2003 Gulfstream Class A (freightliner chassis). This morning when I checked my turn signals prior to hitting the road I encountered this problem: regardless of whether signaling a left or right turn, both sides of my lights flashed. I double checked to assure the hazard lights switch was in the off position, changed both relays for the hazard lights, replaced the turn signal fuse, all of which failed to solve the problem. Any ideas what to check next other than a possible short in the wiring harness?

Yes, I know exactly what is wrong.The two lights that are flashing are actually flashing the thin running light filament. Take a wire from the battery negative, and connect it to the side of the bulb. If everything starts working again, it's that ground wire that is not properly connected to ground.What happens is like this. The current comes to the base of the thick filament of the bulb, then cannot exit out the side and go back to the negative because that connection is not there. So it then goes backwards through the thin running light filament and over to the other side and through that running light filament, then it goes out the good bulb on the side and to ground.The best way to prove this is what is going on is to take the red lens cover off, and look at the bulbs and the filaments inside them. In your case, the current is going through the thick brake/turn filament, then the thin filament on both sides. You will likely see the thick filament barely glowing, but the thin ones are more resistive to current so more voltage drops across them and they glow brighter.

Are you 100% certain you don't have a bulb burned out? I've had trucks give me some strange indicators when a bulb was burned out.

Artum Snowbird wrote:Yes, I know exactly what is wrong.The two lights that are flashing are actually flashing the thin running light filament. Take a wire from the battery negative, and connect it to the side of the bulb. If everything starts working again, it's that ground wire that is not properly connected to ground.What happens is like this. The current comes to the base of the thick filament of the bulb, then cannot exit out the side and go back to the negative because that connection is not there. So it then goes backwards through the thin running light filament and over to the other side and through that running light filament, then it goes out the good bulb on the side and to ground.The best way to prove this is what is going on is to take the red lens cover off, and look at the bulbs and the filaments inside them. In your case, the current is going through the thick brake/turn filament, then the thin filament on both sides. You will likely see the thick filament barely glowing, but the thin ones are more resistive to current so more voltage drops across them and they glow brighter.

Artum Snowbird wrote:Yes, I know exactly what is wrong.I believe he is saying the right and left turn signal are on, not the parking and signal lights.I would put a ohm meter between the right and left turn signal 12v supply lines and see if there is a short. The problem could be a short in the wire loom or with the turn signal switch which is in the steering column and they are not fun to take apart. One other thing to check is if you have a toad or trailer hooked up. Not sure of the circuit on your chassis but a mis wired toad or trailer could cause problems. Good Luck.

Artum Snowbird wrote:Yes, I know exactly what is wrong.The two lights that are flashing are actually flashing the thin running light filament. Take a wire from the battery negative, and connect it to the side of the bulb. If everything starts working again, it's that ground wire that is not properly connected to ground.What happens is like this. The current comes to the base of the thick filament of the bulb, then cannot exit out the side and go back to the negative because that connection is not there. So it then goes backwards through the thin running light filament and over to the other side and through that running light filament, then it goes out the good bulb on the side and to ground.The best way to prove this is what is going on is to take the red lens cover off, and look at the bulbs and the filaments inside them. In your case, the current is going through the thick brake/turn filament, then the thin filament on both sides. You will likely see the thick filament barely glowing, but the thin ones are more resistive to current so more voltage drops across them and they glow brighter.Follow Mr. Snowbird's advice and check out your problem as he describes. This is one of the best explanations I've ever seen about how an ungrounded/improperly grounded bulb will backfeed current thru another bulb, seeking a ground.

Turn signal problem | Good Sam Community

WHITE_TOP
Explorer
Mar 19, 2014
Hope this helps.I once had a Chevy truck with similar issues and turned out to be the emergency flasher switch was bad,may be the same thing garry1p stated.Good luck. (spell check not working hope I did not need it ha ha)
garry1p
Explorer
Mar 19, 2014
This problem is known to happen when the flasher module fails on GM vehicles.

I found this when researching a weird problem with my turn signals.

New module is over $100 salvage yard $20.

May not be your problem but worth checking out if your RV uses the same module.
fcooper
Explorer
Mar 19, 2014
I agree with TucsonJim

I think you have a broken turn signal filament that is hanging down and touching the run light filament in the same bulb, therefore backfeeding current into the run light circuit. I'd check all the turn signal bulbs before doing a lot of trouble shooting.

Let us know what you find as the solution.

Fred
OnaQuest
Explorer
Mar 18, 2014
Artum Snowbird wrote:
Well now, if thin filaments were actually less resistive as you say Mr OnaQuest, why would is be necessary to use thicker and much less resistive wires for battery jumper cables.

In the back of every electrical code book is a chart that gives you the resistance of wires over a distance. It states very clearly that a thin wire has a lot more resistance than a thick wire.

