Hi All,

Having some issues with the brake lights on my 70 which is a SBC conversion.

My brake lights are dual filament bulbs. When I turn the parking lights on both sides illuminate. When I depress the brake pedal with the car on or off, the brake lights do not illuminate. I checked the bulbs and they looked fine.

My next thought was a bad switch at the pedal assembly so I ordered a reproduction switch, installed it, and it didn’t work. I checked both the old and new sensors with a volt meter and they both seem to be working correctly.

When I moved to the fuse panel I noticed the previous owner had installed all 16 amp fuses except for terminal #9 which had a 25 amp fuse. There’s a Tach-Adapt module running through that terminal after following the wire. I moved the fuses around and reversed them and no luck either. I ordered a variety of the fuses just in case and am waiting for their arrival.

I was thinking the non-insulated quick connect terminals may be bad/corroded going to the brake switch itself? Snipping those terminals and crimping some insulated ones on is worth a try.

I didn’t mess with the master cylinder button because that sounded like a constant brake lights on problem.

Anyone run into this issue before or have an idea where else I can troubleshoot?

Try checking for continuity in the second filament of the bulb. I’m thinking that just because the tail lights work doesn’t mean the brake lights do. Sometimes a filament appears fine but actually has a break in it.

I might just try new bulbs since they are inexpensive. Wouldn't hurt to have some extras anyway.

With the key on, if you put the car into reverse, do the reverse lights come on?

The wires for the reverse lights are fed from the wires going to the brake light switch, which, I believe on 1972 and previous, comes from fuse 8. (Fuse 9 for 1973 and later.)

So if you have reverse lights with the key on, then you at least have power going to the switch.

Are you able to probe both sides of the switch with the multimeter to see if you have 12v with the key on and the brake depressed? Likewise at the brake lights themselves if you probe the socket.

Another thing to check is just switch adjustment, is it on the right side of the washer on the brake actuating rod, is nothing interfering with the switch arm's movement when moving the pedal, and is the screw adjuster on the brake pedal switch adjusted so it closes the contacts before the brake pedal runs out of travel? etc.

I replaced the brake lights and no dice. Still did not work.

With the key on, the car off, and in reverse the back up lights did not come on. My back up light on the passenger side is missing. Looks like the plastic broke around the socket. The driver side is intact but did not turn on.

When I installed an reinstalled the switch twice I was careful to put the sensor arm on the driver side of the washer. When I connected the switch, it would have had full and constant break lights on.

It is sounding like I have no power going to the switch, possibly bringing me back to a faulty fuse. When I get my fuses replacing them all will be my first course of action.

Is there any other failure you can recognize with this information?

If you have no reverse lights it might be a bad Fuse 8 then. Are you able to take a peek under there and see if it's making good contact and not blown? Give it a jiggle and spin in the holder.

To know for sure though I'd use your volt meter, or a test light to test. Hook the negative lead up to ground, and with the key on probe the switch terminals (one terminal should have voltage, and the other shouldn't, if the brake isn't depressed) and fuse 8 in the fuse panel (both sides of the fuse should have 12v.)

I try to avoid replacing items w/o knowing what is wrong. You could end up replacing several things just to find maybe a wire loose or a bad connector.


Your best bet to start with is to measure the voltage at the switch with the switch in place. If you have 12V then your fuses are ok. If not, then check the fuses. Did you measure the voltage at the fuse?

Exact same thing happened to me, bought a new switch, installed that and same thing. Ultimately it was just a loose fuse. start there. In fact, they are so cheap if you have not changed them recently you could update them all. Good luck.

Thank you for the input. I am going to check the voltage this weekend. Hopefully it is as simple as a fuse.

I tested the voltage at the switch and it was 0.13 volts. I pulled the switch and checked at the wire terminals and it was at 0.13 volts also.

Strangely enough I checked all the fuses and they all had continuity. Also the new brake switch I purchased did as well.

Possibly a bad ground? Or a broken wire/short?

Could be a broken wire. When you tested continuity at the fuses, did you also test the voltage at the fuses?

The power should flow:
Battery -> Ignition Switch (On position) -> Fuse (8 or 9 depending on year) -> Brake Pedal Switch.

You should have a solid 12v at each of those locations. If not, go from where the voltage stops and work backwards from there to inspect the wires between there and the last good connection.

When I checked the voltage at the fuses, while still in the fuse box and the ignition on, it was 0.00. This threw me off.

It was like that for every fuse. I probed at the metal tabs holding the fuse.

I checked the voltage from a manual fan switch that is connected to the battery and it is 12 volts so I know the meter is working properly.

I'll take a look at the wiring today going backwards like you suggested. Thank you for the information.

Do you think there could be a ground issue?

Hmm. A ground issue shouldn't cause there to be 0v at the fuse panel with the key on...

If you have no voltage at the fuses, next step would be to keep working back further up the chain from there: ignition switch.

Try using the multimeter to probe the fuse terminal while slowly turning the key from Off to Run, Off to Run, Off to Run, over and over, see if the voltage jumps up to 12v while you're rotating the key, if you still get no voltage with the key in the Run position, or only briefly get little blips of 12v while you're rotating the key that goes away when you hit the Run position, you might need a new ignition switch.

Just to clarify, you're saying to clip the multimeter wires on the fuse #8 terminal holder tabs, and rotate the key as you stated above?

I think you might be right it being a ground issue.
The backup and brake lights use the same brown wire for ground.

A quick test to confirm run a test wire from a good ground point and clip the other end to the brown wire on the rear taillight connection point and see if the lights work.

The brown wiring color is ground according to my 1971 diagram.

The grounding point for the taillights is at Point A which is under the regulator board.

I also have a 72 and I have found that they are like a hybrid of 72 and 73 if they were built late in the year. On Mine the yellow/Black wire is the brakes to fuse 9. however it does share the ground with the license plate lights and those are known for grounding out where they go through the body. Check that.
You should have 12 volts with key on at fuse 9 with a yellow/black wire if you probe the either side with a known good ground. That's your starting point. Work out from there.

Good luck..

Yep, exactly! Clip the positive multimeter lead to the fuse terminal tab, and clip the negative multimeter lead to ground, and rotate the key.

Well it ended up being a fuse running from the Tach-Adapt module. Looks like the previous owner spliced the black and yellow wire and ran a separate blade style fuse.

I have a feeling I'll probably be running into a lot of that considering it is a conversion.

I appreciate everyones input! On to the next repair! Thank you lierofox for bearing with me. I owe you an 805 next time I'm in Paso.

No problem! I don't do alcoholic drinks but I'd be down for a Cactus Cooler!