No Voltage on the LH/RH indicator circuits on 7-pin trailer connector

Hey, I just spent 4 hours trying to figure out why there is no voltage on the trailer turnsignal and brake light cicuits on my Dodge Ram 1500. It has the factory tow package that includes the 7-pin (and 4-pin) trailer interface.

I understand electrical troubleshooting somewhat and have checked all of the connections with a probe starting from the 7-pin connector back to the fuse box. All fuses are good. I even attempted to check the output of the fuse box at the 20-pin connector that appears to feed all of the indicator light circuits. These pins are pretty small and I am not sure which pins are which. Still no :excuseme: voltage on the trailer circuits.

All fuses marked trailer lights are fine. There are a number of empty relay sockets but none of these are marked trailer anything. I am stumped.

Is it possible these circuits aren't even hooked up at the source?

Probably not. Do you have a brake controller? Most newer trucks are pre wired for one. Usually the wires are tucked up inside under the dash for it. Check your owners manual.

I am pretty sure the connector under the dash is for a brake controller. Shouldn't the lights work even if I don't have the controller?

I am pretty sure the connector under the dash is for a brake controller. Shouldn't the lights work even if I don't have the controller?

Yes, they should work. Does the turn signals work on the truck? Sometimes there is fuses to insulate the trucks wiring from the trailers wiring. This is because so many trailers wiring is unkept, especially those little home made jobs and electrical issues can pop fuses making the trailer and truck go out at night. This way, the truck will still have light. Look for some separate fuses somewhere. IS the running lights for the trailer working?

You never said if they ever worked....Is this the first time you went to use the 7 pin connector, and found this problem, or did all the lights work before....

The reason I ask is that many times someone will buy a used truck that prior owner had a hitch installed but didn't install all the correct tow light relays for al the lighting...

If you know for sure all the lights have worked in the past, I'd take apart the 7 pin connector to insure all the wire to pin connections are still there and not corroded...and of course check for proper voltage to each circuit...

That's exacty what happened. I purchased the truck 2 years ago and never even gave the trailer electrical a second thought until I had the bright idea to rent a tent camper to keep my wife and kids warm out in Oco Thanksgiving weekend.

I show up to pick up the trailer and "DOH" the the lights aren't working. I really feel like an idiot for not putting a probe to the connector.

Anyway, thanks to some buddies, we all have them, the guys that have been at this their whole lives, we discovered an aftermarket bank of 5 Bosch relays screewed to the plastic frame of the lower dash panel under the steering wheel. My truck has an after-market keyed ignition lock-out installed that appears to have something to do with these relays.

It appears that they may have had some sort of trailer security system and/or brake controller installed as well because in addition to the 7-pin in the bumper there is an in-line 4-pin hanging behind the bumper.

Does this sort of set up sound familiar to anyone?

And yes, my truck lights work fine, fuses are all good. The 7-pind and 4-pin meter out the same. I have +12v on the pos pin and +12v on the running lights but everything else is dead.

This is consistent all the way forward to the 2 connectors behind the front driver's side wheel. I am pretty sure the problem is behind the dash.

It appears that someone has jacked up your wiring...maybe there was some sort of relay system that blinked the lights on & off and has malfunctioned causing no lights...

I have to say this...cause it happens everyday...Why do people ask for help but don't give you the most important info needed...Make, model and year...

I was going to try and help by looking up the factory wiring diagrams that I have access to....only I don't know what the heck to look up...

2004 RAM 1500. I'm taking it to my mechanic today.

2004 RAM 1500. I'm taking it to my mechanic today.

A better place to post this question would be on a Dodge truck specific forum such as

A search on ( ) came up with this:

"Keep in mind that an 04 does not have a TIPM like the 06 and up trucks. On the 03-05 trucks, there is the FCM (Front Control Module), PDC (Power Distribution Center) and the IPM (Intergrated Power Module) which is the combo of the FCM and PDC. The problem in the 03-05 trucks is frequently a bad, non-servicable relay on the bottom side of the PDC. Fixing the problem typically requires replacement of the PDC. Often, the 06 and up trucks can have the reflash done to set the breaker rating higher for the trailer circuits."

Edited by oldmanx
additional info

I just went through this same issue with my 2004 ram 2500. After researching the Dodge forums, I installed a Modulite lighting module last weekend and everything works fine now.

Thanks oldmanx. I think I have found the PDC and it appears to be an unserviceable blob as your post suggests. I'll try to find out more about testing it.

I'll look into the modulite. Does this replace the white blob under the left side of the lower dash oldmanx is referring to?

The Modulite splices into the trucks brake/tail/turn wiring. It also needs 12v from the battery. The module is similiar to the old converters for imports.

After checking every wire/fuse relay/ connector I could find on my own I finally had to accept defeat. I took my truck to a shop here in town and a few hundred bucks later I drove home with a new IPM installed. I guess there was a problem on one of the circuit boards inside. Thanks for the quick responses everybody.

Ok just to bring closure to the subject. I finally took the truck to an automotive electrical specialist. They thought the problem was an FPM. After they replaced the part they discovered that they were wrong and the problem was in the IPM ...the box in the driver's front corner of the engine compartment that contains a couple of circuit boards and all of the fuses/ relays plug into. The guy at the shop tells me that the problem was a short in one of the circuit boards in thE IPM. After $600 replacing the IPM I have all trailer

light circuits working as they should.

I am pretty sure the connector under the dash is for a brake controller. Shouldn't the lights work even if I don't have the controller?

Yes, does the lights work OK on the truck itself?

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