In a series circuit at 12 volts with three 2 ohm filaments, each will glow equally bright. In a series circuit with the same overall resistance of 6 ohms, but with two thin elements of 2.5 ohms each, and one thick element of only one ohm, the wattage of each element is determined by using current squared times resistance. Wattage in each element then is

thick filament, 2Amps times 2 amps times 1 ohm = 4 watts
thin filament 2 amps times 2 amps times 2.5 ohms = 10 watts

this is why the thin filaments glow brighter in the series circuit when power goes through the thick turn filament, then the two thin run filaments.
You are absolutely correct. My old professor would be ashamed of me.:)
Artum_Snowbird
Explorer
Mar 18, 2014
Well now, if thin filaments were actually less resistive as you say Mr OnaQuest, why would is be necessary to use thicker and much less resistive wires for battery jumper cables.

In the back of every electrical code book is a chart that gives you the resistance of wires over a distance. It states very clearly that a thin wire has a lot more resistance than a thick wire.

In a series circuit at 12 volts with three 2 ohm filaments, each will glow equally bright. In a series circuit with the same overall resistance of 6 ohms, but with two thin elements of 2.5 ohms each, and one thick element of only one ohm, the wattage of each element is determined by using current squared times resistance. Wattage in each element then is

thick filament, 2Amps times 2 amps times 1 ohm = 4 watts
thin filament 2 amps times 2 amps times 2.5 ohms = 10 watts

this is why the thin filaments glow brighter in the series circuit when power goes through the thick turn filament, then the two thin run filaments.
OnaQuest
Explorer
Mar 18, 2014
Artum Snowbird wrote:
Yes, I know exactly what is wrong.
The two lights that are flashing are actually flashing the thin running light filament. Take a wire from the battery negative, and connect it to the side of the bulb. If everything starts working again, it's that ground wire that is not properly connected to ground.

What happens is like this. The current comes to the base of the thick filament of the bulb, then cannot exit out the side and go back to the negative because that connection is not there. So it then goes backwards through the thin running light filament and over to the other side and through that running light filament, then it goes out the good bulb on the side and to ground.

The best way to prove this is what is going on is to take the red lens cover off, and look at the bulbs and the filaments inside them. In your case, the current is going through the thick brake/turn filament, then the thin filament on both sides. You will likely see the thick filament barely glowing, but the thin ones are more resistive to current so more voltage drops across them and they glow brighter.

The blurb does correctly explain how a missing ground on one side would cause the appearance of flashing on both sides, but the OP says that turning on the "turn signal" for the opposite side cause the same symptoms. This would not be the case if the problem was as described.
gfs1943
Explorer
Mar 18, 2014
Artum Snowbird wrote:
Yes, I know exactly what is wrong.
The two lights that are flashing are actually flashing the thin running light filament. Take a wire from the battery negative, and connect it to the side of the bulb. If everything starts working again, it's that ground wire that is not properly connected to ground.

What happens is like this. The current comes to the base of the thick filament of the bulb, then cannot exit out the side and go back to the negative because that connection is not there. So it then goes backwards through the thin running light filament and over to the other side and through that running light filament, then it goes out the good bulb on the side and to ground.

The best way to prove this is what is going on is to take the red lens cover off, and look at the bulbs and the filaments inside them. In your case, the current is going through the thick brake/turn filament, then the thin filament on both sides. You will likely see the thick filament barely glowing, but the thin ones are more resistive to current so more voltage drops across them and they glow brighter.

Follow Mr. Snowbird's advice and check out your problem as he describes. This is one of the best explanations I've ever seen about how an ungrounded/improperly grounded bulb will backfeed current thru another bulb, seeking a ground.
zman-az
Explorer
Mar 18, 2014
Artum Snowbird wrote:
Yes, I know exactly what is wrong.

I believe he is saying the right and left turn signal are on, not the parking and signal lights.

I would put a ohm meter between the right and left turn signal 12v supply lines and see if there is a short. The problem could be a short in the wire loom or with the turn signal switch which is in the steering column and they are not fun to take apart.

One other thing to check is if you have a toad or trailer hooked up. Not sure of the circuit on your chassis but a mis wired toad or trailer could cause problems.

Good Luck.
GNR
Explorer
Mar 18, 2014
Artum Snowbird wrote:
Yes, I know exactly what is wrong.
The two lights that are flashing are actually flashing the thin running light filament. Take a wire from the battery negative, and connect it to the side of the bulb. If everything starts working again, it's that ground wire that is not properly connected to ground.

What happens is like this. The current comes to the base of the thick filament of the bulb, then cannot exit out the side and go back to the negative because that connection is not there. So it then goes backwards through the thin running light filament and over to the other side and through that running light filament, then it goes out the good bulb on the side and to ground.

The best way to prove this is what is going on is to take the red lens cover off, and look at the bulbs and the filaments inside them. In your case, the current is going through the thick brake/turn filament, then the thin filament on both sides. You will likely see the thick filament barely glowing, but the thin ones are more resistive to current so more voltage drops across them and they glow brighter.
TucsonJim
Explorer II
Mar 18, 2014
Are you 100% certain you don't have a bulb burned out? I've had trucks give me some strange indicators when a bulb was burned out.