Brake Light Switch, Again?

[rokemester]

Tested 87 and 87a for continuity to 30. No continuity at 87a, continuity at 87. I’m confused by 12V to both 30 and 86. Do I need to add a 12V wire to the circuit? I thought if I followed your schematic the 12V would come from the black green at the white OEM round two spade connector.

[wazzz]

You need 12V both for the relay coil and to power the brake lights. You don't need to pull a second wire. Bring the 12V wire to terminal 30 and loop with a small piece of wire between 30 and 86.

[scarceller]

OK, so that relay is normally close (continuity 0ohms) from 30 to 87, so 87 is normally closed.

This means 87a is normally open, you need to wire the brake lights to 87a. This relay is not like the standard relay I used. My relay was flipped with 87a NC and 87 NO.

As for the 12v source that comes from one of the pins on the connector that was on the stock switches on the MC.

Please measure voltage at those white connectors, one of the 2 pins always has 12volts that's your power source that needs to then feed power to pin 30 and 85.

Basically my schematic will work but only difference is the brake lights will be wired to 87a instead of 87.

Pay close attention to how pin 86 is wired to the 2 switches on the MC, all that wiring is new. Notice how one side of each switch is wired to ground, you need to do that. Then the other side of each switch is wired to pin 86 on the relay. When you step on brake pedal it causes both switches to close, whatever switch closes first then provides ground signal to pin 86 on relay and this activates the relay, you will hear it click closed if someone applies the brakes. That then causes pin 30 to make contact with 87a and the brake lights activate.

[rokemester]

Thanks!! I think I’m tracking this!

“Please measure voltage at those white connectors, one of the 2 pins always has 12volts that's your power source that needs to then feed power to pin 30 and 85.”

On my relay I believe 85 is ground. Did you mean pin 86 Sal? The trigger wire feed from the brake light two pronged switches.

[scarceller]

85 and 86 are interchangeable, it does not matter which one goes to ground. If 85 on your relay is marked ground feel free to use that one for the ground that goes to switches and use 86 as 12vdc side. 85 and 86 simply go to a coil winding, polarity does not matter unless the relay has a diode.

[scarceller]

What these cars really need is a mechanical switch some thing like this one. I've used this in VW Bus applications in my Son's 2 busses he has. Used a switch like this and a relay as well as the switches on the MC. Simply added a 3rd mechanical switch in parallel to the 2 hydraulic switches.
Just need to figure out how to connect such a switch to the MC on the 911?
https://www.amazon.com/Alpha-Rider-Switch-Universal-Kawasaki/dp/B07DJ9NVMS

[rokemester]

Thanks to your explanation this is starting to get through my thick skull. Appreciate your and others help and patience! I’ll report back.

[scarceller]

This AI explanation is helpful, read the last sentence.

A standard 40A 12V relay has five pins: 30 (power input), 85 (coil ground), 86 (coil power), 87 (normally open output), and 87a (normally closed output). The 30 pin receives a fused 12V power source, while pins 85 and 86 are the low-power control circuit; 85 connects to ground and 86 connects to a switch that provides the 12V power. When the coil is not energized, 30 is connected to 87a; when energized, 30 switches to connect to 87.

[rokemester]

Originally Posted by wazzz

You need 12V both for the relay coil and to power the brake lights. You don't need to pull a second wire. Bring the 12V wire to terminal 30 and loop with a small piece of wire between 30 and 86.

The one thing that I found most confusing is the wiring providing 12v and where. The above post helps.

[trials935]

There are several threads about adding a mechanical switch here on pelican. I did it on my 88 930.

[rokemester]

Once I’ve been able to get this relay and brake lights operational, the next challenge will be fishing two hoses back to their resting spot without disturbing the wiring harness. Tight quarters!

[scarceller]

One thing you need to be 100% certain about is that pin 30 to 87 continuity. Every relay I ever worked with always has those 2 pin normally open (no continuity). Read the last sentence of the AI description above.
I'm really surprised that your relay has 87a and 87 functionality swapped?
Even if swapped it's not a big deal as it will simply result in brake light being on all the time and going off when stepping on the brakes. If that occurs you simply move your brake light wire from 87a to 87.
No harm done other than the brake lights working backwards (ON when they should be OFF).

[rokemester]

That seemed odd to me as well. I easily could have made a mistake. I will double check! The schematic on the relay indicates 87 is normally open.

[rokemester]

I confirmed that 87 is NO just as the schematic indicates.

[scarceller]

If 87 is NO then you follow my wiring instructions as in my hand drawing.
Pin 87 feeds the brake lights and nothing is connected to 87a.

[wazzz]

Yes but in post #81 above, you said the following: "Tested 87 and 87a for continuity to 30. No continuity at 87a, continuity at 87. "

So either you confused both states, or you did this test while energizing the relay.
States NO and NC are specified at rest, ie when the device is not energized.

[rokemester]

I hooked everything up and I added 12v to 86 with a spade connector to the open brake light plug. Turned the ignition key one click to right and the relay clicked and the brake lights came on. I pressed the brake pedal and blew the fuse. Clearly I’m not bringing power to the relay properly.

[scarceller]

30 and 86 get tied together and get 12vdc from the Green wire in the white connector.
Then 87 goes to brake lights Brown wire in the white connector.

At this point if you then ground pin 85 it will light the brake lights. Test this first before you wire pin 85 to the existing switches on the MC. keep in mind that both white connectors are now disconnected from the MC switches, the white connectors remain unplugged from the switches once you use the relay.

[scarceller]

You did unplug both white connectors from the MC switches? they need to remain unplugged.
And what do you mean you brought power to the relay pin 86? where did you get power from?
You get 12v power from the green wire on one of the unplugged white connectors.

And you have no need to turn ignition key, the brake lights will work with key completely out of ignition.

[rokemester]

Got it Sal. I was bringing the 12v to the relay improperly.