Electrical: Erratic Battery Voltage Caused by Brake Light Switch

[t1003nl]

I was driving our '64 convertible home the other night and the amp gauge showed "discharge" when I turned my headlights on. Once home, I hooked a voltage meter to the battery and the voltage was all over the place with the engine running. 13V, 0V, 6V...all over the place. Hoping for an easy fix, I disconnected the voltage regulator and the alternator but the voltage was still erratic. I even swapped the battery with no change.

I checked with a friend who recommended I look for a wiring short. I got under the dash and one of the fuses was blown. The blown fuse controlled the interior lights and the brake lights. I pulled both wires from the fuse block and put a fresh fuse in. I hooked up the interior lights and everything worked well. That made me think the issue was with the brake light system.

Our car has power disc brakes so the brake light switch is up top in the engine compartment and easy to access so I started there. I took a continuity meter and checked the two posts of the brake light switch to see if either was grounded. It turns out that one of them was grounded. I grabbed another brake light switch from the garage and installed it. I reconnected the brake light wiring to the fuse block and started the car. Everything worked correctly. The voltage was steady and the battery began to charge as soon as the engine started. Brake lights work, interior lights work, voltage is steady and no blown fuses.

[TWChamp]

Are you using a digital meter? They go wonky just staring at them cross eyed. I only use analog meters around cars.

[StudeRich]

Your "Interior Lights" are just the One Dome Light.

[stude1964]

The conv. has 2 lights under the dash! I always hit the brakes before starting the car to see if the ammeter discharges and if the brake lights work-esp.when it's too bright to see by yourself. As for a short I had an issue with the wiring going up the windshield post on my '64 HT for the interior light and was blowing fuses which as Neil stated also took out my brake lights. It was dumb luck I found it and isolated it after pulling off the post trim. I replaced the wiring when I redid the car.
Rob in PA.

[nvonada]

More specifically digital meters have trouble with the mechanical voltage regulator. The VR works by turning on and off rapidly so the "average" voltage is correct. That plays hell with many (all?) digital meters.

[wittsend]

I'd advise going to a mechanical brake light switch actuated off the brake pedal. The cars I own that have the hydraulic switch are so frustrating. The one pictured is off a Mitsubishi SUV. But, other similar switches will do. The mounting bracket is a small aluminum plate.

Most hydraulic switches are mounted downward suspect for debris to fall into them. Has anyone ever mounted the switch upward and had better success? I'd think a "U" shaped piece of brake line with the proper fittings would do.

[GrumpyOne]

The only thing that a digital meter has is measuring battery voltage prior to starting...

[stude1964]

I run silicone fluid and when installing a new switch add a couple of drops of DOT 3 fluid into it-if it takes it-before I install it. I find the switches last longer. Yes all my switches are located at the bottom of the master cylinder.
Rob in PA.

[Stan.Gundry]

Yes, I have tried mounting the switch upward and it did not improve the longevity of the switch. A mechanical switch is the only way to go.Inexpensive and easy to do.

[wittsend]

Yes, I have tried mounting the switch upward and it did not improve the longevity of the switch. A mechanical switch is the only way to go.Inexpensive and easy to do.

Thanks. And, Yes, I agree. Not having the switch in moisture absorbing fluid, in the hot engine bay is a plus right there.