Driver side marker light not working

[79Vette]

I've been looking at F-13 and F-14 in the -20 TM for about 2 hours, but I dont understand what the diagram is saying.

All passenger side marker and turn signals on the truck work.
The driver side parking light and rear turn signal works, and the side marker light flashes when the turn signal is on. However it does not illuminate when the headlights are on without the turn signal being selected. Also the left turn indicator in the dash never comes on, and the bulb is known to be good.

When the headlights are on, I have 12v on .8 BRN-9B and .5 LT BLU 14B is floating. Obviously this prevents the bulb from illuminating.
When the turn signal is engaged .5 LT BLU 14B is momentarily grounded to illuminate the side marker, and then connected to 12V to turn off the side marker and flash the parking light.

On the passenger side I have 12v on .8 BRN-9G and .5 DK BLU 15B is grounded, so the passenger marker light illuminates. It flashes the same way as the driver side, and works properly.

What is supposed to ground .5 LT BLU 14B when you turn on the parking lights or headlights? Is this one of the contacts in the turn signal switch (top left of F-14)?


Update: I tried replacing the turn signal switch because I had one laying around, but that didnt fix the problem. I dont have any idea what else to try. If anyone understands how this system works, I;d appreciate any tips.

 

[ezgn]

I've been looking at F-13 and F-14 in the -20 TM for about 2 hours, but I dont understand what the diagram is saying.

All passenger side marker and turn signals on the truck work.
The driver side parking light and rear turn signal works, and the side marker light flashes when the turn signal is on. However it does not illuminate when the headlights are on without the turn signal being selected. Also the left turn indicator in the dash never comes on, and the bulb is known to be good.

When the headlights are on, I have 12v on .8 BRN-9B and .5 LT BLU 14B is floating. Obviously this prevents the bulb from illuminating.
When the turn signal is engaged .5 LT BLU 14B is momentarily grounded to illuminate the side marker, and then connected to 12V to turn off the side marker and flash the parking light.

On the passenger side I have 12v on .8 BRN-9G and .5 DK BLU 15B is grounded, so the passenger marker light illuminates. It flashes the same way as the driver side, and works properly.

What is supposed to ground .5 LT BLU 14B when you turn on the parking lights or headlights? Is this one of the contacts in the turn signal switch (top left of F-14)?


Update: I tried replacing the turn signal switch because I had one laying around, but that didnt fix the problem. I dont have any idea what else to try. If anyone understands how this system works, I;d appreciate any tips.

Check the bulb sockets, put in new bulbs, check and clean all grounds going to the frame, and check for loose connections.

 

[79Vette]

Check the bulb sockets, put in new bulbs, check and clean all grounds going to the frame, and check for loose connections.

The bulb works in the socket on the other side of the truck. The grounds are good, and as far as I can tell there are no loose connections.
The lamp is a single filament, 2 wire bulb and flashes when the turn signal is engaged. Unless I am overlooking something, this means the bulb, socket, and connections are all good otherwise it not work at all since any broken connection would also affect the turn signal. It does not illuminate when the headlight switch is on, so there seems to be an issue somewhere with the switches/flasher/cab wiring.

Do you understand how the turn signal switch/flasher and the headlight switch work? From my previous post, probing the functional passenger side wiring indicates that the blue wire at the bulb is grounded and the brown wire is at 12v when the headlights are on, but I do not see where the ground path for that blue wire is on the diagram from the TM and so I don't know where to troubleshoot next.

 

[ezgn]

The bulb works in the socket on the other side of the truck. The grounds are good, and as far as I can tell there are no loose connections.
The lamp is a single filament, 2 wire bulb and flashes when the turn signal is engaged. Unless I am overlooking something, this means the bulb, socket, and connections are all good otherwise it not work at all since any broken connection would also affect the turn signal. It does not illuminate when the headlight switch is on, so there seems to be an issue somewhere with the switches/flasher/cab wiring.

Do you understand how the turn signal switch/flasher and the headlight switch work? From my previous post, probing the functional passenger side wiring indicates that the blue wire at the bulb is grounded and the brown wire is at 12v when the headlights are on, but I do not see where the ground path for that blue wire is on the diagram from the TM and so I don't know where to troubleshoot next.

Honestly, I have never had an occasion to dig that deep into the electrical diagrams. Those side markers I thought, have separate ground connections on the frame close to the actual markers. Sometimes the ground connections upstream or downstream on the lighting circuits can affect other areas. I wish I could be of more help to you, but thoroughly check all the grounds, and clean them up if you see corrosion.

 

[Guyfang]

Have you ever seen this work right? Or did this problem just pop up new?

 

[dmetalmiki]

Just Hot wire the + input wire and see if it lights up. (AND what else corresponding to that circuit ).
Check the Voltage - IN , - in, the 'supposed' operating position.

 

[79Vette]

Have you ever seen this work right? Or did this problem just pop up new?

It never worked right since I bought the truck, and I have just been ignoring it.

However, I think I figured it out and I am embarrassed to admit it took me this long. The ground path for the blue wires on the side marker bulbs is THROUGH the park lamp bulb. This is why the bulbs flash at opposite times when you use the turn signal. When you turn on the headlights the entire .8 BRN-9x circuit branch gets power, applying 12v to the side marker bulb and one of the 2 filiments in the park lamp bulb. The park lamp bulb has its own ground (.8 BLK-150A), and the side marker bulb grounds through the unpowered turn signal filament in the park lamp bulb (LT BLU 14x on the driver side and DK BLU 15x on the passenger side).

