Strange brake light issue

[Dornon13]

Hey guys first post here just been lurking.

I have searched and read and tried everything I can find. I feel like I understand the circuits but still cannot beat this problem.

The issue is that the brake lights do not work. I can measure 12v at the brake light pin in the bulb contact and I can jumper the bulb housing directly to the battery negative but it does not light.

I cannot understand why I am getting voltage everywhere I am supposed to but not getting light! I checked for shorts with my multimeter and could not find any that way.

One interesting find which may help diagnosis
Brakes lights won't come on but when I turn the hazards or turn signals on the bulb lights without being grounded (touching the bulb only to the socket without being in the housing). This doesn't seem right to me but maybe it is

Also fyi I put all new switches in including turn signal and the firewall to rear harness is. new.

[RustyPile]

What technique are you using to measure voltage at the bulb socket?? What ever instrument you're using must have a ground path in order to show a voltage.. If you have voltage at the socket, but bulb doesn't illuminate, you have only 2 possible faults.. Either no ground path or the bulb is bad...

[Dornon13]

I am using a volt meter. I am using the same ground to measure the voltage and try to light the bulb. The technique has worked for other lights in the truck.

The light tests fine and comes on if I use a dedicated battery or if I use a different circuit in the truck ( front signals). Also the light works with the turn signal on as I mentioned and doesn't even use the ground. Then when I touch the ground to it, lights up brighter. I should note the brake switch is removed from it's bracket so it stays on.

I have tried to eliminate as many variables as possible by removing the housing from the equation. I am pressing the bulb against the 2 pin spring contact and using a bare wire to ground the side of the bulb directly to the battery negative.

The light should come on, I really don't understand. Lol

[HO455]

You have a bad connection in the circuit. It takes a minimum amount of current to operate the bulbs. Your multimeter requires far less current to work so the meter works as it should indicating 12 volts. Because the bad connection in the circuit is restricting the flow of current the voltage in the circuit drops below the minimum level needed to light the bulb.
Sort of like a rotating lawn sprinkler. If you kink the hose the sprinkler will quit turning but there is some water flowing.
Since you said the turn signals are working it is likely that the problem would be in the brake light circuit before the turn signal switch, although the bad connection could be at the turn signal switch.
First thing I would try is putting a jumper wire across the brake light switch and see what happens. The switch is a mechanical item that will fail with use. While you doing this do a visual inspection looking for rust and corrosion (on wiring harness & components) as they are a common cause of this type of failure.

[Dornon13]

I did try sourcing power from the white wire at the TS connector and still no light.
I am suspecting the brake switch as it is the only one that isn't new.
I'll try that tonight.

[Dornon13]

Update:
Replaced the brake switch with no luck.

Tried sourcing power from the orange wire at the brake switch...no luck
Tried sourcing power from the top of the brake light fuse...no luck
Tried sourcing power from the red wire on back of the fuse block...Got LIGHT ON
Tried sourcing power from the bottom of the brake light fuse...Got LIGHT ON
Disconnected everything that uses the orange wire
Brake switch
Dome light
Ignition switch
Heater fan switch
Heater resistor
Tried sourcing the same places but no change.
Keep in mind I still have voltage at all these places.

I think the contacts in the fuse block are not getting good connection. The fuse tested good. I put half of it in the bottom and sourced power from the other side without clipping it into the fuse block.

[Dornon13]

Bump

[dmjlambert]

If your tail light glows with headlights on, and glows brighter with the turn signal or hazard flasher on, then you are completely done with troubleshooting at the back of the truck, go ahead and screw it back together and the problem would be in the cab, either the brake switch or turn signal switch or in the wiring up there in the cab.

[toolboxchev]

When the turn signal switch goes bad it usually shows the problem first in the brake lights.

I had a truck that got corrected by running a new brake light wire directly to the housing from the switch. I was just to cheap at the time to replace the turn signal switch.

