Bad Ground ?

jhpfennig · 10-23-2009, 10:45 PM

I have a 2005 3500 4x4 towing an All American Sport 5er toyhauler. With the trailer unhooked but with the 7 way light cord plugged into the truck , I seem to have a feedback in the tail & marker lights. With the tailights and the left or right signal on, I get a dim flash in the opposite side tailight. The rear marker lights also flash dimly. It sounds like a bad ground to me. I checked all the trailer light grounds with no success. I had the truck checked by a Dodge dealer and said it was ok. I also hooked my truck to another 5er and the same condition persisted. I also attached a temporary ground between the truck and the pin box, still no success. I'm convinced the problem is in the truck and not the trailer, but where ? Any help would be appreciated, thank you.

PBJ2500CTD · 10-24-2009, 12:50 AM

How does it act with the trailer hooked to the hitch? If the symptoms disappear you probably have a bad connection at the plug. Speaking of plug...are you connecting to the factory plug in the bumper or an auxiliary plug set up for the 5'r? The truck's receptacle may be the problem. Using an ohmmeter do you get < 1 ohm from the ground pin to a good chassis ground?

jhpfennig · 10-27-2009, 06:20 PM

Thanks for your response. The lights act the same when truck is hooked to the trailer. I've tried connecting into both the stock bumper plug as well as the auxilliary receptacle that was installed. I removed the tape where the two plug harnesses join and found no evidence of corrosion. I also cleaned the battery terminals as well as any ground wires that I was able to find. I followed the trailer wiring harness to behind the left front wheel well. It joins another harness at that point and goes God knows where. Where is the trailer harness normally get it's ground ? When I test the ground pin in the receptacle to a good chassis ground I get 4-5 ohms. If you have a Verizon cell phone and want to call me there would be no charge to either of us. My number is 716-628-5886 and am on Eastern time, 3 hours later than you. If I'm unable to locate the problem, I'll have to take it to a dealer, OUCH. Thanks again. Oh by the way, my name is John Pfennig and live near Buffalo, NY.

B.G. Smith · 10-27-2009, 07:26 PM

You will probably have to remove the factory installed receptacle to get at the problem, it is probably corrosion in the rear where the wires are connected to the receptacle.(oxides growing back there) bg

jelag · 10-27-2009, 07:32 PM

You are describing a bad ground, but I think its in the trailer and not the truck... I'd suggest you open one of your tail/stop lights and run a test jumper from the ground wire in that light to the truck... I'd just use a small vice grip to clamp the wire for testing to the connectors... I don't think your 4-5 ohms is out of line for a ground circuit... but remember your not testing the bad connection... With the tail light open you might also try the ohm test from the ground in the tail light to the truck... REMEMBER not to have the circuit powered at this time...

We've had problems with trailers as well... We've run an 8 gauge wire from the trucks (12+) battery to the trailer plug through a 40 amp fuse... We also ran the same gauge wire from the trailer plug to ground...

We've also ran the same size wire in the trailers....

We've found that the trailer batteries charge faster, and hold a higher voltage.. on one of our work trailers we've installed a relays to run both stop/tail/running lights instead of the factory wiring... using the factory wiring to trigger the relays.. and drawing power from the 12+ lead in the trailer battery box..

One last thought... when we get the time we remove all the scotch lok connectors and use a good butt connector and shrink tube.. the shrink tube we use has an adhesive that is heat activated so the connection becomes water tight...

jhpfennig · 11-01-2009, 08:04 PM

Hi Jim, thanks for the response. I've tried hooking my truck to another trailer with the same results. Also have hooked another truck to my trailer and it acts the same. I haven't tried any of your suggestions yet as I'm stumped why it happens with different trucks & trailers. I did install 2-6 volt golf course batteries hooked in series in the trailer, maybe that is causing it. I also wired in 2 stop/turn/tail lights at the rear of the trailer although I've unplugged the pigtails from the light bodies. Any other suggestions would be appreciated. Thanks.

jhpfennig · 11-02-2009, 10:30 PM

That wasn't it. It would have been too easy ! Still looking for the cause. Thanks.

jelag · 11-03-2009, 12:03 AM

No, I still think you have some kind of bad ground... I'm afraid with out the truck to do some basic tests I just think I'll lead you down a path.... that might consume hours...

Maybe sand off a bear spot on the trailer frame, and run a set of jumper cables from the - side of the battery to that spot you just sanded off... see if a really good ground solves the problem..

With happening with 2 trailers to the same truck makes me believe that your right and the problem is in the truck... and I'm still thinking ground.. but can only assume its a connection at the frame somewhere...

Also pull the female connector from the truck, check those connections and while its loose from the truck plug the trailer into it.. than consider trying your jumper from the ground at the back of the plug to ground...

Good luck..

PBJ2500CTD · 11-03-2009, 02:20 AM

Your symptoms are typical of a bad ground. We are just having a difficult time identifying its location. What I find odd is the symptoms are occurring with your truck and your trailer; with your truck and another trailer; with another truck and your trailer. That last combination would indicate a bad ground in both your truck AND trailer, which can be a possibility. The only combination not given was how the other truck and other trailer acted.

