1979. Randomly my ‘stop light’ warning has been coming on. Yesterday they both went out completely.

Pulled both bulbs. They are fine. #10 fuse has power to it, and has continuity. Grounds to the rear light housing have been cleaned and de-ox’d.

So I suppose I am looking at the stop light switch on the brake M/C. This has 3 connections. I am not sure of the wire connections to test it.

Is there anything else I am missing before ordering a switch?

TIA

 

buy 2 new quality switches screw them into the MC the lights should work

 

Ok. Now I am stumped. Got a pair of switches from Pelican. Fun time installing them. i have 12v to each of the plugs. Still no brake lights.

New bulbs, fuse is good. Pulled and cleaned the connector to the lamp computer. Good grounds to the taillight housings.

What am I missing here?

 

Put a rod to the brake pedal an use the seat to hold the brakes on leave it like this overnight

this will compress any air bubbles in the system they will have an easier time to rise

Note disconnect the battery first

 

I have done that for 3 days. I have a security bar that locks the wheel to the brake.

But I bled the brakes again today amd got some more air out. I will give it another day and see what happens

 

UMM does the lock bar press the brake pedal or pull it?
It has to press

 

Even with one circuit low on pressure, the otherswitch will light the brakelights. If you have any pedal, there's enough to get at least one switch to close for the brake lights.

The circuitry is described accurately in the wiring diagrams. Commonn faults include damaged wire and connectors at the switches. Bake fluid is tough on the insulation, and the wiring gets corroded and breaks wen it gets twisted durig switch work.

Always worthwhile checking stewpid stuph like fuses and bulbs, rear wiring and socket at the lamps.

 

The lock bar presses.

i have checked all the wiring. Everything is in good shape.

From what I see on the wiring diagram, the current for the loghts themselves do not pass thru the light computer. Correct? That is why there are 3 wires on the sockets at the MC switches

 

Yes the power to the brake lamps does go through the bulb check controller and they do fail.

Here is what you can do to jump connections and bypass the bulb check controller.

1. Jump one of the brake lamp switch connectors with the ignition on. Terminals 81 to 82a or black/red to black wire. Brake lamps still out? Continue on to next step.

2. Unplug the 12 pin connector at the bulb check controller. Test pin 1 to ground with volt meter - this will be 12v from the jumpered brake light switch. If yes skip to step 5, if no, continue to next step.

3. If you found no voltage in the step above, you have a break in the wiring. There are two terminals you can check at the CE panel. The black/red wire from the brake light switch runs to F7. The plugs at the bottom are labeled A through Z and this is how the terminals are numbered.


4. You should be able to test without unplugging by backprobing F7 to ground with your voltmeter. Behind the CE panel is simply a jumper that runs to S6 and test here as well. The spade connectors sometimes push out the back of the panel and lose connection. The wire out of S6 goes to pin 1 at the bulb check controller.

5. Jump pin 1 to pin 12 for left brake lamp and pin 1 to pin 9 for right brake lamp. This will light up each brake lamp if the wiring, sockets, and bulbs are good.
If you do not have voltage at pin 1 but still want to test the rest of the brake lamp wiring you can jump pin 10 (fed from X bus, key on accessory position) to pin 12 and 9 (left or right brake lamp).

The switches are wired parallel. This is the power flow from fuse to bulb.

 

"Grounds to the rear light housing have been cleaned and de-ox’d."

Just to be clear, I assume that means you removed the trim panel and cleaned the ground point in front of the right side tail light.

 

Yep

 

MM,

Perfect. Thanks so much for this. My next step was to dive into this.

 

Alright. I am back stateside.

After pulling the controller out, the bulbs out, checking the grounds, I jumpered the new switch at the front of the M/C, and the brake lights work.

Both switches are new from Pelican. Yet when I step on the brake pedal, no lights. Obviously points to the switches. The brakes have been bled and are firm.

I guess I need to try pressurizing the brake system again for a few days.

 

NOTE you also could have a bad switch

 

Update. Brake lights are working again.

Lesson learned. The plugs for the switches, because of their leads being led out at right angles, only want to be oriented one way. The switches must be aligned to the plug pin orientation.

Forcing the plugs to be oriented with the switches resulted in a bent spade terminal on the new switch.

Not sure I have ever had a worse repair on this beast. The brake pipes between the MC and the proportioning valve mist be removed, as do the vacuum lines from the brake booster. Then there is still no room to swing a wrench. You must use a 24mm crows foot with a multi jointed long extension.