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calico190xt68 
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Location: Frankton, IN
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 Topic: Replacement LED Light Help
Posted: 10 Jun 2026 at 5:45am |
This isn't an Allis issue but there are a lot of electrical experts on the forum. I continue to try and improve my 87 GMC General semi. I installed LED front fender mount pedestal turn signals and LED headlights. When the headlights are off, the turn signals flash correctly. When I turn on the headlights, then turn on the left turn signal, the right turn signal has a faint flash seen because the running light is on. The running light doesn't work on the right turn signal so it doesn't have a faint flash like the left one. That is something I am going to have to figure out separately. The problem is I don't want someone mistakenly think the semi is turning the wrong direction due to that faint flashing.
This didn't happen with the old non-LED headlights and non-LED turn signals. I put a 50w 6ohm resistor inline right before the turn signal light on both sides thinking this would mimic having the old headlights but it didn't change anything. I put a diode in place right before the turn signal and it didn't change anything.
I don't know how the turn signal circuit and the headlight circuit can affect each other but it does. My guess is that I need to place a diode earlier in the circuit to isolate these two circuits. Don't drive it much at night so not a huge problem but it is perplexing. I am in need of some ideas on how to debug this? I have seen online where motorcycles with one turn indicator dash light have this problem, but I have two turn indicator lights in my dash so not the same situation.
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PaulB
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Posted: 10 Jun 2026 at 6:58am |
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Having LED turn signals requires a special flasher compatible with LED bulbs. Also I suggest you have a ground problem as the symptoms will occur with incandesent bulbs and bad grounds.
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If it was fun to pull in LOW gear, I could have a John Deere.
Real pullers don't have speed limits.
If you can't make it GO... make it SHINY
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Les Kerf
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Posted: 10 Jun 2026 at 8:49am |
 calico190xt68 wrote:
... I put a 50w 6ohm resistor inline right before the turn signal light on both sides thinking this would mimic having the old headlights but it didn't change anything... |
Ummm... this will actually reduce the current through the circuit and also will reduce the voltage available to the LED; some LED's will operate on a fairly wide range of voltage so you apparently did not go too low or they wouldn't light up at all.
I too suspect grounding issues; there is no such thing as too much ground  Best wishes on your project.
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AC7060IL
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Posted: 10 Jun 2026 at 7:56pm |
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X3 that your truck has a ground issue. Guessing that it’s doghouse (hood/grill/fenders/ect.) probably hinges & tips forward? If so, then it’s electrical circuits for lights will need to have a designated ground wire going from hood circuit back to trucks main frame, or even all the way to batteries negative bar.
You could do a quick ground test by running a temporary ground wire from hood’s negative to batteries negative? If then it’s lights operate correctly ~ problem solved.
Also make certain that the hoods positive circuitry isn’t bared/slightly grounded? LEDs pull a much lower amperage. So if it has a slight grounding in its positive runs, it might not be enough pull to trip fuse / breaker ?
Edited by AC7060IL - 10 Jun 2026 at 8:01pm
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calico190xt68
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Posted: 11 Jun 2026 at 9:16am |
AC7060IL wrote:
X3 that your truck has a ground issue. Guessing that it’s doghouse (hood/grill/fenders/ect.) probably hinges & tips forward? If so, then it’s electrical circuits for lights will need to have a designated ground wire going from hood circuit back to trucks main frame, or even all the way to batteries negative bar.
You could do a quick ground test by running a temporary ground wire from hood’s negative to batteries negative? If then it’s lights operate correctly ~ problem solved.
Also make certain that the hoods positive circuitry isn’t bared/slightly grounded? LEDs pull a much lower amperage. So if it has a slight grounding in its positive runs, it might not be enough pull to trip fuse / breaker ? |
Yes, it is a hinged hood and tips forward. I am using the existing ground wires that worked for the incandescent lights.
I am going to do as you suggest and run a dedicated ground wire from the battery to each set of lights as a starting point. Will also inspect the wires but they are in a harness so may be hard to see everything without undoing the harness.
The head lights have a 3 wire plug with one of them containing the ground wire. My LEDS came with the 3 prong plug that fit exactly like the incandescent lights. I thought that was going to make things easy. i will have to rig up test wires that plug into the prongs to do this test.
I did inspect the ground wires closer. For the light that is blinking when it shouldn't, I do see a different ground wire than the original one one from the 3 wire prong. That could be the culprit.
As always thanks for the ideas and direction from everyone. I will report back after I dig into it. I thought this truck light fix was going to be super fast and easy, but of course not. I still have an air conditioner to fix on my 7010 that I need to get back to.
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80 7010, 80 7020, 68 190XTD, 67 190XTD, 500 Loader, AC 2000 Plow, Member Indiana A-C Partners, Member TAC
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Jordan(OH)
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Posted: 13 Jun 2026 at 7:23am |
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Resistor needs to go across from hot to ground not in line.
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calico190xt68
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Posted: 13 Jun 2026 at 4:37pm |
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I obviously need to learn more about resistors. Thanks for the info. I guess i need to know where exactly I wire from hot to ground?
