Brake Lights not working.

[Spool]

Hey Guys.
73 C20 Here.

I noticed my break lights have stopped working.

I've been cleaning up my wires to my starter recently when I accidentally bumped an inline fuse of a wire going to my junction box. In doing so, the case opened and I lost the fuse in the grass.

In following the wire, it led me all the way under the truck where it goes to the rear end, which makes me think this fuse may be the culprit of my now inoperable break lights.

For the time being, I had a 20amp fuse that I slipped into the casing but that doesn't seem to do the trick.

Does anyone have any ideas of how to address fixing the break lights?
It's been keeping me off the roads at night for now.

Thanks for any help.

[Spool]

Idk if this will help at all, but my buddy replaced his turn signal switch and the mechanic didnt tighten the wire harness all the way together on the collumn, sobit popped halfway off and meased with his lights. He had running lights, no brake lights, only the right blinker. With his running lights off, he had the left brake light and nothing else. Make sure that plug is secure.

I'm having a similair problem now with my brake lights
When you mentioned tightening the wire harness all the way, is this referring to the bulk head connector?
I recently tried to take my bulkhead connector off with no success and put it back for the time being. This was around the same time my brake lights went out.

[bd]

Ahhh!  The case of the broken brake lights....  Do the rear turn signals flash?

[Spool]

BD! Thanks for your time yet again!

The rear flashers do work.

When I was cleaning up the starter, I accidentally bumped an inline fuse going to my junction box that runs from the rear of the truck. I lost the fuse and replaced it with a 20amp fuse I had around.

Maybe this was the culprit?

[bd]

The rear flashers do work.

If the rear turn signals flash the wiring between the turn signal switch and the rear turn/brake lamps is functioning.  Check the brake lamp fuse in the fuse box.  Use a test light to probe the metal clips that retain the fuse.  Oxidation between the fuse and the fuse box terminals can impede current transfer by an otherwise good fuse.  If the fuse is okay, check the brake light switch and adjustment.

The fuse that's hiding in the grass is not tied to the brake lights.  Is the truck fitted with a trailer hitch?

[Spool]

There was a 30amp fuse that I replaced with a new 20amp fuse as labeled fuse box.
Brushed it clean and tested both terminals.
The Bottom terminal read 12 volts but the top terminal went crazy without a proper reading.

I located the brake light switch (very cool to see how that works by the way)
But how do I test it?

I do have a hitch on the truck.

Thanks again.

[bd]

There was a 30amp fuse that I replaced with a new 20amp fuse as labeled fuse box.
Brushed it clean and tested both terminals.
The Bottom terminal read 12 volts but the top terminal went crazy without a proper reading.   Be explicit.  What do you mean by, "the top terminal went crazy without a proper reading?"  When the brake fuse is installed, both fuse box terminals should measure battery voltage.  Power runs from battery positive through the fuse to the brake switch then proceeds on to the turn signal switch.  The brake lights won't illuminate if power is interrupted anywhere along that route.

I located the brake light switch (very cool to see how that works by the way)
But how do I test it?   The brake switch has two terminals.  One terminal should measure constant B+.  The remaining terminal should measure zero until you depress the brake pedal, then measure B+.

I do have a hitch on the truck.   The fused wire that runs rearward from the junction block may be for an electric trailer brake, or a charge lead for a trailer mounted battery.

[Spool]

Sorry. What I mean't by crazy without a proper reading was that when I tested the top terminal, the numbers rapidly went as high as 80 and as low as negative 16 on my multi-meter.
I hadn't tested the terminals with the fuse in.
I tested the fuse itself, which was .04 and the terminals separately without a fuse in place.
Is the top or bottom of the fuse holder positive?

I'll find the terminals on the brake switch and test both.

[Spool]

When testing the brake switch, do you test the wire itself coming out the back of the terminal (as seen in the first photo) or pull the connection apart slightly to test the actually prongs of the connection?

[bd]

It's probably easier (more convenient) to probe the wire entry point of the connector, no?  Orange is B+, white is load.

There sure is a lot of wire sectioning under the dash.  What is that all about?

[Spool]

It's probably easier (more convenient) to probe the wire entry point of the connector, no?  Orange is B+, white is load.

There sure is a lot of wire sectioning under the dash.  What is that all about?

Ha... I wish I could tell you. I'm in the process of learning, cleaning and redoing most of everything wire-wise as of now.
I bought the truck 2 years ago from someone who had it a about a year and sold low around the holidays.
Story has it, that he bought it from two teenagers who gave up on their dream of redoing the truck themselves. I find many things to be strange.

So far I've had the carb rebuilt, master cylinder replaced, new disc brakes on the front, window regulators swapped, horn installed, new battery, exhaust pipes extended to the end for registration, starter replaced along with the rewiring as you know and I've been eyeballing a coolant flush, hose replacement and coolant reservoir install as my next project along with seatbelts which arrive on the 11th.
(Got to admit though. I love it. It's fascinating)

I'll take the reading on the switch this afternoon. Thanks BD.

[Spool]

Alright.

Orange read 11.44 with the truck off and 14.55 with the truck on.
White read nothing other than .06 pressed or depressed.

Does this indicate that my brake switch is broken and needs to be replaced?

[bd]

Jumper orange to white and check the brake lights - they should illuminate.  Then replace and adjust the brake switch.

[Spool]

Thanks BD.

Are there any safety precautions when jumping?
I read online I could use a paper clip; is this so? Should I attach the paper clip to a splinter of wood to avoid shock?

Think it's a good buy? Seems straight forward, no?
https://www.am-autoparts.com/1973/Chevy/suburban-c20/brake-light-switch/AM-2886901275/840689.html?rawFit=840689-1973&gclid=Cj0KCQjw5arMBRDzARIsAAqmJewijFpc8X-jh5jJAPkdKu9t9QqyyiZL6fRfhL0adkem58w1jDHDhccaAgzhEALw_wcB
I've seen the same one for $25 instead of $5

[bd]

Thanks BD.

Are there any safety precautions when jumping?   Don't let the jumper arc to ground (any adjacent metal).
I read online I could use a paper clip; is this so? Should I attach the paper clip to a splinter of wood to avoid shock?   ...paperclip, wire, even a small screwdriver placed across the prongs of the switch.  As long as your hands aren't sweaty and you aren't wearing any metallic hand/wrist jewelry or watch your chances of receiving a "shock" are about the same as handling a 9-volt transistor radio battery.

Think it's a good buy? Seems straight forward, no?   Seems that way.
https://www.am-autoparts.com/1973/Chevy/suburban-c20/brake-light-switch/AM-2886901275/840689.html?rawFit=840689-1973&gclid=Cj0KCQjw5arMBRDzARIsAAqmJewijFpc8X-jh5jJAPkdKu9t9QqyyiZL6fRfhL0adkem58w1jDHDhccaAgzhEALw_wcB
I've seen the same one for $25 instead of $5