When you turn on the turn signal the flasher applies 12v to the blue wire and illuminates the turn signal filament in the park lamp bulb, and consequently removes the ground from the side marker bulb and causing it to go out (like how the GEN lights go out when the alternator is supplying power). This is why the side marker and park lamp flash at opposite times when the turn signal is on.

This truck had LED bulbs when I bought it, and the LED in the park lamp slot does not provide a low resistance path to ground for the side marker bulb. Removing the LED park lamp bulb and replacing it with the correct incandescent bulb causes everything to work properly. Measuring everything with a meter, it turns out there is a 90-100 ohm short to ground somewhere in the blue wire to the passenger side marker light that allowed that bulb to work even with an LED park lamp installed. If the wiring was undamaged, neither side would have worked. I'm going to find and fix that short tonight.

In conclusion, if neither side worked I think I would have immediately realized the problem was the LEDs. However with the passenger side appearing to work properly it was very confusing. I still don't understand exactly how the marker light was flashing with the turn signal engaged (since it couldn't ground through the park lamp bulb) but I assume there must be a low quality ground path somewhere inside the turn signal switch and/or flasher (because its only available with the turn signal switch closed) which is not important to the factory circuit design and not shown on the diagram but happens to sink enough current for the LED to operate (10-15mA).

I want to keep the LEDs, so I connected a 50 ohm 10w resistor from the blue turn signal wire to ground. This provides a ground path to illuminate the side marker when the turn signal is off, and sinks ~5w of power when the turn signal is flashing on). It turns out there are commercial products available to solve this problem for people doing LED conversions (literally a fancy plastic box with wire connections and a resistor inside it), but they cost 50x more than just getting the resistor...

Putco 240008 Putco LED Light Bulb Load Resistor Kits | Summit Racing

Free Shipping - Putco LED Light Bulb Load Resistor Kits with qualifying orders of $109. Shop Light Bulb Load Resistor Kits at Summit Racing.

www.summitracing.com

As an off topic rant:
In my opinion this is a pretty half-baked approach to circuit design. If another foot of 16 gauge wire and another 2-filament bulb were used, the side marker could be connected in parallel with the park lamp bulb and not use this absurd ground design. That would be a simpler and more robust system and add negligible cost, but I don't answer to the GM bean counters...

 

[ezgn]

It never worked right since I bought the truck, and I have just been ignoring it.

However, I think I figured it out and I am embarrassed to admit it took me this long. The ground path for the blue wires on the side marker bulbs is THROUGH the park lamp bulb. This is why the bulbs flash at opposite times when you use the turn signal. When you turn on the headlights the entire .8 BRN-9x circuit branch gets power, applying 12v to the side marker bulb and one of the 2 filiments in the park lamp bulb. The park lamp bulb has its own ground (.8 BLK-150A), and the side marker bulb grounds through the unpowered turn signal filament in the park lamp bulb (LT BLU 14x on the driver side and DK BLU 15x on the passenger side).

When you turn on the turn signal the flasher applies 12v to the blue wire and illuminates the turn signal filament in the park lamp bulb, and consequently removes the ground from the side marker bulb and causing it to go out (like how the GEN lights go out when the alternator is supplying power). This is why the side marker and park lamp flash at opposite times when the turn signal is on.

This truck had LED bulbs when I bought it, and the LED in the park lamp slot does not provide a low resistance path to ground for the side marker bulb. Removing the LED park lamp bulb and replacing it with the correct incandescent bulb causes everything to work properly. Measuring everything with a meter, it turns out there is a 90-100 ohm short to ground somewhere in the blue wire to the passenger side marker light that allowed that bulb to work even with an LED park lamp installed. If the wiring was undamaged, neither side would have worked. I'm going to find and fix that short tonight.

In conclusion, if neither side worked I think I would have immediately realized the problem was the LEDs. However, with the passenger side appearing to work properly it was very confusing. I still don't understand exactly how the marker light was flashing with the turn signal engaged (since it couldn't ground through the park lamp bulb) but I assume there must be a low-quality ground path somewhere inside the turn signal switch and/or flasher (because its only available with the turn signal switch closed) which is not important to the factory circuit design and not shown on the diagram but happens to sink enough current for the LED to operate (10-15mA).

I want to keep the LEDs, so I connected a 50 ohm 10w resistor from the blue turn signal wire to the ground. This provides a ground path to illuminate the side marker when the turn signal is off and sinks ~5w of power when the turn signal is flashing on). It turns out there are commercial products available to solve this problem for people doing LED conversions (literally a fancy plastic box with wire connections and a resistor inside it), but they cost 50x more than just getting the resistor...

Putco 240008 Putco LED Light Bulb Load Resistor Kits | Summit Racing

Free Shipping - Putco LED Light Bulb Load Resistor Kits with qualifying orders of $109. Shop Light Bulb Load Resistor Kits at Summit Racing.

www.summitracing.com

As an off-topic rant:
In my opinion, this is a pretty half-baked approach to circuit design. If another foot of 16 gauge wire and another 2-filament bulb were used, the side marker could be connected in parallel with the park lamp bulb and not use this absurd ground design. That would be a simpler and more robust system and add negligible cost, but I don't answer to the GM bean counters...

Very interesting and informative post. I'm sure it will help someone in the future. Good work and effort figuring it all out.

 

[WWRD99]

LEDs can work sometimes like a filament bulb and sometimes not...have no idea why...I use the led relay instead of using the individual resistor for each one...some are adjustable for flash speed as well...not sure how well they'd work with no leds or a mixture of them...I used led 194s all over mine but did not change the dual filament ones to led yet. Superbright led is a great site that has all that stuff.