Headlight Problem

This one is driving me nuts [short drive].
My passenger side headlight on dim went out, this has happened in the past but came back on before I could fix it. This time it didn't come back on. I replaced the bulb, socket and switch but didn't help.
I checked volts at socket and got 11.2 V. I checked with wire from battery ground and its the same.
The strange part is that I get the same volts with the switch off. There must be a explanation, does anyone have any ideas.
2004 2500 Ram

 

You need to check with the bulb in place, otherwise you are measuring the bulb-out pulses. Which are sent out all the time, no matter if the switch is dialed to On or Off.
No bulb - no power to the socket.
And it takes a couple seconds for the BCM to recognize the bulb and switch on the power to that circuit.

So, in your case the BCM isn't happy with the resistance readings he gets from that circuit and therefor doesn't switch it to ON. Very likely you have a short somewhere or a big resistance at a connection from power or ground.

Replacing the switch was a waste of money as this tells only the BCM to switch the lights to the desired position. There is no hard wiring from switch to bulb.

FYI, disconnect the switch illuminates the driving lights and taillights?&src=tdr">tail lights by default to ON as soon as the engine is started. A fail safe feature.

 

As a first I'd recommend a hard reset of the systems, if not already done, by disconnecting the battery's for some time. This cures many glitches.

 

The explanation as to why you are getting voltage at the bulb socket with the switch on or off is that the switch tells the BCM to switch the GROUND side, so there is always 12v to the bulb and if you are checking socket to the battery ground, then your DVM will show the 12v, NOT off.

That said, 11.2 volts is kinda low...what are you reading at the batteries?? Should be at least 12.6 standing volts.

 

At the batteries I am getting 12.2 volts this is after I have had the lights on and off for a while checking wiring.
I am going to do the hard reset and see if that helps but will be a couple of days.

 

Maybe your battery's are on the end of their lifetime also, shouldn't be so low by just playing around with the lights for some time.
I recommend a load test.

And the voltage to the headlights is regulated through the BCM, they can show different then what the Car has with the engine running.

A good healthy battery has a resting voltage of 12.8V the next morning.

 

You should be working with Ram's engineers to build a more reliable truck. I'm always impressed with you and others on this site.

 

Thank you sir, very much appreciated.

 

I got around to working on truck again and checked battery voltage at 12.7so that is good. I still can't find problem with right headlamp I think its in the front control panel. Will a OBD scanner pick up on this and can it be repaired. I know nothing about this.
Thanks for everyone's help.

 

I had a similar issue caused by a poor quality connection between the BCM and the fuse block. I removed the fasteners connecting the BCM to the fuse block and gently moved the BCM into several different degrees of connection to the fuse block. What I found is that the connection was improved by keeping the BCM out from the fuse block about 3/16" of an inch. I placed 2 washers on each fastener between the BCM and the fuse block to act as a spacer. Once tightened down, a reliable connection has been maintained.

The other issue I've run into is poor quality control with LED light bulbs and the required "decoders" vendors offer. If you're running LED bulbs, try switching the bulbs around and see if it follows the bulb or the "decoder". I've been warned that these LEDs can cause catastrophic failure of a wiring harness. I've not experienced that and have been running such bulbs for 3 years now. I have had a poor quality "decoder" that would overheat and cause a similar issue, only with one high beam instead of your low beam.

Good luck!

 

With wiTech or AutoEnginuity that circuit can be examined.

 

Like said, as long as this circuit isn't in perfect shape, electrically, BCM will not feed it. As long as it senses a fault, there is no current to the bulb.
This is how it works.

 

There is one thing you can try as it came up years ago quite often.

You have your fuse box, and directly attached to it is the control module - it's somewhat hidden inside the fender. So the fuse box needs to come out to come to it.

Unplug that module from the fuse box and spray the contacts on both sides with electronic cleaner, then re-attach after dry.
As this connection sometimes oxidized.
Some people had good results with putting small washers in between the two units so that the connector rested in a different position, reestablished a formerly broken connection.

Also check the bottom connectors at the fuse box for corrosion. Make sure to reattach them correctly otherwise they will fall out of the connection after a couple miles.

Battery's need to be disconnected for that!

 

OP - In case you're still working through your headlight issue, I've tried to attach a couple of pictures that may help you visualize the potentially problematic connection.

One picture is the fuse block side of the connection. The other is the FCM side. I mistakenly called it the BCM in my earlier post. I hope I added the pics correctly.

As mentioned earlier, clean the connections with some electrical cleaner. I cleaned the connections up with some gentle use of emery cloth, then electrical cleaner again, and then a sparingly light coat of dielectric grease to help keep corrosion at bay. Last, I experimented with spacing and found I needed 2 washers under each screw securing the FCM to the fuse block to get a reliable connection.

As Ozy mentioned, also check the connection in the fuse block for continuity. The power goes to and through the fuse block, then through the FCM, and finally out to the bulb. The FCM is a gatekeeper, so if you have power through the fuse block to the FCM, but not out of the FCM, you will have to determine if it's a connection issue or if your FCM is malfunctioning. Likewise, if power in but not out of the fuse block, the issue is likely a connection inside the fuse block. #ad
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Thanks Bryan and all for the photos and great info. I had to stop with the head light problem but getting back on it next week. I'll let everyone know when and if I find the problem.

 

I was able to get back on the headlight problem, had to put a new radiator in it with hoses and stat. At 81 I don't move as fast as I did 10 years ago.
I cleaned all grounds I could find and pulled the FCM and sprayed with contact cleaner. I then put 2 small stainless washers to bring it out to another contact point and headlight now works. that seams to be a poor location for the electrical but its worked for 250,000 miles so must be ok.
Thanks to everyone for your great help I could not fixed it without your help.

 

Awesome that you could solve it.

 

Nice work! Glad to hear you got the headlight working again.