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Old 06-03-2021, 01:30 PM   #1
Peepaw
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Wiring turn signal question, purple wires

Good afternoon guys.

I have a question regarding my 72 C10. I am rewiring entire truck, and the diagrams that have been posted have been HUGE help, but I have a question regarding the wires coming from the turn signals.

The purple wires go into the plug/junction box at firewall.
then....they come out on other side as one brown wire? And then it goes to fuse box?

Then the dark blue wire and light blue wire goes into the "u" shaped connector under steering wheel. Then they go into the connector that plugs into instrument cluster.

What has me confused is there is really not a clear (that I have been able to find) explanation for the connector at instrument cluster connector and wiring for blinkers/hazards and the relays assocaited with those. Anyone have a picture of wiring for that?

Thanks, Derrick
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Old 06-03-2021, 01:54 PM   #2
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Re: Wiring turn signal question, purple wires

There's information on the gauge cluster connector pinout in this thread:
https://67-72chevytrucks.com/vboard/...d.php?t=645940
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Old 06-03-2021, 03:32 PM   #3
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Re: Wiring turn signal question, purple wires

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Originally Posted by pjmoreland View Post
There's information on the gauge cluster connector pinout in this thread:
https://67-72chevytrucks.com/vboard/...d.php?t=645940



I will check it out. Thank you sir.
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Old 06-03-2021, 06:18 PM   #4
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Re: Wiring turn signal question, purple wires

Still not finding what I need.

That link had some info in regards to the instrument cluster (and how to convert it), but still missing info I need.


I am looking for the wiring schematic for turn signal/flasher/relay.

In particular, the 2 blue wires from turn signals up front, run to firewall junction and then come out I believe in a single brown wire.
I THINK that wire runs to light switch. But Im not sure if that wire is key on power (pretty sure it is), but also the brown wire out of fuse box and runs to the "U" shaped connector under steering column.

The wiring diagram posted up is PERFECT detail for everything, except turn signals and flasher relays.
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Old 06-03-2021, 06:40 PM   #5
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Re: Wiring turn signal question, purple wires

Schematics that show the functionality of the various switches and relays are a little harder to come by. You might take a look at the wiring diagrams available for download here:

http://67-72chevytrucks.com/vboard/s....php?p=5781194

You'll find stuff like in the attached screenshot.
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Old 06-05-2021, 11:32 AM   #6
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Re: Wiring turn signal question, purple wires

Thank you for your help, but honestly, I still got nothing. The link works, but it might as well be in Greek.


Still looking for wiring routing for turn signals , in particular the "U" shaped plug under the steering column and the connector for the instrument cluster.

Wording such as "brown wire goes here" would be preferred LOL
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Old 06-05-2021, 11:36 AM   #7
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Re: Wiring turn signal question, purple wires

Let see if this works:

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Well, sort of. Ok, in this diagram the top right plug...goes to instrument cluster with gauges. I have a 72 C10 instrument cluster with oil pressure, temp gauges. My harness has 2 TAN wires in same pin location. On the schematic above, it does not show the 2 tan wires.


Also, the U shaped connector on the left...the purple wire, yellow, etc...all say "goes to directional hazard...." but I dont see where they actually go
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Old 06-05-2021, 11:59 AM   #8
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Re: Wiring turn signal question, purple wires

Maybe you need to track down where those two tan wires are coming from in order to figure out where they need to go.
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Old 06-05-2021, 12:55 PM   #9
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Re: Wiring turn signal question, purple wires

Headlights and side marker lights work fine. Bright lights also work fine.

Right hand turn signals work fine (as does right rear tail light and right front turn signal lamp)

Left hand tail light and left front turn signal bulb are very dim. When I turn on left hand turn signals, both tail light and front turn signal indicator both go off.

With no turn signals on, if I hit brake pedal, the left rear tail light goes off (and brake light does NOT illuminate). The right rear brake light works just fine.
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Old 06-05-2021, 12:56 PM   #10
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Re: Wiring turn signal question, purple wires

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Originally Posted by pjmoreland View Post
Maybe you need to track down where those two tan wires are coming from in order to figure out where they need to go.