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80 7010, 80 7020, 68 190XTD, 67 190XTD, 500 Loader, AC 2000 Plow, Member Indiana A-C Partners, Member TAC
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8070nc
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Posted: 13 Jun 2026 at 6:04pm |
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Where you put the resistor in line with the light just hook the hot wire and the wire going to the light and one end of the resistor together. Hook the other end of the resistor to ground
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calico190xt68
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Posted: 15 Jun 2026 at 8:22am |
Thanks for the resistor install help. I watched a detailed video on resistors and now I understand more how they are used. I did temporarily attach correctly in parallel but it did not help the problem.
I also ran an isolated ground wire from the battery terminal directly to the ground of the turn signal light. It did not stop the problem though which really surprised me.
I think I am going to have to now go back to the firewall, disconnect the wires running from the firewall to the turn signals, then run a wire from there to the light and see what happens. Essentially bypassing the wiring harness that runs in the engine bay. This truck has a wiring bar across the top part of the firewall where the dash wiring harness terminates and then the engine wiring harness takes over. If this doesn't fix it, then I have something wrong under the dash is my guess.
It is quite bizarre to see it. When I turn on the low beams and then use either turn signal, the high beam lights have a slight flicker to the cadence of the turn signal. As well, the left turn signal running light also has the same flicker when the headlights are on. It also doesn't matter if the the high beams are on or just low beams. Same thing happens. If I do not turn on the headlights both turn signals work fine.
I will also say that after I bought the truck two years ago, I found a mouse nest in the tachometer area behind the dash. A mouse may have chewed a wire causing this under the dash possibly. To me it has to be a bare wire related to the turn signals somehow touching the headlight or running light wires whether it was caused by a mouse or not, since I think I isolated the ground wire not being the problem. Because both head lights blink, I think it has to be under the dash or early in the circuit but I could be wrong too. I think I can also check the ohms on the wires to see if any are out of line but I haven't done that kind of testing before. Any ideas on additional debugging techniques would be appreciated. I am still learning.
Edited by calico190xt68 - 15 Jun 2026 at 8:23am
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TedN
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Posted: 15 Jun 2026 at 9:34am |
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Reading your original post I was in agreement with the thought you had a ground issue, but reading your testing results I think I would unhook the right turn signal and see if the problem goes away. I think your issue with the right running light and your flicker may be related, and since they weren't there before you installed new lights I think you may have a bad light.
Ted
Edited by TedN - 15 Jun 2026 at 9:35am
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190XTD seriesIII, 190XTD seriesI, maroon belly 7000, 7030, 7045, D14, 190XTD series??? project(or maybe parts)
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PaulB
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Posted: 15 Jun 2026 at 9:49am |
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Are you using a proper & correct flasher specificly for LED lights?
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If it was fun to pull in LOW gear, I could have a John Deere.
Real pullers don't have speed limits.
If you can't make it GO... make it SHINY
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calico190xt68
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Posted: 15 Jun 2026 at 10:14am |
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I disconnected the right turn signal and I am still getting blinking in the low beam and high beam headlights on the right side and left side when I turn on the headlights on low beam and I turn on the left turn signal. No matter which turn signal I turn on, the headlights have a faint blink in cadence with the turn signal blinker.
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80 7010, 80 7020, 68 190XTD, 67 190XTD, 500 Loader, AC 2000 Plow, Member Indiana A-C Partners, Member TAC
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calico190xt68
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Posted: 15 Jun 2026 at 10:21am |
I am not using a new LED flasher. I do not have hyper flashing occurring though. I still have enough incandescents in the circuit going to the rear of the truck that it is preventing hyper flashing, I believe. I do need to find the flasher module in the truck. It might be worth a try to replace it. But, that wouldn't explain the headlights blinking along with the turn signals. Some day, I may have all LEDs and then I would need to replace.
Even though I replaced the ground for the turn signal, I didn't replace the ground for the headlights. I am thinking it is a grounding issue on the headlights and not on the turn signals. The turn signals cause the problem and they aren't the issue. I will have to try and isolate the headlights and those ground wires. A little harder to do because they use the 3 prong plugs but I can rig some wires up.
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80 7010, 80 7020, 68 190XTD, 67 190XTD, 500 Loader, AC 2000 Plow, Member Indiana A-C Partners, Member TAC
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TedN
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Posted: 15 Jun 2026 at 11:20am |
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If your headlights are on a common ground you can pull one and hook a ground wire into the harness there, then just test with one light out. I also wonder if there is something elsewhere in the truck that might be causing the issue but it wasn't noticeable with incadescent lights? Really hard to know. If you don't have a multimeter I would recommend getting one, maybe two. One digital to get precise voltage readings and one analog to see flickers. You can also get test leads, Harbor Freight has a set that winds up so they are tidy to store. With those tools you should then be able to string your test lead to the battery and test at the headlight to see which side is giving you your problem. If the ground stays steady while the hot fluctuates there is no sense chasing a ground issue. You are past the common issues, now you need to start narrowing down where to look.
Ted
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190XTD seriesIII, 190XTD seriesI, maroon belly 7000, 7030, 7045, D14, 190XTD series??? project(or maybe parts)
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Les Kerf
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Posted: 15 Jun 2026 at 12:55pm |
calico190xt68 wrote:
... No matter which turn signal I turn on, the headlights have a faint blink in cadence with the turn signal blinker. |
I have found that LED's are ever so much more sensitive to slight voltage variations compared to incandescent bulbs; complete isolation is about the only cure, and may not be possible on a vehicle due to the very slight differential in potential from one end of the truck chassis to the other end.
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