Greetings Dodgeforum,

This is my first tech post, but I assure you there are many to come. Here's what I've got going on with my lights:

Problem 1: The headlights usually don't work, but sometimes kick on after idling or slow rolling

I can hear you shouting "HEADLIGHT SWITCH!!!" already. But wait, I have a voltmeter! Let's confirm.

When the lights aren't working: with the switch off and F6 removed, there is 0v across the terminals. With it pulled to low-beam position, there is +12V, and yet the lights are dark. The failure affects both high and low beams simultaneously, so it can't be a short on L3 (highs) or L4 (lows), which means there's a short to ground somewhere between C206 (headlight switch) and C210 (column-mounted multifunction) (See service manual 8W-50-4). Does this sound like a bad switch or a mangled wire? I don't understand where that power is going if not through the headlights.

Problem 2: The left tail light goes out when the brake is pressed

I am very confused with this one. When unplugged, the socket 4-pin harness connector reads +12 on the running light normally, but shows divided voltages (7.5, 4.5) when the brake is depressed. It seems like there's a problem somewhere between the switch and the end of the harness, but where? With a bulb in, it runs dim normally and dies when the brake is depressed. Where the hell is the voltage divider if the bulb is out? Something is bridging these wires, but I don't know where. I can't be 100% sure my measurements are right here, it's possible that I was checking the wrong pins since I didn't have the service manual at the time. One license plate light works so at least I know the ground (Z13) and running light power (L7) are good all the way down. I do not know if the license plate light goes out when braking.

Auxiliary problems and hints

Some other issues that may relate but probably don't:

• The underhood light was unplugged when I bought it, but it works fine. Suspicious.
• The PDC has the trailer light fuse (I think) value scratched out and replaced with "40A"
• The radio doesn't work, and notably shares circuit L7 with the tail lights
• ABS is always triggered, such that it actuates even under light braking with the fuse installed. I pulled the fuse for now.

 

Easy stuff:
Underhood lite tends to like to stay on, and runs the battery dead in a few hours. There is a mercury switch in there that controls it, maybe not as reliable as it could be.
Trailer fuse is indeed 40 amps. It's for trailer brakes.
Does the clock on the radio work? Do the dome lights work they way they are supposed to?
May wanna replace the rear wheel speed sensor. (on top of the rear diff) Not that the RWAL on these trucks was much good brand new.......

Not so easy stuff......

Pull the headlight switch, and have a look at it. Saw this problem in another thread recently, and it was indeed the headlight switch that was bad. It's relatively cheap, just replace it. (common problem on these trucks.)

Rear lights.....

Have a trailer harness on that thar truck? Check where the trailer harness hits the main rear harness. Also a common problem area.

Welcome to DF!

 

HeyYou work fast, man! Thanks!

For the easy stuff, that seems pretty reasonable.

• There is nothing displayed on the radio (zero power whatsoever, switches do nothing). Maybe a bad fuse, this is on the backburner.
• The dome seems to work fine, comes on when I twist the lamp switch and when I open the door.
• RWSS is probably bad, given the state of the rest of the axle. It's FWAL if that matters, hope I don't have to do the FWSS too.

For the complicated stuff:

• Will do on the headlight switch. Strongly considering an upgrade to relay-driven HIDs soon, so it'll be a good fresh start
• Yep, there's a trailer harness. It's nasty too, looks like the cap stopped closing about 10,000 miles ago, and some of the wires are clipped.

Unfortunately, I left the Ram at work tonight for logistical reasons, but I'll have it home tomorrow.

What should I be looking for where the trailer wires meet the harness? Burns? Cuts? Bad workmanship?

 

Ok, do you have a factory service manual? If not, get one here. (should put that in my first post.)

Think I would pull the radio while you were playing with the headlight switch, and see what's up back there. Make sure you are getting power where you should, when you should. Might simply be a failed head unit.

4WAL? Ah, such fun. Before replacing anything, I would hook it back up, drive it for a while to get the light to come on, then get the codes read, and see what it is whining about. RWSS is cheap and easy, not so sure about the fronts... (my truck doesn't have 4WAL....)

Usually, where the trailer harness hits the rear harness, the splices are really poor quality, like to corrode, break, and cause all sorts of other havoc. Have a look, see what ya think, take it from there.

Get to it when you can, and let us know what ya find.

Good Luck.

 

Thanks for the SM links, I actually downloaded them from the same guy before posting to look up those circuits. Wiring diagrams make everything more visible compared to crawling around under a dirty truck at night!

WRT the ABS: I was originally driving on 4 mismatched tires (lol farmers) and just yesterday replaced them with 4 new Duratracs. I didn't put the fuse back after the tire shop, but I wonder if it fixed anything. If not, I have no problem blowing $15 on a sensor that's known to fail, even if it doesn't fix the problem outright.

I'll see you tomorrow night, hopefully with good news.

 

If it had four different sized tires on it before, yeah, that will confuse the snot out of the ABS computer...... Give you the electronic equivalent of throwing up its hands, and hollering "I QUIT!!" Plug the fuse back in, see how it behaves. The new tires may just solve the issue.

 

I didn't get around to as much as I would have hoped in the cold, dark weather, but tonight I got the headlight switch out. It's the original Chrysler unit with no visible burns or marks and prior to removal the switch was solid. I'm a little skeptical, but I ordered a super cheap knockoff replacement just to see if it's a switch problem anyway. That arrives tomorrow, along with an OBD2 scanner to troubleshoot a surprise CEL. Can these things be damaged internally and not show burns or melts?

 

If the connector looks good as well, you are likely going to have to look elsewhere for the problem..... put in the new switch when it shows up anyway, and see what happens. Take it from there.

 

Miraculously, it was the switch. There must have been something burnt or worn internally, because the crappy new one worked right away. Add this to your catalog of mysteries - the HL switch can fail internally without any signs of burns.

It's freezing rain here today, so no chance to get under and look at the trailer splice. I can at least check out the OBD codes while waiting for it to clear.

 

Update - Switch is still working, and gets super hot under normal operation. Really scary design, ordering the LMC Truck harness kit tomorrow.

Also, I got under and looked at the hitch harness. Not only was the hitch itself a mess, the main connector was completely ruined. One of the tabs had broken and turned into a giant wad of green copper rust. I need to cut it, but I'm not sure where yet. The grounding issue is still present with the hitch unplugged, so it's either at the connector or the splice. Hoping it's the connector.