That is one of my problems. I have no idea where they go. It's a long story but the harness has been cut and those 2 tan wires only have @ 6" wire coming out of the connector.
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Old 06-05-2021, 01:10 PM   #11
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Re: Wiring turn signal question, purple wires

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Here is my connector.

Pin 6 should be temp gauge.(it is)

Pin 5 should be empty (it is)

Pin 4 should be tan for fuel gauge (it is)

Pin 3 Should be Pink fuel gauge feed (it is)

Pin 2 should be Tan and for brake lamp warning (why do I have 2 ?)

Pin 1 should be black/white and goes to alternator (it is)
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Old 06-05-2021, 01:31 PM   #12
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Re: Wiring turn signal question, purple wires

Oh, I forgot to add:

My interior dome light works ONLY when the headlight switch is installed and in place (ie grounded to body). BUt if I take head light switch off the body, my interior dome light does NOT work.

Im trying to add as much info as possible
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Old 06-05-2021, 01:37 PM   #13
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Re: Wiring turn signal question, purple wires

There are minor differences in the wiring harness from one year to the next, and variations depending on options. You can find year-specific schematics in chapter 12 of the factory service manual for your truck (available for download in the link I sent previoisly). Does/did your truck have a second fuel tank?
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Old 06-05-2021, 01:45 PM   #14
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Re: Wiring turn signal question, purple wires

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Originally Posted by pjmoreland View Post
There are minor differences in the wiring harness from one year to the next, and variations depending on options. You can find year-specific schematics in chapter 12 of the factory service manual for your truck (available for download in the link I sent previoisly). Does/did your truck have a second fuel tank?
No sir. 72 C10 402, LWB, 2WD, Turbo 400, AC, 12 Bolt, single fuel tank.
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Old 06-05-2021, 01:51 PM   #15
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Re: Wiring turn signal question, purple wires

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Originally Posted by Peepaw View Post
Headlights and side marker lights work fine. Bright lights also work fine.

Right hand turn signals work fine (as does right rear tail light and right front turn signal lamp)

Left hand tail light and left front turn signal bulb are very dim. When I turn on left hand turn signals, both tail light and front turn signal indicator both go off.

With no turn signals on, if I hit brake pedal, the left rear tail light goes off (and brake light does NOT illuminate). The right rear brake light works just fine.
Probably not much help, but my son had a very similar issue with his 69. We grounded the tail light housing to the frame and all his light issues were fixed. At least for now. LOL. He has a stepside, don't know if that matters or not.
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Old 06-05-2021, 03:06 PM   #16
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Re: Wiring turn signal question, purple wires

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Probably not much help, but my son had a very similar issue with his 69. We grounded the tail light housing to the frame and all his light issues were fixed. At least for now. LOL. He has a stepside, don't know if that matters or not.


Well, that actually HELPED! I have the bed up on jack stands and realized I didnt have bed grounded!!!!!

I still have ONE problem:

All my lights work (on the rear), EXCEPT when I have the RIGHT turn signal on, and I hit the brakes the LEFT brake light does not come on.

If I turn the turn signals OFF, and hit the brake pedal, both brake lights work.

If I turn turn signal on (left) and hit the brake pedal, the RIGHT brake light works.

So again (for the rear), if I have right turn signal on, the left brake does not work.

