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Sat Jul 05, 2025 2:00 pm

Drive thru deep road water in traffic — enough to completely drown the brake light switches. My brake cylinder pressure is probably not optimal but does provide enough pedal to brake easily.

I can easily replace the stitches but remember hearing that sometimes faulty master brake cylinder pressure can affect the circuit. Is there a way to test for this?

Sat Jul 05, 2025 3:51 pm

Simple test light should work.
Point on terminal wire/switch tab & clip grounded.
Key on & press brake, both if dual circuit but should be redundant.

I'd make a test light from speaker wire & a bulb then set bulb on frunk hood - my wife rolls her eyes sometimes when I ask.

Good luck ... stay safe

jinx

Sat Jul 05, 2025 4:06 pm

3-minute diagnosis.

Of course you didn't state what year......

Sat Jul 05, 2025 4:59 pm

It’s a 1973 std. Each switch has 3 prongs — test each one with the light, ignition on, brake pedal pushed? Does fluid pressure in the master cylinder have anything to do with the continuity?

Sat Jul 05, 2025 5:18 pm

pernest wrote: It’s a 1973 std. Each switch has 3 prongs — test each one with the light, ignition on, brake pedal pushed? Does fluid pressure in the master cylinder have anything to do with the continuity?

The two parallel spades on each switch are continuity switches for the brake lights. Pressure in the system when brake pedal is pressed down "closes" the circuit and brake lights operate, so -yes - pressure in the brake system is extremely important ! There are two switches, so a redundancy.

If your horn works -these share a fuse - then the fuse is OK.

Turn on ignition/"run" position. Ensure that one of the parallel spades has positive voltage (use 12 volt test light or DC voltmeter). Have helper push down on the brake pedal; now the parallel spade to this should also have positive voltage. From there the wire goes back to the brake lights.

The middle terminal on the brake switches is for the semi-useless warning light.

Sun Jul 06, 2025 8:14 am

I installed this in the trunk for troubleshooting brake lights/switches. I can check for voltage coming from the switches with a VOM or apply voltage with a jumper to 'force' the brake lights on.

Use this 'affordable' clamp from HF to depress the brake pedal against the seat when no help is around.

https://www.harborfreight.com/36-in-ratcheting-bar-clampspreader-64152.html

Tue Jul 08, 2025 8:57 am

Connected the two parallel spades with ignition turned on — no brake light. Blinkers work so fuse must be okay. Will check to see if brake light connection somehow became disconnected from fuse box connection but I think it’s still connected.

So if the light doesn’t work even when the parallel switch spades are connected, what else should I consider?

Tue Jul 08, 2025 9:02 am

Quote: So if the light doesn’t work even when the parallel switch spades are connected, what else should I consider?
Burnt out light bulbs? Using a VOM would tell you what's wrong in minutes.

Tue Jul 08, 2025 10:04 am

runamoc wrote: Quote: So if the light doesn’t work even when the parallel switch spades are connected, what else should I consider?
Burnt out light bulbs? Using a VOM would tell you what's wrong in minutes.

What about the inline fuse for this circuit, near coil positive terminal #15.

Diagnosis is a couple of minutes at most for this !!!

Tue Jul 08, 2025 10:05 am

I don’t thing it’s a burned out taillight bulb — normally one would work. Car has a three bulb assembly. The middle bulb is two filament and works with the lights switched on. I don’t think the master cylinder is involved because even when the two I parallel spades are connected, there is still no light. I could not find a correct diagram for the 1973 std beetle three bulb taillight assembly so I’m assuming it’s the middle bulb.

Tue Jul 08, 2025 11:14 am

We assume they worked before this puddle.

Things I'd check :

Clean grounds, water can get between screw/washer & metal if not clean, shiny, & tight to begin with. Also scrape pin on bulb & recessed U-cup where pin rests in holder housing.

Jumper wire from known hot to + of bulb.

If brakes used while in puddle - chance the wire/terminal connection shorted the switches as it mite be exposed if the rubber terminal boots are torn/missing.

"I don't think it's a burned out bulb".
You won't ever know until you test. Rule this out.
All we can do is make suggestions, it's up to you to verify a
PASS or FAIL.

Good luck ... stay safe

jinx

Tue Jul 08, 2025 11:24 am

pernest wrote: I could not find a correct diagram for the 1973 std beetle three bulb taillight assembly so I’m assuming it’s the middle bulb.
Use the 1972 wiring diagram, much simpler.

Tue Jul 08, 2025 1:49 pm

Jinx

Yes, the boots around the switch were totally missing exposing the connections to water. I somehow got thru water at least 12 inches deep. The street is notorious for water retention and I had to use it to make an important medical appointment.

I verified cionductivity connecting the parallel wires. Will get a switch tomorrow and install. Thank you (and the others) for verifying my suspicions that the water experience might be the culprit.