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Old 03-02-2018, 10:21 PM   #1
SabbathFanatic
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Headlight/ Turn Signal Bug

OK, so I'm getting close to done with all the wiring on my project. Everything under the hood works great, my LS swap fired up first try.

I've got several issues with the original wiring in the interior though, and I'm not sure where to start with diagnostics. I can read a wiring diagram and follow LS swap wiring instructions just fine but I have no idea how to diagnose a headlight bug.

Here's what's going on:
  • Headlights only come on with brights
  • Turn signal indicators both come on steady when headlight switch is on
  • Gauge lights will not come on with headlight switch
  • Nothing happens when you go left with the turn signal lever, right works fine
  • Hazards work fine
  • Reverse light fuse is burnt out but the rest are good
  • Flashers are probably pretty old
  • Buying new fuse and flashers tomorrow, kind of hoping that magically fixes everything but looking for some advice in case it doesn't

Thanks in advance!
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1971 C10 - LQ4/4L80E swap in progress
1988 K3500 - 8.1 swap planned

Last edited by SabbathFanatic; 03-06-2018 at 08:03 PM.
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Old 03-03-2018, 07:08 PM   #2
dmjlambert
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Re: Headlight/ Turn Signal Bug

Take it one problem at a time. Search this forum using the search field at the top of the page, for each problem and see what others have found. This was my favorite answer to turn signal problem:
http://67-72chevytrucks.com/vboard/s...d.php?t=645408
I ran an alligator clip jumper wire between the purple wire on the connector to the other color wires as described and verified the various lights worked and blinked, and narrowed the problem to the turn signal switch. So I replaced the switch.
Hi-beam headlights only could be bad headlights, or bad dimmer switch. You can use a probe type light-up tester to see where you are getting 12V in your wires, or use a multimeter set to a DC volts scale. Connect the negative lead of the tester to a good ground, shiny metal of a bracket or bolt under the dash somewhere.
The bulbs can be burnt out for the instrument cluster lights. They are replaceable by reaching up behind the instrument cluster and twist to unlock them and pull them out, and push in new bulbs. The problem with instrument cluster could also be a bad headlight switch, or the switch is simply turned to the full dim position.
Wiring diagram:
http://67-72chevytrucks.com/vboard/s...&postcount=302

Tackle one thing first and then post back with any specific questions about that item. Good info to include in your post is whether you are using regular bulbs or have LED bulbs.
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Old 03-03-2018, 07:22 PM   #3
dmjlambert
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Re: Headlight/ Turn Signal Bug

Other posts you may find useful.
If you have a gauge cluster, here is the diagram:
http://67-72chevytrucks.com/vboard/s...d.php?t=709847
Fuse block diagram:
http://67-72chevytrucks.com/vboard/s...d.php?t=741245
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Old 03-06-2018, 08:09 PM   #4
SabbathFanatic
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Re: Headlight/ Turn Signal Bug

Ok, I replaced the flashers and dimmer switch. Here are the new symptoms:

Headlights off:
  • Turn signal lever either way causes indicators to flash for both directions, and gauge lights flash too
  • Hazards do the same thing

Headlights on:
  • Gauge lights and both turn signal indicators on steady
  • Turn signal lever and hazards do nothing

To answer a few of your questions, the turn signal switch and headlight switch are both brand new. I am using Dakota Digital gauges.
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1971 C10 - LQ4/4L80E swap in progress
1988 K3500 - 8.1 swap planned
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Old 03-07-2018, 07:32 AM   #5
dmjlambert
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Re: Headlight/ Turn Signal Bug

I think you have arrived at the point of needing to check grounds. The front turn signal housings need to be grounded (attached to clean metal somewhere on the body or frame), the engine needs to be grounded with several straps, the cab needs to be grounded to frame, the instrument cluster needs to be grounded to the cab, the rear turn signal/brake light housings need to be grounded to frame and body. You can check which the culprit is by using alligator clip wires for troubleshooting, and get some ground straps and make the connections permanent. The wires in the truck are 1/2 of the job, the return of the current flow to the battery is provided by the body and frame.
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