Baby steps. LOL
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Old 06-05-2021, 03:17 PM   #17
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Re: Wiring turn signal question, purple wires

This sounds like maybe your turn signal switch is gummed up. With your right turn signal on and the brake pedal pressed, does the left brake light come on if you wiggle your turn signal or move it slightly away from the fully on position?
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Old 06-05-2021, 04:51 PM   #18
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Re: Wiring turn signal question, purple wires

In addition to good bed to frame grounds, you might consider a ground from each housing to the bed or frame. That's what I did on mine just to make sure there were no gremlins messing with the tail & brake lights. It may not fix the strange issue you're having, but who knows. You probably already checked, but make sure you have the correct (1157?) bulbs in each socket. Maybe also check the sockets to make sure they're clean and the contacts at the base are lined up correctly. Edit: I see REDROCKER already suggested the extra ground straps.
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Old 06-05-2021, 08:27 PM   #19
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Re: Wiring turn signal question, purple wires

The second tan wire goes to the ignition switch. I'm thinking it is a bulb check circuit.
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Old 06-05-2021, 08:31 PM   #20
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Re: Wiring turn signal question, purple wires

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Originally Posted by pjmoreland View Post
This sounds like maybe your turn signal switch is gummed up. With your right turn signal on and the brake pedal pressed, does the left brake light come on if you wiggle your turn signal or move it slightly away from the fully on position?

Not sure. I will check on that.
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Old 06-05-2021, 08:37 PM   #21
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Re: Wiring turn signal question, purple wires

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Originally Posted by tim_mc View Post
In addition to good bed to frame grounds, you might consider a ground from each housing to the bed or frame. That's what I did on mine just to make sure there were no gremlins messing with the tail & brake lights. It may not fix the strange issue you're having, but who knows. You probably already checked, but make sure you have the correct (1157?) bulbs in each socket. Maybe also check the sockets to make sure they're clean and the contacts at the base are lined up correctly. Edit: I see REDROCKER already suggested the extra ground straps.

New sockets and yes, correct new bulbs 1157.

All my marker lamps and sockets are new, including tail light assemblies.

95% of the wiring on the truck is new. The only left overs, are the socket that goes into the instrument cluster, and the "U" shaped plug under steering column....and the turn signal switch.

Ignition switch, head light switch, etc....all new.


The old harness was rotted/exposed and is why its all new and using as little from old stuff as possible.

The thing that is baffling me is:

I had EVERYTHING working just fine. Removed bed from frame, and BAM. This problem.

So you would THINK it would be something l did like remove bed from frame, but now that I have ground problem fixed, its still a no go.

I guess it's POSSIBLE I have bad luck and turn signal switch took a dump and pure coincidence, but part of me is telling me its something else.
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Old 06-05-2021, 08:38 PM   #22
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Re: Wiring turn signal question, purple wires

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Originally Posted by BigBird05 View Post
The second tan wire goes to the ignition switch. I'm thinking it is a bulb check circuit.

Do you have any idea if leaving it unhooked would/could cause problems I am having?
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Old 06-05-2021, 10:38 PM   #23
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Re: Wiring turn signal question, purple wires

The terminal with two tan wires in it is for the brake warning light. As BigBird05 pointed out, it is connected to the igniting switch so you get confirmation that the bulb is working when you are starting the engine. I doubt these wires have anything to do with your brake light issues.
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Old 06-06-2021, 09:31 AM   #24
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Re: Wiring turn signal question, purple wires

Good morning. I am going to post some pics here in a little bit, and I think/maybe/hopeful I have found some issues. The wiring schematic I have been going by (provided by this great forum), shows some wires that are slightly different locations off ignition switch/pigtail and wiper switch.
HOWEVER, you do not notice it the differences unless you are REALLY looking at the harness and trying to do what I am doing, and now having problems like I am.

SO, I am studying EVERYTHING trying to find out why I have this problem, and I am now on a new computer and can zoom in on the wiring schematic and see possible issues. Again, I will take pictures of what I am talking about as pics are easier to describe the problem.

Not saying I have FOUND the problem, but I am hopeful after spending all night last night studying the schematics and comparing to my truck and the aftermarket components I have installed. I will post pics, then today I am going to put my theory to test and report back to yall.

I can not state it enough on how much information I have found on this forum (even if some of it may have taken me in the wrong direction LOL).
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Old 06-06-2021, 10:01 AM   #25
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Re: Wiring turn signal question, purple wires

Ok, here we go. I will post pics, then will add descriptions and my questions comments after.
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factory pigtail ignition switch.
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