Left blinker stays on with running lights.

Stuxnet1492

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Mar 27, 2019
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Just bought 1999 ford mustang gt. They had some ugly tail lights and it had gotten some front end damage so the car has all new headlight and taillight assemblies. The passenger side taillight was filled with water, so the reverse light and the light with only 2 wires going to it were rusted. I replaced the plug for the bulb, new bulbs all around, and the issue i have is that the blinkers dont work like they should. When in daylight and i have the headlights or running lights off, the blinkers work perfect. All lights work perfect. But when i turn the running lights on, the driver side blinker dash lights stays solid, but will blink when i turn the blinker on. The blinkers in the front and back work but they blink fast. Right side works perfect. If i turn hazards on, the rear blinkers work right, but the front blinkers alternate so it blinks left, right, left, right instead of at the same time. I turn off the running lights and its back to normal. Any clue what may be going on? I have replaced fuses as the water in the taillight blew the fuse. All fuses are good, i havent changed the flasher modules in the dash, or relays. How should i go about finding the solution?
 
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wmburns

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If this were my car one of the tests I would do is to CONFIRM that both tail lights have a solid ground path that is capable of carrying a current. Best to use a test light that will "load" the circuit.

What I suspect is happening is one side has a weak ground and the brake lights are back feeding through the common terminal for the parking lights.

Howto perform charging system voltage drop test
https://www.stangnet.com/mustang-fo...perform-charging-system-voltage-drop-test.56/
 

Stuxnet1492

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I will have to find a wiring diagram of the rear tailights and verify the wiring is correct. Do you know what amperage flows through the wires when under load? The multimeter i have is able to measure current up to 10amps max.
 

wmburns

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Aug 14, 2009
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Recommend that you not over think this. It's possible to confirm a ground with a simple Volt-Ohm meter (VOM) or a test light.

Besides taking DC current measurements with an old style VOM is a PIA because the meter has to "pass through" the VOM shunt. Much easier to use a clamp on style current meter.

With a VOM meter set on DC, put on lead on one of the ground wires (Black or black with a white stripe). Put the other lead on battery negative. May have to add a piece of wire long enough to reach battery negative. Read the voltage drop directly off the VOM.

If the voltage drop is high (anything over 0.5 volts), try using the scrap wire going back to battery negative to "boost" the existing ground. If the problem improves then you know you are on the right track.

If getting a long piece of wire is too hard, then try maybe using a shorter wire to the rear trunk ground. Or use the ground on the other tail light as it's likely working.
 
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Stuxnet1492

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Mar 27, 2019
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So ive tried to check everything, nothing is out of the ordinary. I havent posted lately, so sorry for the delay, but i had a thought about 5 minutes ago that i forgot to mention that there are led bulbs in the mustang. I have not tried halogen bulbs yet but would it cause that to happen? The car got into a wreck, very small vender bender and busted the driver side headlight assembly before i bought the car. I replaced the front end, no fram damage, and installed led healight bulbs i had for my 97 f150. Would led bulbs cause a power difference for the blinker? The blinker only blinks fast with running lights or headlights on. Blinks like it should when them off.
 

Stuxnet1492

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Mar 27, 2019
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Just drove it again and noticed the headlights acting up as well. Parked behing other cars at night or at a store front at night i can see the reflection on my car, so i tried to change between high and low beams, and theres no difference on the driver side. On the passenger side, changing from low to high, the led gets dimmer. I dont have fog lights installed either. I will check to see if new halogen bulbs changes anything tomorrow morning and post and update.
 

Stuxnet1492

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Mar 27, 2019
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Stuck once again. I unplug the driver front binker bulb and the interior dash light turns off. I plug it back in and it comes back. I have cleaned and tried to check wires with a multimeter and check fuses and i have found nothing yet. The plug is a little burnt but there is no crossed connections in the plug. The flasher switch works like it should and even when unplugged, the light stays on. Unplugged the flasher relay under the dash and no change. I have swapped both turn signal bulbs in the front and have found nothing new. What do i check next?
 

wmburns

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Aug 14, 2009
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I still think that you have a bad ground. Either in:
  • Rear trunk
  • Front radiator core support (both sides).
  • behind center console.
Since you know where the flasher module is I would suggest confirming there a good ground path back to battery negative.

