondering about. WHAT IF, there is not a good ground available on the prop valve? then, even if the shuttle valve displaces, the light wont illuminate, right? Maybe not realistic, because the prop valve is connected to the MC with hard steel lines, and so ground should pass through from MC - even if not available at the prop valve bolting to firewall.

But if there is not a good ground, the light wont work, right?

I agree Jbojo, when Randy said MC, he meant prop valve above. And, it is also amazing that the diagram you provided SHOWS the plunger in contact with the shuttle valve when centered even though it can't be touching in that condition - otherwise "light". AND the wording on the diagram implies it is a usual switch related to the plunging, when in fact is significantly different from a regular switch....

Bottom line is this: If my switch is working, and my dash bulb & wiring to the bulb are good - to test the switch, I need to provide a good ground to the "plunger" or "copper nub" on the switch. When I do that, I should get a brake warning light illumination on the dash if the key is in the RUN position.

All of this also means that the "plunger" on the switch is "hovering" in open space when the shuttle valve is centered. The only reason it is on a spring is so that it can slide back and let the shuttle valve pass by - whenever there is an imbalance in the front/rear brake line pressures and the valve moves sideways.

So, I'm gonna go see if I can find any voltage in my line when the key is in run. Right now, no matter what I do, my light won't work. But I know the dash bulb is ok (or at least it was only a week or two back).

I may be looking at a bad ground, or something not properly energizing the bulb circuit with 12ish volts.

I don't know, but I have found this conversation to extremely helpful... even if I don't fully understand the comments above related to the CRANK key position....

THANK YOU guys.

 
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Wow. I'm amazed how complicated the function of this little switch is. It is literally one of the simplest systems on the car, but I had my mind blown looking into the switch and realizing both wires were hooked together inside the switch.

So, there is another case I am wondering about. WHAT IF, there is not a good ground available on the prop valve? then, even if the shuttle valve displaces, the light won't illuminate, right? Maybe not realistic, because the prop valve is connected to the MC with hard steel lines, and so ground should pass through from MC - even if not available at the prop valve bolting to firewall.

But if there is not a good ground, the light won't work, right?

I agree Jbobo, when Randy said MC, he meant prop valve above. And, it is also amazing that the diagram you provided SHOWS the plunger in contact with the shuttle valve when centered even though it can't be touching in that condition - otherwise "light". AND the wording on the diagram implies it is a usual switch related to the plunging, when in fact is significantly different from a regular switch....

Bottom line is this: If my switch is working, and my dash bulb & wiring to the bulb are good - to test the switch, I need to provide a good ground to the "plunger" or "copper nub" on the switch. When I do that, I should get a brake warning light illumination on the dash if the key is in the RUN position.

All of this also means that the "plunger" on the switch is "hovering" in open space when the shuttle valve is centered. The only reason it is on a spring is so that it can slide back and let the shuttle valve pass by - whenever there is an imbalance in the front/rear brake line pressures and the valve moves sideways.

So, I'm gonna go see if I can find any voltage in my line when the key is in run. Right now, no matter what I do, my light won't work. But I know the dash bulb is ok (or at least it was only a week or two back).

I may be looking at a bad ground, or something not properly energizing the bulb circuit with 12ish volts.

I don't know, but I have found this conversation to extremely helpful... even if I don't fully understand the comments above related to the CRANK key position....

THANK YOU guys.
Just to add, do a continuity check between the valve body and ground, if there is continuity then there is a proper ground.

 
Good point jbojo. I checked ground at prop valve.... I have GOOD ground at both the MC and the prop valve... Sooooo... thats not it.

I checked some other stuff. Interesting data:

Fact: both wires into the brake warning light switch are purple (I think - I am essentially color blind) with white stripe.

Note, since both wires into the plug are the same, I arbitrarily chose to refer one side as "a" and the other side as "b". For all checks, key was in "run" position:

Resistance Check:

Ground -to- wire a = 0.0 Ohms (these are clearly connected)

Ground -to- wire b = 1.2 Mega-Ohms (there is a big resistance, almost like being unconnected)

wire a -to- wire b = Infinite Ohms (these are definitely unconnected)

Voltage Check:

wire a -to- wire b = 0.5V

ground -to- wire a = 6.0V

ground -to- wire b = 1.9V

Interesting side notes:

- generally when key in "run" (but engine not running) ALT & OIL lights are ON

- BUT, when "wire a" short to ground, OIL light goes out, ALT light is brighter! (Isn't that whacky!)

So, I don't really understand what the above means. Maybe someone can enlighten me. Hope so.

Thanks in advance!

Jay

 
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That is the right color for the 2 wires coming into the connector.

Try this:

1. Connect the connector to the switch, doesn't matter if it's installed.

2. At the starter solenoid disconnect the right push in wire (red-light blue). This will disable the starter so the motor will not urn over.

3. Go into the car and turn the key to start and hold it there. The motor should not turn over, if it does you disconnected the wrong wire.

4. While the key is held in the start position look at the brake light, did it come on? It should.

 
You have voltage leading to the oil lamp. If you can, remove the oil lamp on the dash and re-test your voltages. The voltage leakage is expected because the alternator and oil lamp share a common signal (RUN-only line). The bulb filament allows continuity between the two systems.

 
John- I tried what you said, just this am. Sure enough starter did not engage.

My light on the dash still does NOT illuminate. Not sure what all this means.

Maybe nothing more than a burned out bulb in the dash, but I know it was working recently....

I'll try what Randy suggests later today. It's a little more involved. Is it the same test to disconnect the oil light sensor? or do I really need to remove the bulb from the dash cluster? I maaaay be able to pull that bulb by reaching around the dash, or I may not, pulling the dash cluster is not somewhere I'd like to go right now. idk, maybe it's not too bad... but I'm hoping it can be reached from beneath.

Jay

 
I think you have a bad bulb or a broken trace on your circuit card.

 
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