Working on a 2001 Dodge Ram 2500 with 5.9 Cummins Diesel.

History: Left front turn signal quick working a few weeks ago. When left turn signal turned on, instrument cluster lamp stays lit. A quick look outside reveals that left front bulb is not flashing. Running light works, all other turn signals work.
First I replaced the bulb. Still did not work.
Second, I replaced the bulb socket. Still did not work.
Third, I checked every ground I can find, from the ones to the engine block to the one coming out of the lamp assembly to the frame. All were secure and wires were in good order, no sign of corrosion or insulation defects.
Fourth, I replaced the turn signal multi function switch. This is where it got weird.

Now, after replacing the mutli-function switch, all turn signals and hazards work correctly, IF the running/headlights are off. If I turn on the running lights or headlights, the left turn signal indicator on the instrument panel stays lit. If you turn on the left turn signal while the running lights are on, the instrument cluster light glows irregularly and the actual signal sometimes will work, sporadically, and sometimes not at all. Right side turn signals work correctly. If I turn on the hazard lights, with or without running lights, all lights flash correctly inside and outside the vehicle.

So now I am in a place where I have no idea what is going on. This is a very weird and very specific set of symptoms.

I should note when I replaced the multi-function switch, I inspected the connector and wiring harness there, and it all looked fine.

Any ideas?

 

trying to do some more troubleshooting after work, I tried jumping the ground wire on the bulb socket to the battery negative terminal, and now I have no running lights and no dash lights. I have looked at wiring diagrams, what I am seeing here should not even be possible. If anyone has any ideas, i would love to hear it this is totally screwed up and I really don't want to have to take it to the shop to get this fixed.

 

You have a short somewhere. My first guess would be the trailer harness, if you have one.

 

I do, but I recently (about 6 months ago) re-wired a new trailer plug, So Think that's ok. Upon inspection it does no look like anything is going on there in the way of frayed or melted wires. I wouldn't think that would cause an issue with the left front turn signal anyway, but then again electrical issues are notoriously difficult and weird.

With the running lights and dash lights not currently working, the turn signals are working great right now, even if I turn on the headlights, which work. So right now only the running lights and instrument cluster lights are not working. Could this be a bad headlight / dimmer switch?

 

Ok, running lights and dash light issue was a blown parking lamp fuse under the hood. I missed that because it was mislabeled on the fuse box lid. Anyway...

So now I am back to the original issue I listed in the post I started with. The only thing I can figure based on the behavior I am seeing is not so much as a short circuit, but voltage from the running light circuit is cross feeding into the left turn signal circuit. Any idea where that could be happening?

Headlamp switch is new. Multi function switch is new, bulb socket is new, bulb is new.

One thing I was wondering about is a junction block labeled 8w-12-3 on the wiring diagram. Any idea where that is on the truck?

 

Junction box is in the left side of the dash. 8w-12-3 is the page that has the wiring diagram of it.

A bad bulb would give you the symptoms you are seeing. check both front and rear bulbs. (they are cheap enough, just replace them.) What happens is, one of the filaments burns out, breaks, and falls across the other one.... and weirdness ensues.

 

FIXED IT!

I am a little embarrassed by the cause here, but in the interest of helping someone else in the future, I will post the solution here.

So my actual problems where a burned out bulb and a bad turn signal multi-function switch.

That said, I made thing much MUCH worse for myself when I replaced the bulb socket. In my defense, I did not have a wiring diagram for the truck at the time. What I realized last night is that I had wired the ground on the bulb socket to the running light wire on the truck, and the running light wire on the bulb socket to the ground on the truck.

I was studying the wiring diagrams from the service manual (thanks much for that by the way!) and I was looking at the wire color codes... and had that dawning moment... son of a.....! DuOh!

So switched those two wires, a bing! it worked.

Anyway, its all working correctly now. Thanks.

 

Hey man, in the end, getting the problem solved is all that matters. We all have our 'moments'.

Glad ya got it fixed.