Brake Light won't go OFF!

[geezer]

Forget the Ebrake handle. It's fine. We know this because you say the light gets brighter when the brake is applied. I don't have these features in my 1970 Z and I don't have a wiring diagram. I went to the AtlanticZ site and had a look at the diagram they have for a 1977 Z. (By the way, it has a ClassicZCars logo on it) I found one thing odd, as if this diagram was for a RH Drive although it didn't state that. The "Brake Indicator Switch" has a label which says "Engine Compartment Firewall Passenger Cabin". On a LH drive the Brake Master Cylinder is on the left side. Since you are the guy with the car in front of you, you will have to confirm the physical locations of the components. They are not shown on a wiring diagram.

What I gathered from trying to read the schematics on my computer screen is this. The light can be activated by two methods. The first is when the Park Brake is engaged. I believe the engine must be running because the "Brake LampCheck Relay" gets its 12V power directly from the alternator field wire. By applying the Park Brake you are supplying a ground to the relay. You earlier said the light became brighter. Fact is, the light should have been this bright even before the Park Brake was applied. This means you have a bad ground connection on the black wire, coming from the "Brake Warning Lampcheck Relay" that is screwed/connected to the body.

On to the wires at the Brake Master Cylinder. As I've said, I don't have a '77 Z to look at so you need someone else that does to jump in to help. In the past I've worked on two types of Brake Warning Systems. One would supply a ground signal when the fluid level was low in the Brake Master Cylinder and the other would supply a ground signal when the pressure fell below a set parameter in a Brake Pressure Switch on the brake line. We need a FSM. Either way it is just a ground signal being sent to the relay. The other thing I don't know is the nature of the Brake Warning Lampcheck Relay. Is it normally open or closed? This is why you need the vehicle in front of you or at least a FSM to find all the answers. I hope this in some way, helps you understand a little more of how this system works. We need a little input from someone with a '77 to identify the wire(s) at the Brake Indicator Switch.

[sblake01]

More info, if I disconnect the relay under the passenger seat the light stays on, and there is wires on the master cylinder, but they are not connected to

anything, my mechanic could find nothing that they could connect to, maybe

this is the issue? If the wires were connected then that would solve the problem? Does anyone know where they go?

Thanks,

Jim

There should be a brake line pressure warning switch on a 77 which is different from the fluid level warning switches in the master cylinder caps on a 78. It's under the master cylinder.

[geezer]

Stephen - That's what I didn't know, having never looked over a '77. It's now up to you Jim. I can tell you what to look for. Start at the Brake Warning Switch that is illustrated in Fig. BE-81 that Stephen provided. Using a test light or multimeter, connect to Bat.(+) and use the other probe to test the switch with the wire disconnected. You should not detect a ground. If you do, the switch may be defective, or it is simply doing it's job, warning you of a drop of residual pressure in either the front or rear brake circuits. Check for leaks.

If no ground is detected, next test the Brake Warning LampCheck Relay. The black wire should have a ground, the yellow should have 12V only when the engine is running and the green/yellow stripe wire should have a ground only when the Park Brake is applied or a ground signal is recieved from the Brake Indicator Switch (name used in the schematic) or the Brake Warning Switch as it's called in Fig. BE-81 (same thing, different name). If all tests are OK up to this point, the relay itself is suspect.

Check it out and let us know what you find.

[rhochisociety]

Geezer:

Sorry it has taken so long to reply, but we had a fierce cold front come thru, and crawling around an unheated garage is a daunting task. But here is what I found when I followed your tests.

1.Using a test light or multimeter, connect to Bat.(+) and use the other probe to test the switch with the wire disconnected. You should not detect a ground

----I did not detect a ground with my continuity function on my multimeter----

2. test the Brake Warning LampCheck Relay. The black wire should have a ground, the yellow should have 12V only when the engine is running.

----Here it gets interesting, I tested DC voltage of the yellow wire in the harness and it read 6.8 Volts DC, I tried again and got a 7 Volt read. I thought my meter must be screwed up, but I tested the battery itself and it came in a 12 Volts. I don't know if this has anything to do with my problem, but it is to me an interesting finding!!!!

3. green/yellow stripe wire should have a ground only when the Park Brake is applied or a ground signal is recieved from the Brake Indicator Switch.

----I did only get a ground when the parking brake was applied-----

Well, I hope this helps narrow down the problem, but it has me scratching my head. My sincere thanks for all of your help up to this point!

Jim

[sblake01]

Does your car have the master cylinder caps with the wires for the fluid level switches? I'm only asking because looking at the wiring diagram in 77 FSM I'm seeing conflicting information as to which system it has.

[geezer]

That does narrow it down. 1. No gound detected = good, no fault with front & rear brake circuit and the switch seems normal. 2. I take it you had a ground on the black wire? The voltage readings you had on the yellow wire are not unusual because it will be dependant on RPM. 3. The green/yellow stripe wire having a ground with the Park Brake applied is normal. This narrows it down to only two possibilities that I can think of. TXZ1956 could very well have nailed it from the get go attributing the problem to a bad relay, but I prefer to track down a problem without spending money on parts you may not need. The Brake Warning Lamp Check Relay is "normaly open", meaning if it was defective it could remain closed causing the light to recieve a constant ground. The only other way a constant ground could cause the light to stay lit would be, a short to ground on the green/yellow stripe or the yellow wires. To test the relay, disconnect the green/yellow stripe wire from the relay and use your test probe on the relay where you removed the wire and check for ground. If you have a ground, the relay is defective. If there was no ground and the light is still on, there is a short to ground on the green/yellow stripe wire.

[het976]

I think I have the caps that have the sensor wires for fluid level if you need them. They are from a 78. Send me a PM if I can help.

[rhochisociety]

sblake:

Yes my master was replaced by the previous owner, and it does have 2 wires that are attached to it, but they are just wrapped up and are not attached to anything. My mechanic could not find anything that they should attach to, this has puzzled me all along. In theory they should go somewhere, but they don't. There is a wire that is attached to the brake warning switch, but I cannot tell from where it originates.

[rhochisociety]

Geezer:

I will check this green/yellow out on Saturday (I think I will be shoveling snow for the next couple of days). 2 days ago it was 62 in the morning and in less than 20 hours it was 2, with a wind chill of -15. Lord I love the weather in Missouri, if you don't like the weather today, you will hate it tomorrow!

thanks,

Jim

[sblake01]

If you have the brake warning switch, there would be no place for the wires from the caps to plug in as they are not used in the 77 system. 78 and later had the fluid level switches in the caps with no brake warning switch. Different systems. My 78 had a bad switch in one of my caps and it caused the same problem you're having so pehaps the problem is in your brake warning switch.

[geezer]

Since both of these warning systems send a ground signal in order for the warning light to come on, it is concievable either or both could be wired into the circuit at the same time. Then you would have the best of both worlds. When the light came on you would then have to determine if the Brake Master Cylinder has a low fluid level, or if the Brake Warning Switch is warning of a pressure differential in the front or rear brake circuits. Either way, it would warn of a problem that needs attention.

[onesexZcar78]

I know I'm new here and don't have very much knowledge to the indepth parts of my Z but I own a 78 280z and I've had the brake light stick on before...I have reason to belive it is associated with being a check engine light as well as a brake warning light. So, with that said is there anything else going on with your car? Just another idea to think about.:nervous: