The Stovebolt Site Forums The Shop Area The Electrical Bay Chasing brake light demon - What am I missing?

I'm trying to get brake lights on a 1950 truck operational again and I'm stumped. I'm probably missing something obvious. But I readily admit wiring is not my strong suit.

Here's what I've done so far.

Tail lights work.

Swapped bulbs from tail light to stoplight, both bulbs work in tail lights.

Checked fuse. It's good.

Started at brake light switch.

Discovered no power to switch.

Rewired from fuse box to switch.

Checks to make sure voltage passes through to other pole on switch when pedal is depressed. Meter shows consistently just under 6V.

Ran a new 16-gauge wire from the switch to the back of the truck.

Confirmed end of wire has voltage with a meter when pedal is depressed.

Hooked up wire to correct wire coming out of tail light assembly.

Pushed pedal and cannot get the bulb to light.

I thought, perhaps, the assembly itself or the wire feeding it was bad. Checked resistance from the tail light assembly wire to the terminal inside the bulb socket and the meter pegged.

Very confused. Voltage to the wire, good resistance to the bulb socket and a good bulb should light up.

Is there something obvious that I'm missing?

Thanks in advance for the wisdom of your experience.

are the sockets grounded to the housing? And is the housing grounded to the the frame?

Good thought. Not sure. Wouldn't the housing be grounded through the bolts that attach the light housing to the truck?

Pardon my ignorance, but what's the correct way to check this?

You have proven that there is power to end of wire. That's good. I do not know what this means
"Checked resistance from the tail light assembly wire to the terminal inside the bulb socket and the meter pegged." I think you have checked that power "would be there" BUT You must prove that there is power to the brake socket's inside center contact While power is on.

If you do have power there: (some of this is just reiterating what Rusty Rod said)
1. Be sure you have the correct bulb and that it's center tap is touching the socket contact.
2. If it's the correct bulb and is touching, you have proven the ground path is interrupted.
3. With bulb out, check continuity of socket shell to chassis ground. THIS MUST be confirmed.*see below.
3. I assume you have a separate brake light and tail light. If tail light works. Install a test ground wire from tail assy clean metal to brake assy clean metal. Bulb has to light then. You can actually touch the bulb case, while it is installed, with a test wire probe to provide an alternate test ground to chassis or a known good ground.

Never, ever assume this in any wiring troubleshoot: "Wouldn't the housing be grounded through the bolts that attach the light housing to the truck?"
The number one cause with lighting troubles is GROUND. Paint, loose screws, loose assy, rust, corrosion, oxidation.... all cause loss of ground.
* Do a continuity test from socket to light housing, light housing to brackets, brackets to truck chassis, truck chassis to neg battery post.

If you can not figure out why assy is not grounded after checking and cleaning and tightening. Add a permanent ground wire from assy clean metal spot to chassis clean metal spot.

The #1 cause on almost ALL electrical problems are GROUNDS!! Check grounds first and then fuses. Grounds have to be CLEAN and TIGHT!! When it comes to electrical issues start with the "keep it simple" procedure then move on from there, or you will end up chasing your tail and needlessly throw "C" notes and parts that won't fix the problem. As Bartamos stated run a grounding strap from the light housings to the frame, clean and tighten.

Tim

Thanks for the advice. I know you must be correct on the ground being the problem, but I couldn't get it resolved today.

In response to your checklist:

1). Doublechecked the bulb was correct. Center contact was touching socket contact.
2). Logically, you must be correct on the ground, though, if the tail lights work fine my brain is trying to tell me that if it's grounded enough for the tail lights and the brake light and tail lights share the same piece of metal, the brake light housing should be grounded too. Perhaps that's incorrect though. Brake light and tail light sockets are inside of the same housing, bulb sockets share internal bracket.


3). Struggling on this one a bit. I showed continuity within close proximity of the housing (bracket holding it, lower portion of bed.


In digging around under the truck (it's new to me so I don't know it's history) I found in the battery box what appears to be the remains of a chassis ground strap. It was a large, battery-cable sized wired bolted to the passenger side frame, but the other end was cut off and bent back up into the battery box.

I'm guessing that may be at least part of the culprit. Exactly where and how many ground straps are needed.

At one point I tried running a ground wire from the back of the light housing all the way back to the firewall, where another wire is grounded into the sheetmetal.

Again thank you for all the help.

On a brighter note, I was chasing another wiring problem on a different old vehicle today (ironically, also with a brake light issue). This one turned out to be a wire that had been pulled out of a connector. One down, one to go.

If your tail lights work you don't have a big strap problem. Understand you have one housing, one lens with two sockets inside. Each bulb is a single filament NOT a dual filament, correct? I see two power wires enter housing, one from stop switch, one from tail light fuse block.

Please advise: When you test the BRAKE light socket (not the contact) does it have continuity to the tail light socket? The socket is the part that the bulb fits down inside. The part that the bulb case pins touch.
Obviously these two sockets must have continuity, so check that. Touch one socket with one test lead and touch the other socket with the other lead. Any ohm reading is continuity. Set meter to "continuity". If there is continuity between sockets, I resign.
If there is no continuity between sockets and you said both contacts are live then:

With brake switch activated, Touch a wire to the BRAKE bulb case (the case of the bulb is the brass part with locking pins), while bulb is in, and touch other end all the way to battery negative post. You HAVE TO establish a ground to the side of the bulb by BYPASSING every wire, every bracket, every housing, every socket, everything, in order to start somewhere. The case of the BRAKE light bulb needs to be grounded by you in this way. IF there is 6V at it's contact, bulb will light. Remember: bulb is IN the socket during this test.

Please report the results of these exact tests. This is not that hard to diagnose. I may not be communicating properly or not understanding your tests.
I am pin pointing you to exact tests now. No other tests. I have clarified my terms so we are talking about the same things. Let's forget housings and brackets and the strap you found for now. Unless there is an unknown dedicated ground wire to the battery, your chassis is grounded. Your problem is localized at the socket. It's simple, each socket needs power and ground. Power to the center contact, ground to the socket and into the bulb case.
The fix is next. After the bulb lights.

Not sure about this:
"Confirmed end of wire has voltage with a meter when pedal is depressed."

"Hooked up wire to correct wire coming out of tail light assembly."

"I thought, perhaps, the assembly itself or the wire feeding it was bad. Checked resistance from the tail light assembly wire to the terminal inside the bulb socket and the meter pegged."

Does this say there was voltage on the end of wire at the tail light? The wire going to the contact in socket?
If so; instead of checking the terminal inside socket for voltage, you checked resistance between wire and the terminal inside socket? If so; that should prove terminal "would have" voltage. But you still need to actually check that the contact does have voltage.

This whole thing could be you just need to sand paper the socket wall and the contact terminal. The above tests will tell.

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Replying to myself here.

I've fixed the problem, but I don't understand WHY what I did resolved the problem. Perhaps someone else can help educate me.

Here's what I did:

After re-reading Bartamos' good instructions, I realized the best approach was to systematically check everything step by step.

After wiring up the bulb directly to the battery and confirming the bulb would light, I began backtracking to the brake light switch and connected wiring and trying to manually hold positive and ground wires onto a loose bulb.

I determined although the positive wire coming from the fuse box and connected to one of the brake light switch terminals showed 6.04 volts on my multimeter, it would not light the bulb when manually connected to the bulb.

I then unhooked the other wire going into the fuse box and touched it to the bulb and it lit.

I checked the voltage and it showed only slightly higher. 6.18 volts.

After cleaning all the contacts coming into and out of the fuse box, the wire coming out of the fuse box began lighting the bulb again.

After reconnecting all the wires to the brake light housing, it started working again.

Obviously, something in cleaning the connections (all of which seemed tight) and were good enough to pass through 6.04 volts was the difference.

Could that minor voltage difference be the factor?

Thanks in advance for any replies. The wisdom of those who are more knowledgeable is extremely helpful to the rest of us.

"Could that minor voltage difference be the factor?"

No

Here is my guess for what it's worth: If I understand all this. There was ground to the socket because the socket in question showed continuity to another socket that had a lit bulb. There was power to the center contact inside the socket in question, according to you.
Therefore the bulb has to light IF.
1. IF Area of socket that touches bulb bayonet pins is clean.
2. IF Contact spring has good pressure and contact is clean. The spring provides pressure for power AND ground. Pushing center contact against bulb tap and pushing bayonet pins against socket slots/sides. You need to feel a spring pressure when inserting bulb. If not, replace socket.
3. IF Bulb is clean and tests good.

Therefore; One of the above was fixed, or temporarily fixed, by your fiddling/cleaning, cycling the contact spring.
All in all, you did as good as anyone could. It was/is one of those frustrating electrical issues. Systematic troubleshooting is all you can do. Reasoning it out after that.... and then magic luck

Warning: Not sure how socket is attached to housing. If it is "clipped in" using fingers or crimped on, that is a place to lose ground intermittently. If socket it soldered or otherwise integral, may not be a problem.