Brake Warning Light Won't Turn On - Stumped

[ktm]

Now, I consider myself to be fairly mechanically an electrically inclined. I am not an expert nor a veritable Z'sondabrain, but I have done some extensive electrical work on my car.

Saying that, I can not, for the life of me, get my brake warning light to turn on. Yes, the switch is fine. The bulb is not burned out. The wiring diagrams are a JOKE. I have a 1972 240z.

The FSM shows a Y/G from the hand brake switch going to the lamp. Check. It shows a Y/L coming from the brake block. Mine is Y/G. The Chassis Supplemental section shows the light connection as G/Y and Y/G. Check. The wiring diagram shows jack for this connection.

How does the light get power? The light works by switching ground. Where is the power coming from? Does anyone know the circuit and could give me some insight?

It worked before I did the extensive modifications to my dash lighting system, swapping out gauges, etc. I know I did something and I know I can fix it, but without a correct wiring diagram I am blind.

Thanks.

[ktm]

Bueller?

[Nissanman]

I am using the Haynes Manual, circuit diagram for USA model S30, on pages 168 and 169.

The BRAKE IN, which I think is the fluid level or pressure fail lead is YL, connects to the HANDBRAKE SW lead, also YL, and t's off to the speedometer binacle as a YL lead.

This is the earthy side of the warning lamp circuit.

The warning lamp is in the speedo binacle.

The supply side of the warning lamp is an RL lead.

This lead also supplies the +ve to the tachometer and mysteriously becomes a G lead.

This G lead ultimately gets back to the fuse block, specifically the 20A IG fuse.

So, that is the power source:devious:

Actual wire colours may differ but that is how it works:surprised

My guess is that you may have put a hole in the supply lead somehow when you fitted your new tachometer:nervous::nervous:

[ktm]

I have an FSM and a Haynes manual and know those wire colors. However, the wires for the light itself is G/Y and Y/G. There are TWO wiring diagrams in the FSM for the 1972 240z. One shows the hand brake with a Y/L wire and the other with a Y/G wire.

There only place in the circuit with the R/L wire can provide power is the G/Y wire on the light lead. The Y/G and Y/L wires (one on the handbrake, the other on the distribution block) are grounds. The R/L wire only gets power when the dash lights are turned on.

Odd.

[EScanlon]

Bo;

Nissanman said it, although in reverse order as far as the circuitry goes.

The Green wire is powered through the fuse box when the Ignition Switch is in the RUN position.

That Green wire goes directly to power the Tach and the Turn Signal Flasher (after it goes through the Hazard Switch, which powers the Hazard Flasher from a different power source that isn't slaved to the Ignition Switch).

The Green wire has a connection to it, a Red / Blue (RL wire) that although it is the same color as the lighting circuit is NOT a part of that circuit.

That RL wire connects to one side of the bulb and the other side of the bulb connects to a Yellow / Blue wire (YL) that can complete the circuit by either the Parking Brake Switch or the Brake System Switch in the Brake Master.

I don't have the FSM for your year so I can't verify the YG wire you mention. You might look and see if one is the U.K. Wiring as the Haines calls it, or the Basic S30 wiring and the other is the later S30. However, whether it's YG or YL matters only in tracing it back through the harness.

HTH

E

[EScanlon]

You might also check that the parking brake switch is actually in good condition too. I've seen the wire get disconnected and / or break off at the switch and NOT make ground...and therefore no Brake Indicator.

(Pretty sure you wouldn't go to the trouble of purposely setting up a brake system inbalance just to check out a warning lamp.)

E

[TomoHawk]

The electrical section of the FSM has many of the electrical circuits detailed by themselves with a diagram of the circuit and its associated wiring diagram and usually a diagnosis chart, and has been very useful for solving things on my 280Z. It may be the same for the 240Z.

[ktm]

Escalon, thank you for the clarification. I know that there is really no difference between the Y/G or Y/L wires on one side of the bulb. The wiring diagram shows the R/L wire connecting to the other side of the bulb, however, both the supplemental chassis section and my own observations show that a G/Y wire is connected to the otherside of the bulb. Therein is my problem. The G/Y wire is definitely factory. I do not see a R/L wire connecting to the otherside of the bulb. This is why I am confused. What I am seeing does not match the wiring diagramsl, but it is confirmed by the supplemental chassis section in the FSM.

The switch is fine. I checked continuity on it while rotating it around its base and I have full continuity.

I can always run my own power supply to the bulb and replace the G/Y connector, however, I am reluctant to do so until I know the routing of this wire.

I truly wish I could follow the circuit in the harness, but the harness is in the dash and taped up.

Tomohawk, unfortunately the 240z FSM does not have such diagrams. As a matter of fact, I do not think it even has a section on the hand brake switch. It mentions it once or twice, but does not go into any detail.

[EScanlon]

OK, there are two possible explanations.

One is that your car is a "transition" car, neither a 72 or a 73 (if it was produced late in the year) or 71/72 (if early in the year).

The other, is that either the wire fell off the bulb socket and is waiting to short out on you if you should try to start the car with the parking brake on or a brake system warning sensor on.

The only extreme other possibility is that you're not looking at the Brake Warning Light.

In the diagrams I have for the 71, 72 and 73, the only GY wire I see near that area of the dash is connected to the Stop Lamp Switch...i.e. the Brake Switch. But that is a PAIR of wires and they end in Bayonet connectors (iirc).

What's the date of manufacture of your car, and are there other wires running around loose in your dash area?

E

[ktm]

I'll post pictures tonight of the light as well as the supplemental chassis section. My build date is 5/72, so it could be a transition car.

I am looking at the Brake Warning Light. I know the Brake Switch has two GY wires connected to it. I saw the same thing in the wiring diagrams. It's because the Brake Switch has the GY wires that I am reluctant to remove the GY wire attached to the brake light.

Both the GY and YG wires at firmly attached to the light. I've been up under my dash quite a few times routing wires, etc. and investigating this matter.

Thank you for all of the help so far.

[geezer]

Allow me to jump in here and try to simplify the solution. Instead of trying to follow the routing of the R/L or whatever color your 12V feed wire is to this light, look at this from a different perspective. Two conditions must be met to light the bulb. You need a 12V power source which will be constant as long as the key is in the ignition run position only. Otherwise, whenever the handbrake was applied, the light would remain on. Next you need a ground supplied. This ground is supplied by two different methods. The first is by completing the ground path by applying the handbrake. The other is a ground supplied by what is called in the Haynes manual as a "BRAKE IN". I believe that is short for what should be called a BRAKE WARNING INDICATOR SWITCH.

I agree with Nissanman's assessment. It would be either a low fluid level switch or a pressure fail lead. Now to find the problem. First, with the key in the on position check for 12V with your tester. If it's not there you can either track down the break in the circuit (my first choice), or supply another ignition on power source. If you have the 12V to the light and it does not work, the ground is at fault. A simple continuity check on the ground supplied by the handbrake switch or the other ground source which could be a fluid level switch or pressure fail switch, will be needed. The odds of both ground sources being defective are less likely than the problem being with the 12V ignition on feed. Check that first. I know how to find and fix these wiring problems, but have a harder time explaining it. I hope I was able to help.

[geezer]

I just read through this thread one more time and found the answer to the only question I had. In post #4 you say the other ground comes from the distribution block. I missed that previously. It doesn't matter. It's just that I'm not familiar with a '72 and didn't know if the ground derived from a distribution block (which would warn of low pressure) or a fluid level switch on the master cylinder reservoir. It doesn't change my evaluation and I'm stickin' to it!:laugh: Good Luck