No driver's side rear turn signal or brake light, because no power at bulkhead

Hi, thanks in advance for help. Sorry to post this but I couldn't find my particular problem.

Everything works for both front and rear signal lights, all markers, signal indicator light on dash, hazards, and rear brake lights and backup lights, except for the rear signal light and brake light on the driver's side. These are powered by wire #18 Yellow as seen in the GM 1980 Wiring Diagram (ST_352_80_1980_Chevrolet_Light_Truck_Wiring_CK_10_30_Only.pdf). I unplugged the 4 prong rear light wiring harness from the bulkhead on the engine side of the firewall. Since the passenger side works, and after a bunch of testing, I tried connecting the passenger side tab from the cab (#19 Dk Green) to the #18 circuit in the harness going to the rear of the truck. The rear driver's side signal and brake lights worked fine. I also confirmed with the meter that while I was getting 12V from the #19 Dk Green tab from the cab, I was getting nothing from #18 Yellow. So I need to know why there's no power at the #18 Yellow tab from the cab when the signal is on or when the brake is depressed.

The picture I clipped and attached from the Diagram, "bulkhead_cab_side.jpg", shows the bulkhead and 4 wires exiting the firewall into the cab with #18 yellow (physically) the second plug from the top. The wire continues uninterrupted to a connector in "termination_of_18_yellow.JPG". What is that connector, and where is it physically? Is it on the steering column? Where exactly? I don't think that connector is the signal switch... I believe the signal switch is at the far right of the picture, and is inside the column near the steering wheel. I think the connector I'm asking about is near the bottom of the column and outside of it. Correct?

I also labelled the wire from the brake pedal switch, to illustrate that it goes directly to the signal switch. I think that confirms it is not part of the problem, since the signal light also fails. Yes, the signal switch could be the problem, but I should confirm if I have power at the connector I'm asking about first.

Look at the opposite side of the steering column from the electrical part of the ignition switch. You will see a long skinny electrical junction. You can get to all the signal switch wires there for testing.

You probably need a turn signal switch. There are two dogs in the switch that move (literally) between the brake and turn signal input and complete the circuit when the lever is moved. The right side dog is probably burned out, i would say this with near certainty if the front blinker works fine as it operates independently from the dog, and you obviously have voltage from the brake pedal switch since the other side works. The yellow wire could be broken under the dash, but that rarely, if ever happens since its not a nasty environment under there

Mark80K15 said:

I think the connector I'm asking about is near the bottom of the column and outside of it. Correct?

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Attachments show what it looks like and the area it should be at.

Thank you all. @Jgonick, "turn-signal-switch-plugged-in.jpg" is very helpful. Are you showing the connector that I'm asking about or the signal switch? Or are you saying they are the same thing? I think @Ricko1966 refers to the connector in your picture as a long skinny electrical junction, so I think he is referring to the connector I'm asking about, not the signal switch. I ask because of the name of your picture.

Does the harness on the left side of the picture go up the column to the signal switch, or the harness on the right side?

The colors on the harness seem to match the schematic I posted in "termination_of_18_yellow.JPG". It has #19 Dk Green and #18 Yellow as the 2nd and 3rd wires from the top. Is that correct?

Does the connector have to be removed from the column in order to unplug it for testing? Once I unplug it I should be able to do a continuity test on #18 Yellow from there to the bulkhead, because it is continuous between those two spots, correct? And if the continuity is good, the problem is from the connector up to and including the signal switch, in which case it is most likely the signal switch itself as @77Dmax says, since it's not a nasty environment up the column?

Mark80K15 said:

What is that connector, and where is it physically? Is it on the steering column? Where exactly? I don't think that connector is the signal switch... I believe the signal switch is at the far right of the picture, and is inside the column

Click to expand...

I reread your question- If I understand what you are asking- It is just the inside portion of the bulkhead. You are correct that the turn signal connection is on the column.

Jgonick said:

It is just the inside portion of the bulkhead

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No, I think the connector in "bulkhead_cab_side.jpg" is the inside portion of the bulkhead. I just included that to help my narrative. I'm not asking about that at all. I'm asking about the connector in "termination_of_18_yellow.JPG".

Mark80K15 said:

Does the harness on the left side of the picture go up the column to the signal switch, or the harness on the right side?

Click to expand...

the longer connector goes up the column to the the turn signal switch.

The inside of the bulkhead is behind the fusebox. It generally isn't the problem. (last place I would check) I would check the connection in the attachments. -much easier to get to.

Mark80K15 said:

I'm asking about the connector in "termination_of_18_yellow.JPG"

Click to expand...

yes- that is what is in the attachment pictures.

we are playing tag.

Once I unplug it I should be able to do a continuity test on #18 Yellow from there to the bulkhead, because it is continuous between those two spots, correct? And if the continuity is good, the problem is from the connector up to and including the signal switch, in which case it is most likely the signal switch itself

Click to expand...

yes you are correct

Ok, @Jgonick thanks very much.

If you have a tilt column the wire might be broken at the swivel point. Usually there isn't enough space to repair it though. A new switch comes with that harness.

Yes thanks, @77Dmax , I have a tilt column. I see the switch on Rock Auto and it clearly illustrates the switch together with the harness, right down to and including the connector I was asking about. The wires are colored correctly as the GM original. I'm going to disconnect the connector at the base of the harness and test for continuity from there to the bulkhead. If that works, I will connect #19 Dk Green at the base of the harness to #18 Yellow on the other side of the connector (which goes to the bulkhead) and operate the right turn signal. If the rear left signal operates, that means something is wrong with #18 Yellow in the signal switch/harness/connector assembly, which can be replaced with the switch from Rock Auto. I'll let you know what happens.

Ok, got a happy ending. I tested for continuity from the firewall side of the connector (at the steering column) to the bulkhead in the engine side of the firewall, and there was continuity (thank god). Then, with that connector disconnected, I switched to IGN on and was going to jump #19 Dk Green to #18 Yellow as described above. But I found that the signals are inoperative when the connector is disconnected. So I couldn't do that final test and concluded the signal switch was toast as as @77Dmax suggested.

Then I plugged the connector back together (good and engaged, with di-electric grease), and tried all functions to make sure I didn't break anything, and the driver's side rear turn light and brake light WORKED! Very surprised, I guess the truck gods decided I had screwed around enough. I'll tear the column down and replace that signal switch if it happens again, though. Thanks all!