You have to expect there might be "issues" when using LED bulbs as many flashers units are calibrated to the lamp load.
 

Stuxnet1492

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Mar 27, 2019
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What would be a good way to check that the flasher has a good path to ground? Also another issue has arose that i do not like but i know why its there. When checking the plugs for the turn signal switch on the steering column, i removed the ignition cylinder and there is a small strip of metal that goes along that ignition cylinder that broke off. There is a small contact pin that goes into that cylinder that controls when the "alarm" beeps in the car. When my door is open, the car constantly dings. The same ding you hear when you open the door with headlights on to alet you to shut them off. So the pin that goes into the cylinder doesnt have that metal strip to contact for the dinging to make the sounds at the right time. Could i rewire that pin so the sound doesnt chime with the door open? It isnt a big issue, im just wondering. But as far as the blinker goes, what would be the best way to check the ground to the flasher and check the ground around the radiator core?
 

wmburns

SN Certified Technician
Aug 14, 2009
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Houston Texas
I gave you a detailed link on how to perform voltage drop tests. There are how to videos with examples and TONS of explanations on voltage drop. Did you study it?

But just in case it was TLDR; take a test light with the pig tail on battery positive. Test by probing on battery negative. Confirm that the test light lights.

Now go to the flasher module. Test each of the ground wires with the test light probe. The ground wires are black or black with a white stripe. A good ground should brightly light the test light.

As far as the grounds on the radiator core support. LOOK at them. Are the CLEAN (no rust or corrosion). Are they tight? If this were my car and they didn't look perfect I would take them apart and CLEAN everything. The goal is to have clean, bright, shinny, metal to metal contact. Di-electric grease can help prevent corrosion.

I get the impression that you don't see HOW a weak ground in the trunk can CAUSE the brake lights to come on when the running lights are on. Remember that the brake lights and the running light SHARE a common terminal. When that common terminal isn't connected to a strong ground, then electricity will always try to take the shortest path. In the case of a weak ground that path is through the brake lights.

A new lock cylinder is not all that expensive. Almost any lock smith can re-key to match your current key. No house call required. Just take the cylinder to them. Or you you are handy try doing it yourself. Much easier than trying to hack up the wiring of a perfectly good car.

More Information for MOTORCRAFT SW6383
https://www.rockauto.com/en/moreinfo.php?pk=510833&cc=1354722&jsn=436&_nck=ZExt1ysfqhqWnq+EvUNdS3Txyey60IDtnjPWF5UU9J0BWQv7pvqZbkEUk86+AYIh/8vGRbRbhjvDMrbf6g144tO2IQkPfsfBB2DHIp9eavjunNJbNgu5KgiHbtBtUbmsxEU6pTSRTP8vK/1179NkfrKDS3zsuLzNBYkzJMwueB3KD2z9jHHY5oGTy6Wl/8BwIMqI/d1/L3Z29QGQN/w2jeOcgvFytU3sPYVcq/fm83t2FV/uM4rvLiIH/RH8oGhnBvjq1VewfhyZDWHBYCITZKCJg9l3a/mxvFNncSneFdBUTVcSob1Hnqx7WM3wrogcc9G0ggkJ8By3QeTKCW2JrWUt4jxYtSBa
 
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Stuxnet1492

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Mar 27, 2019
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Georgia
Well, today i fixed it. Didnt have much free time from work and college, but i went to a junk yard and cut off blinker light plugs so i had the right color code and new plugs, and replaced them by splicing them in. Also, i replaced my dash lights with led light and changed my headlight bulbs back to leds. All of that fixed it. Mainly the blinker plugs, but when i changed the dash lights in the instrument cluster, the interior left blinker light that was solid was now gone with running lights or headlights on. They still blinked fast with those on so i replaced the plugs and it fixed it. I think the wires for running and blinker was swapped when the previous owner spliced into it for the halos he had. But it is now solved. Thanks for the help
 
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