Turn signals give lamp out message only when headlights are turned on

2003RAM

MEMBER
2003 DODGE RAM
5.9L
175,000 MILES

I have a 1500. Two weeks ago a deer hit the passenger headlamp an busted it. This gave me a lamp out signal when I had to turn to the right side. I purchased new headlamps with new bulbs already installed. I replaced both drivers and passenger headlamps and now when the headlamps are on, and I use either turn signal I get a lamp out signal. First replaced all of the rear lights but no good. Then I isolated the problem to the turn signals only when the headlamps are lit and turn signal is on. I used the original drivers side turn signal bulb (4157 NAK) to replace the (4157Na) bulb that came the new head lamp and that, stopped the lamp, out chime for a few days only on the drivers side. So I thought the bulbs were the issue until tonight while having the lights both turn signals gave me a lamp message again. I don't what tells the truck it has a lamp out or why the turn signals cause a lamp out signal only while the head lights are on. Any ideas?

3 Replies

JACOBANDNICKOLAS

MECHANIC
109,885 POSTS

Hi and thanks for using 2CarPros.

The indicator light you refer to is controlled by the front control module (FCM). This module is located in the totally integrated power module at the left front of the vehicle (under the fuses). I don't think the deer caused an issue with the module since it was the opposite side, but I really question a ground issue. I don't know the extent of the damage or if any of the wiring was damaged. I attached a couple pictures for you to see.

The first picture shows the top of the power distribution box. The second is a portion of a wiring schematic. I am focusing on the front signal lights since that is where the deer hit. If you note, there are two grounds for the lights, one on each side. First, locate the grounds using the attached schematic. Second, and specifically the right side, confirm a good ground and wiring condition.

If they appear good, then I want you to check the splice that is indicated on the right signal wiring. In the picture, it is identified as S101. That is where a group of wires come together for ground. If you look at picture 3, it shows that the headlight uses the same ground. What I suspect may be happening is you have a weak ground there. When you turn the headlights on, the ground fails when the signal is added and it becomes similar to an open circuit which in turn forces the FCM to turn the indicator light on inside. When you replaced the bulb and it worked for a period of time, you may have moved enough things around that it allowed them to work temporarily.

Now, I would focus on the right side ground as well as the splice on that side. Also, check the wiring for damage to ground. I added picture 4 to help you find the splice location. Again, it is S101. Do the same with that, confirm that noting is corroded, loose, or damaged.

Finally, picture 5 shows ground (G105) location. Picture 6 shows ground (G105) position. Make sure they are good. Keep in mind, even of everything appears good, you may want to check for continuity while moving them around as well as when the headlights are on and you turn the signal on too. Ground G103 can be found on picture 4.

Here are a couple links that you may find helpful:

https://www.2carpros.com/articles/how-to-use-a-voltmeter

https://www.2carpros.com/articles/how-to-check-wiring

Let me know if this helps or if you have other questions.

Take care,
Joe

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