You said you removed some tape and did not see evidence of corrosion at the harness addition. How were the wires connected? Were they soldered, or stripped and wrapped together?

I agree with BG Smith's recommendation for a starting point. Disconnect and clean the harness connector to plug...maybe even change the 7 pin plug out. They aren't that expensive.

Have you changed any bulbs in either the truck or trailer lately? Even if you have not changed bulbs I would verify that the the correct bulb is installed in each lamp assembly and that they are clean and properly seated. Check the elements in the bulbs, too. Make sure they are positioned on their respective posts. Another item that has come to mind since the symptoms happen to both left and right is to try disconnecting the brake controller. Now if you could only get access the other "TDR" (time domain reflectometer) we could find your issue in short order.

HCannon · 11-03-2009, 02:45 AM

If I am understanding your problem right, I have the same one. If I take a set of jumper cables and go from a good ground on the truck to a good ground on the trailer everything works as it should. Havent had time to look into my problem, been just putting my jumper cables on and going. Mine will do it on any trailer that I have had it hooked to, so that make me thank that it has to be the ground on the truck. When you get yours fixed, let us know what you find.

TLane · 11-03-2009, 03:53 AM

You need either a meter or a test lamp and good knowledge of how to use them. And be very careful not to short out the pins while probing in the connectors.
If you have a meter:
-Measure the resistance from the ground pin in the connector to the frame of the truck. It should be well under 1 ohm. Many people will measure the voltage on the ground with no load and assume zero volts means "it's OK" - but that isn't a good test.
-Measure the resistance from the ground pin in the connector on the trailer to the frame on the trailer. It should be well under 1 ohm.
-Check to make sure the ground pin on the trailer side is the same pin as the gound pin on the truck side - I've gotten the pins swapped before by turning the plug around.
-With the trailer connected to the truck and as many running/tail/turn signal lights turned on the trailer, measure the voltage between the trailer frame and truck frame. It should be zero.

If you have a test lamp:
-Ground one side of the lamp to the truck frame, check the brightness of the light when you touch the hot pin, the running light pin, and either left or right turn signal in the truck connector. Then ground the light at the truck connector and touch each pin again - the light should be just as bright. Loss of brightness indicates a poor ground, no lamp at all indicates no ground at all.
-If the trailer battery is connected up to the trailer plug such that there's a hot lead in the trailer plug (normally the case), connect the test lamp between the hot lead and ground on the trailer connector. It should be just as bright as when you connect it across the trailer battery.
-With the trailer connected to the truck and lots of lights on, connect the test lamp between the frame of the trailer and the frame of the truck. Any illumination at all indicates a poor ground.

In many cases the test lamp is actually a better instrument than a meter because it draws some current. Many meters will read a voltage at a connector pin when there is very little current available due to poor connections. Meters are great for testing the PCM, where you don't want to draw any current; but poor at diagnosing trailer lights where you want to put a load on the circuit to make sure it works properly. I'd get a test lamp at the local parts store start and start poking around.

Getting a ground connection is a lot harder than it first seems. Dirt, paint, rust, plastic, all kinds of things get in the way when you're trying to make a good ground. You really need to scrub a good clean spot to get a good connection, especially at a connection to a frame for a ground. Many people will drill a hole and install a wire under a screw - it will work for a little while. And just touching some spot with a probe often does NOT result in a ground. Clean off spot to nice shiny metal to be sure.

Don't give up on it - keep testing until you find it.

PBJ2500CTD · 11-03-2009, 05:51 AM

Good thorough post TLane.

RBridenbaugh · 11-03-2009, 09:04 AM

I had similar symptoms when I got my used truck and I traced it to connection /corrosion problems with my 7 pin connector.

The problem was not between the trailer 7 pin and the bumper mounted 7 pin receptacle, rather it was between the back of the bumper mounted 7 pin receptacle and the wiring harness connector that plugs into it. It wasn't seated completely and connections were dirty and corroded.

Removed the receptacle from the bumper, cleaned contacts and wiring harness connector and completely seated the 10 pin wiring harness connector into the back of the 7 pin receptacle. Problem solved.

jhpfennig · 11-09-2009, 02:05 AM

I haven't had a chance to try TLanes' approach to finding the problem. Today however, I used a snowmobile battery to power all the trailer plug positions without the truck hooked up. I still was able to simulate the same problem. Even more weird, the truck tailights act the same without the trailer cord hooked up. Now I'm really baffled ! Are there any TSI letters dealing with this ? Maybe my problem is the truck and not the trailer. That still doesn't account for the trailer screwing up when the truck is not attached. Any ideas ?

Hitec.Redneck · 11-09-2009, 02:24 AM

If you connected a battery to the trailer lights only( truck disconnected I presume) and experienced the same thing it sounds like the ground on the trailer has a bad connection, the symptoms you are describing I have had several times and has always turned out to be a bad ground. You should locate where the ground wire in the wiring harness connects to the trailers frame, remove it and make sure there is shiny metal there, take a wire brush on a drill, sandpaper, grinder, something to get to bare metal, then reconnect it back and test again with the batter, that should eliminate that problem.

If you are saying the truck now has the same problem with the trailer disconnected not sure where to tell you to start on that one.

Good Luck