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Old 07-05-2017, 12:35 PM   #1
JMD
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Rear signals won't flash

Hi all,
Almost finished with my '68 k10 frame off restoration except for an issue I'm having with the rear turn signals. All the wiring is brand new, all the lights are LEDs, and I have a no load flasher for both the turn signal and hazard circuits.

The issue is that the rear turn signals won't flash if the running lights/headlights are on. The front signals work fine, as do the indicators on the dash. If I have the running lights off the rear signals work just fine, but as soon as I pull the headlight switch out the brake light stays solid.

Before I start tracing wires and threatening to light the thing on fire, is there anything I should check right off the bat?
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Old 07-05-2017, 08:47 PM   #2
ray_mcavoy
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Re: Rear signals won't flash

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Originally Posted by JMD View Post
Before I start tracing wires and threatening to light the thing on fire, is there anything I should check right off the bat?
The first thing I would check would be the tail light grounds. Bad / missing grounds can cause all sorts of strange behavior. For example, it is possible for the turn signals to find an alternative path to ground through the running light circuit (when the running lights are off). But that alternative ground goes away when that circuit is live.

The factory ground path for many of the lighting circuits relies on the truck's sheetmetal & frame. And on a fresh restoration, paint could be preventing metal-to-metal contact between some of the parts, resulting in a bad ground.
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Old 07-05-2017, 09:40 PM   #3
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Re: Rear signals won't flash

Thanks for the reply,

I'm sure the grounds are good, but to be sure I ran a jumper wire straight from the negative battery post to the tail light housing and there was no change in performance. I also tried switching the pigtail leads and swapping out a different flasher. No dice. When I put a regular bulb in the socket, it functions properly.

On a hunch I pulled the LED park lights off and noticed they have the same wiring setup as the tail lights. When I plugged the park lights into the tail light socket they functioned perfectly.

So then I plugged the tail lights into the park light socket and had the same issue. Everything works fine until I pull out the headlight knob, then it stops flashing.

So...I'm thinking faulty tail lights? I spoke to LMC and they're sending me replacement tail lights, but they won't be here until for another week.
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Old 07-05-2017, 10:43 PM   #4
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Re: Rear signals won't flash

Okay, the jumper wire directly to the negative battery post is a good way to rule out a ground problem so it sounds like you're okay there.

And with the testing you have done (swapping the park & tail lights) it does sound like you are dealing with some faulty tail lights.
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Old 07-12-2017, 09:48 PM   #5
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Re: Rear signals won't flash

Just for grins how about connecting the jumper wire directly to the tail light socket instead of the housing? I'm wondering if there is a bad connection/crimp between the socket housing and tail light housing.

Or if the tail lights are installed jam a scribe or small screwdriver between the socket housing and tail light housing. That might allow a ground path from the socket to the housing.
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Old 07-12-2017, 10:43 PM   #6
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Re: Rear signals won't flash

Installed the replacement tail lights from LMC, same problem. Doesn't look like the tail lights are the problem.

I think I've completely ruled out a bad ground at this point. I cut the "socket" end of the tail lights off and hardwired it. I ran a jumper wire straight from the negative lead to the negative battery post, and crimped the running and brake light leads to the proper wires coming straight out of the fuse box. No change.

I installed an electronic flasher with a ground wire. No change.

The tail lights are manufactured by United Pacific and feature "sequential flash" and "Brake attention", which apparently means the lights will flicker one time when the brakes are applied. The flicker feature doesn't appear to be working, maybe that has something to do with the problem?

The back of the package says that a United Pacific LED turn signal flasher item #90652 is required (funny how LMC doesn't mention that in their catalog) I looked up that part number on Summit, and it appears to be the same as the flasher I just bought, just a different brand. Maybe there's something special about their flasher?

I'm going to call United Pacific tomorrow, hopefully they can steer me in the right direction.
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Old 07-13-2017, 10:47 AM   #7
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Re: Rear signals won't flash

Looked at your build thread, (nice work BTW,) and saw that you are using an LED array based on the pic of the park lights. Does the array simply plug into the standard 1157 socket base? I haven't tried those before, just LED replacement bulbs.

Try temporarily using a jumper in place of the flasher. That will eliminate the electronic flasher.

Last edited by damnyankee36; 07-13-2017 at 11:09 AM.
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Old 07-15-2017, 11:00 PM   #8
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Re: Rear signals won't flash

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Originally Posted by damnyankee36 View Post
Looked at your build thread, (nice work BTW,) and saw that you are using an LED array based on the pic of the park lights. Does the array simply plug into the standard 1157 socket base? I haven't tried those before, just LED replacement bulbs.

Try temporarily using a jumper in place of the flasher. That will eliminate the electronic flasher.
Yes they have a standard 1157 socket pigtail that just plugs in, VERY simple to install. I got everything figured out, turns out there is something special about United Pacific's flasher, installed the flasher today and everything works great! Thanks for the help guys.
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Old 08-14-2017, 11:11 PM   #9
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Talking Re: Rear signals won't flash

Just a side note, if it wasn't already mentioned, most LED lamps are polarity sensitive, meaning if installed the opposite way in the socket, they won't illuminate.

One more thing to try out, the intermediate harness (the four wire long harness that connects at the firewall and runs to the rear of the bed and attaches to your rear light harness, might be "Crispy" (weathered-dry rotted) and may be grounded somewhere along the chassis. I'd disconnect the harness at the rear tail light harness and probe the sockets for power when you turn on the flashers/turn signals etc...
Hope this helps!
Ben


Sorry, didn't see that you solved your issue.....
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Old 09-06-2017, 11:16 PM   #10
Robznob11
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Wink Re: Rear signals won't flash

The problem is that led flashers (digital )a re polarity sensitive . If you swap the two wires in the flasher it would work with any digital flasher. If you look the prongs are marked . The only other possible problem is that the led tails could be wired backwards for your truck.i have ran across 3 trucks so far where the wires for park and brake/turn are reversed, so when you plug the 1157 in they seem to blink fine until you turn on the lights. Thats because the dim side is blinking. When you turn on the parks the bright side lights up and you no longer can see the blink. One other issue i have ran into with full leds is when someone uses switch back leds in the front. The internal circuitry of the switch back will back feed with the white parking lights on causint the yellow turn to light up but no blink. When you turn the white park light off you kill the back feed and you then have enough load to make the flasher blink. If you ever think you have this issue then add a load resistor to check if it works. If so that is the problem. In this case you can either run a load resistor, or not run switch backs. But with all leds your first check is the flasher polarity. If it is reversed you may have enoung load to flash until you turn on the park circuit. I have seen it happen more than once. Also alot of flashers claim no load ,but they should be labled low load.....
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