[Dornon13]

I know the problem is not in the TS because I am trying to use power before the TS.
Essentially the light works if I use the red wire but doesn't work If I use the orange wire.
It doesn't work even if I have everything disconnected from the orange wire ( including the brake switch that feeds the TS)

[HO455]

Failed connections in the fuse block buss work is not uncommon. The difference pieces of buss work are riveted together and can get loose or corrode.
To identify the location of the failed connection you will have to pull the fuse block out and use your multimeter and trace the path checking for continuity. Use you ohm scale and look at the readings. Don't rely on the buzzer it can lead you astray when looking for a poor connections.
When you find a bad connection the repair is to solder the joint.
Good luck.
Oh and Welcome Aboard!

[Dornon13]

Thanks.
Is it possible to remove the wires from the fuse block? Some of them are simple blades but alot of them go through the fuse block with the fuse clip actually crimped to the wire .

[HO455]

I don't believe that you can remove the ones that are crimped. I had this type of problem many years ago on my GTO and the fix was burnishing the area around the connection to clean it and soldering to reestablish the electrical connection.

[88Ironduke]

A DVM cannot show the health of the wire or current carrying capacity. Seeing 12 volts at a particular point of wire or measuring ground does not show the whole picture. You're going after damaged wire or partial ground.

You need a load test light. Wire up a tail light in a socket, switch your DVM to amps and put the light in series with the meter and run power thru the light at the battery directly post to post. Solder sewing pins on the wire as they work well to back probe connectors. See what amperage you're pulling with the light and the note brightness, you now have a standard to measure. Start at junctions and connectors with your load light. Isolate legs of wire. If you can light the bulb you have verified the wire is solid.

Brake pedal switch is good place to start. The bulk head connection going from inside and outside, rear connection and then the tail light assemblies themselves.

Paul

[dmjlambert]

I'm looking at the wiring diagram and notice the blinker power does not go through a fuse, but the brake light power does go through a fuse, although both of those wires light the same elements inside the bulbs. So, pull that brake light fuse, do an outstanding job of cleaning the fuse holder to get rust off of it. Perhaps use a dremmel tool or little wire brush. Then put a different fuse in there just to eliminate the fuse as a problem.

[Dornon13]

Originally Posted by 88Ironduke

A DVM cannot show the health of the wire or current carrying capacity. Seeing 12 volts at a particular point of wire or measuring ground does not show the whole picture. You're going after damaged wire or partial ground.

You need a load test light. Wire up a tail light in a socket, switch your DVM to amps and put the light in series with the meter and run power thru the light at the battery directly post to post. Solder sewing pins on the wire as they work well to back probe connectors. See what amperage you're pulling with the light and the note brightness, you now have a standard to measure. Start at junctions and connectors with your load light. Isolate legs of wire. If you can light the bulb you have verified the wire is solid.

Brake pedal switch is good place to start. The bulk head connection going from inside and outside, rear connection and then the tail light assemblies themselves.

Paul

Yeah I get it now, I don't know if you read my follow up comments but that's what im doing now. I am using one of the bulbs and jumpering power and ground to it from different sources. I found that the orange wire doesn't provide enough current straight from the fuse block even if I disconnected everything on that circuit.

[Gromit]

Recently I removed my bedsides for the first time on my 1970 C10 and I noticed there can be a less than optimal ground path connection between the sides that house the brake lights and the ground to the chassis. (see photos), and this would be even if you had good ground straps to the bed floor cross members.

Iron oxide is not the best conductor - maybe try running a dedicated ground all the way back to a known good ground?

[Dornon13]

Well I think I have got it sorted out.

The majority of the problem has been in the fuse block. I still haven't been able to get it to work with a fuse (multiple fuses) but I did get them to work with a jumper in place of the fuse.
I also found the two brown wires in the headlight switch were run together. This caused my tail lights to be on with the headlights off.

[firedemon]

being you found your problem something else to look out for , for others reading this thread . on mine the strain relief on the taillight housing was stuck to the wires that go through them and with the bulb having been in there for so long it kept the contact plate where the bulb contacts just enough away from the new bulb that the power wasn't getting to the bulb . I spent the better part of a day testing and putting in new grounds and 4 or 5 bulbs before I found that one .